Meeting the Challenge: 

Invasive Plants in Pacific 

Northwest Ecosystems

United States 

Department of 

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical

Report

PNW-GTR-694

June 2007
TECHNICAL EDITORS

Timothy B. Harrington, research forester, U.S. Department of Agriculture, Forest Service, Pacific Northwest Research 

Station, Forestry Sciences Laboratory, 3625 93rd Ave. SW, Olympia, WA 98512 and Sarah H. Reichard, associate professor,

University of Washington Botanic Gardens, Box 354115, Seattle, WA 98195.

Cover photography (clockwise from top):

Manual removal of garlic mustard (Alliaria petiole) (Karen J. Peterson)

Herbicide application to milk thistle (Silybum marianum) (Patricia A. MacLaren)

Seedhead weevil (Larinus minutus) for bio-control of diffuse knapweed (Centaurea diffusa) (Daniel L. Fagerlie)

Milk thistle (Silybum marianum) (Dennis Chambreau)

Scotch broom (Cytisus scoparius) (Joe Kraft)

Hoary alyssum (Berteroa incana) (Daniel L. Fagerlie)

Mechanical removal of Himalayan blackberry (Rubus discolor) (Ann Risvold)

Japanese knotweed (Fallopia japonica) (Kyle Strauss)

Center image: spotted knapweed (Centaurea biebersteinii) (www.invasive.org, Steve Dewey)

Graphic designer: Jenny Beranek, Beaverton, OR.

Papers were provided by the authors in camera-ready form for printing. Authors are responsible for the content and accuracy. 

Opinions expressed may not necessarily reflect the position of the U.S. Department of Agriculture.

The use of trade or firm names is for information only and does not imply endorsement by the U.S. Department of 

Agriculture of any product or service.

PESTICIDE PRECAUTIONARY STATEMENT

This publication reports research involving pesticides. It does not contain recommendations for their use, nor does 

it imply that the uses discussed here have been registered. All uses of pesticides must be registered by appropriate

state or federal agencies, or both, before they can be recommended. 

CAUTION: Pesticides can be injurious to humans, domestic and wild animals, and desirable plants if they are not 

handled or applied properly. Use all pesticides selectively and carefully. Follow recommended practices for the

disposal of surplus pesticides and pesticide containers.
Meeting the 

Challenge: Invasive

Plants in Pacific

Northwest 

Ecosystems

Timothy B. Harrington 

and Sarah H. Reichard

Technical Editors

U.S. Department of Agriculture

Forest Service

Pacific Northwest Research Station

Portland, Oregon

General Technical Report

PNW-GTR-694 

June 2007
This page is intentionally left blank.
ABSTRACT

Harrington, Timothy B.; Reichard, Sarah H., tech. eds. 2007. Meeting the challenge: invasive plants in Pacific 

Northwest ecosystems. Gen. Tech. Rep. PNW-GTR-694. Portland, OR: U.S. Department of Agriculture, Forest

Service, Pacific Northwest Research Station. 166 p.

During September 19-20, 2006, a conference was held at the University of Washington Botanic Gardens, Seattle, WA, 

with the title “Meeting the challenge: invasive plants in Pacific Northwest Ecosystems.” The mission of the conference 

was to create strategies and partnerships to understand and manage invasions of non-native plants in the Pacific Northwest.

The audience included over 180 professionals, students, and citizens from public and private organizations responsible for 

monitoring, studying, or managing non-native invasive plants. This proceedings includes twenty-seven papers based on 

oral presentations at the conference plus a synthesis paper that summarizes workshop themes, discussions, and related

information. Topics include early detection and rapid response; control techniques, biology, and impacts; management

approaches; distribution and mapping of invasive plants; and partnerships, education, and outreach.

KEYWORDS: Non-native plants, invasive, exotic, weeds, vegetation management, early detection/rapid 

response, biological control, integrated management.

ENGLISH EQUIVALENTS

When you know: Multiply by: To find:

Degrees Celsius (°C) (C*9/5) + 32 Degrees Fahrenheit (°F)

Centimeters (cm) .3937 Inches (in) 

Meters (m) 3.2808 Feet (ft)

Kilometers (km) 0.6214 Miles (m)

Square meters per hectare (m2

/ha) 4.3560 Square feet per acre (ft2/ac)
ACKNOWLEDGMENTS

This conference was sponsored by USDA Forest Service, Pacific Northwest Research Station; University of 

Washington Botanic Gardens; The Nature Conservancy; and the Center for Invasive Plant Mangement. We would like to

thank members of the Conference Steering Committee for their guidance during planning for the conference, the Science

Advisory Committee for their assistance with selection and organization of conference presentations, and students from

University of Washington and other colleges and universities who helped with conference logistics. Special thanks to 

Alaine Sommargren for assistance with manuscript formatting, to Steve Reutebuch for providing critical insight and 

support, and to our keynote speakers, John Randall, Les Mehrhoff, Ann Bartuska, and Doug Johnson, for sharing their

knowledge, experiences, and inspiring ideas. 
Meeting the Challenge: Invasive plants in Pacific Northwest Ecosystems

CONTENTS

CONFERENCE SYNTHESIS 1

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems 3

Sarah H. Reichard

EARLY DETECTION AND RAPID RESPONSE 11

Invasive Species Early Detection Protocol Development in the National Parks: Integrating All the Pieces 13

Susan O’Neil, Brad Welch, Penelope Latham, and Daniel Sarr

Developing Early Detection Networks to Abate the Invasive Plant Species Threat 17

Kyle Strauss

The First Line of Defense: Interceptions of Federal Noxious Weed Seeds in Washington 19

Margaret Smither-Kopperl

Summary of the National Environmental Policy Act Approach for Early Detection/Rapid 

Response to Invasive Plants on the Olympic National Forest 23

Rochelle Desser

CONTROL TECHNIQUES, BIOLOGY, AND IMPACTS 25

Developing a Biological Control Program for Invasive Knotweeds (Fallopia spp.) 27

Fritzi Grevstad, Richard Reardon, Bernd Blossey, and Richard Shaw

Cytisus Scoparius (Scotch Broom) Control Using Sewage Biosolids – Preliminary Results 31

Jacqueline D. Shaben

Establishment of Scotch Broom Seedlings in Douglas-fir Forests: Effects of Overstory 

Retention Level and Seedbed Type 37

Timothy B. Harrington

Biology and Management of Invasive Hawkweeds (Hieracium spp.) in the Pacific Northwest 43

Linda M. Wilson

Fungal Endophytes in Spotted Knapweed: Do They Affect Its Invasiveness? 47

George Newcombe, Anil K. Raghavendra, Alexey Shipunov, Cort Anderson, Hongjian Ding, 

Sanford Eigenbrode, Timothy Prather, and Mark Schwarzlaender

Successful Biological Control of Invasive Plant Species in Washington 51

Jennifer Andreas, Tara J. Zimmerman, Daniel L. Fagerlie, Brad W. Gaolach, Dale K. Whaley, 

and Tyler W. Wilson

Garden Loosestrife (Lysimachia Vulgaris), a Spreading Threat in Western Waterways 53

Katie Sauter Messick and Drew Kerr

Garlic Mustard Control: Is Success a Possibility? Strategy and Potential Impact 59

Karen J. Peterson

The Ecological Consequences of Giant Knotweed Invasion into Riparian Forests 63

Lauren Urgenson and Sarah H. Reichard
MANAGEMENT APPROACHES 65

Strategic Management of Public Invasive Species Programs 67

Steven J. A. Burke

Invasive Plant Management Following the 2003 Okanagan Valley Wildfires, British Columbia 73

Lisa K. Scott

A Summary of Acute Risk of Four Common Herbicides to Birds and Mammals 77

Shawna L. Bautista

Got Milk Thistle? An Adaptive Management Approach to Eradicating Milk Thistle 

on Dairies in King County, Washington State 83

Dennis Chambreau and Patricia A. MacLaren

Controlling Invasive Plants Without Herbicides, Cedar River Municipal Watershed 85

Sally Nickelson, Heidy Barnett, David Chapin, Bill Richards, and Dwayne Paige

DISTRIBUTION AND MAPPING OF INVASIVE PLANTS 93

Non-Native Plants on the Mt. Baker-Snoqualmie National Forest 95

Tracy L. Fuentes, Laura L. Potash, Ann Risvold, Kimiora Ward, 

Robin D. Lesher, and Jan A. Henderson

Is the Spread of Non-Native Plants in Alaska Accelerating? 117

Matthew L. Carlson and Michael Shephard

Challenges in Predicting the Potential Distribution of Invasive Species Using 

Habitat Distribution Models 135

Chad C. Jones, Steven A. Acker, and Charles B. Halpern

The Integrated Noxious Weed Invasive Species Project (INWISP) of Washington State 139

Daniel L. Fagerlie, Jennifer E. Andreas, Tara J. Zimmerman, Brad W. Gaolach, 

Dale K. Whaley, and Tyler W. Wilson

Distribution and Abundance of Invasive Plants in Pacific Northwest Forests 143

Andrew Gray

Herbarium Collections and Invasive Species Biology: Understanding the Past, Present, and Future 149

David E. Giblin, Ben Legler, and Richard G. Olmstead

PARTNERSHIPS, EDUCATION, AND OUTREACH 151

Weeds Cross Borders Project: A Canada – United States Collaboration 153

Lisa K. Scott

The St. Louis Codes of Conduct: Providing a Framework to Prevent Invasions from Horticulture 157

Sarah H. Reichard

Policies to Reduce the Risk of Invasive Plant Introductions via Horticultural Trade: Stakeholder

Perceptions and Preferences 163

Arianne Ransom-Hodges
CONFERENCE

SYNTHESIS

Garden loosestrife (Lysimachia vulgaris) (King County Staff)
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ABSTRACT

Compared to other parts of North America, the Pacific Northwest was settled relatively recently by humans of European 

origin. This more recent population growth and development has resulted in fewer plant invasions and therefore a greater

opportunity to protect still relatively pristine wild areas. This can be achieved by prevention of new invasions, improved

methods of control of existing invasive species, and better coordination of control work within geographic areas to prevent

reinvasion within the areas. However, increased attention must be given to coordination and education over a wider area,

such as the entire Pacific Northwest. This can be accomplished through non-profit organizations such as an Invasive Species

Coalition, similar to efforts developed in other parts of the United States. These non-profits develop educational tools, serve

to communicate about new invasions, and may coordinate control efforts over a broader geographic region. 

KEYWORDS: Invasive plants, coordination, Exotic Pest Plant Councils.

1 Sarah H. Reichard is an Associate Professor, University of Washington Botanic Gardens, Box 354115, Seattle, WA 98195, 

reichard@u.washington.edu

3

INTRODUCTION

The Pacific Northwest (PNW) region of North America 

encompasses two countries and four states in the United

States and one province in Canada (fig. 1). Coastal and

Plains tribes of Native Americans occupied this region for

centuries, but their population numbers prior to European

visitation are unknown. Early explorers and fur trappers

brought smallpox, measles, and influenza which caused

waves of epidemics, decimating the populations before any

type of census was done. There is evidence that these first

peoples altered the landscape by burning areas to increase

regrowth of some desirable species (Wray and Anderson

2003) and may even have moved some species into areas

where they did not naturally occur (Larson 2006). However,

this mostly either maintained a matrix of native species

somewhat more diverse than would naturally have occurred

or created small “gardens” of native species translocated

somewhat out of their native context. 

Although in the mid 1700s several European countries,

including Russia, Spain, and England, recognized the rich

natural resources in the PNW and sent ships to explore, 

the region was considered to be geographically remote from

the rest of the continent and most areas did not see permanent settlements until well into the 1800s, less than two

hundred years ago (Schwantes 1996). Contrast this with the

east coast of North America, where St. Augustine, Florida

was settled by the Spanish in 1565, Jamestown, Virginia

was established by the English in 1620 and Plymouth,

Massachusetts in 1620 by the Pilgrims. While human populations have steadily increased, especially in the coastal

areas of the PNW, the colonization by large numbers is still

fairly recent. 

This region is therefore in a somewhat unusual situation on this continent in still having remaining landscapes

which in many cases are minimally impacted by the

destructive actions of humans. However, for those who

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

MEETING THE CHALLENGE: INVASIVE PLANTS

IN PACIFIC NORTHWEST ECOSYSTEMS

Sarah H. Reichard1
General Technical Report GTR-694

have lived in the PNW for more than two decades, the

increase in urbanization, in particular, has been striking.

With urbanization comes changes in hydrology, increases in

disturbance, and the introduction of plants for ornamental

or other uses (USDA Forest Service 2006). All these

changes can lead to an increase in biological invasions 

of introduced plants (Reichard 2004).

Invasive plants can be defined as non-native species

that have or potentially can establish in wildland or managed ecosystems, develop self-sustaining populations, and

become visually dominant and/or disruptive to those

ecosystems. Other terms are often applied, such as “exotic”

or “alien,” but these terms simply mean non-native and

have other, sometimes value-laden meanings. “Weed” is

another term often used. This simply means that the plants

have a negative impact on a desired management objective,

rather than an ecological meaning.

In the last twenty years we have learned much about

the impacts of invasive species in wildlands. Such species

compete with native species for essential resources, interrupting food webs. They alter nutrient cycling, hydrology,

and disturbance regimes (Mack et al. 2000). Researchers 

in the PNW continue to discover new potential impacts of

long-time invaders (Urgenson and Reichard 2007).

We still have the opportunity to reduce the introduction

and spread of many species. While the United States and

Canada federal governments must take on the role of controlling the entry of species either accidentally or for intentional uses in their countries, agencies, universities, and

engaged citizens should play a key role in preventing the

4

Figure 1—Pacific Northwest Ecosystems. Reprinted with permission from the University of Washington Press.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

spread of existing and new invasive species and the control

of those that have already established. 

THE “INVASIVE SPECIES PROBLEM”

WILL NOT GO AWAY

There are no estimates for how many new invasive species 

are being detected in the PNW, though each state or province regularly detects new species annually. A study done in

California capitalized on the publication of the new Jepson

Manual (Hickman 1993) and found that, while the rate of

introduction of new species appeared to have slowed since

earlier floras, 151 new species were established and another

101 reported earlier might no longer be present (Rejmánek

and Randall 1993). They also found an increase in established species native to North and Central America and to

South Africa, perhaps reflecting the introduction of new

species by the horticulture industry. Another study, however, used herbarium specimens to verify species present 

in California and found an additional 315 species present

that were not listed in the Jepson Manual. Of these, 58 

were found in natural habitats, 53 in disturbed areas, 34 

tenuously established, 13 in cultivated environments, 43

likely transitory, and 110 for which they could not conclusively determine invaded locations (Hrusa et al. 2002).

While the PNW is less densely settled, it likely follows a

similar trend of increasing introduction and establishment

of invasive plants. 

In 1997 a group under the auspices of a non-profit

organization called the Pacific Northwest Exotic Pest Plant

Council (see below) attempted to develop a list of all of the

non-native species known to be surviving outside of cultivation in Oregon and Washington (PNW-EPPC, unpublished list). They found evidence of a total of 173 species

invading, with 100 considered to be potentially or actually

injurious to wildlands and another 73 present but less injurious. Twenty-three had nomenclatural and geographic

issues that needed to be resolved. Since that time, a number

of new species have been detected in wild populations,

including notorious species invading other parts of the 

continent such as Pueraria montana var. lobata (Willd.)

Maesen & S. Almeida (kudzu) and previously unknown

aggressive species, such as Ficaria verna Huds. (fig buttercup). 

These are indications that the PNW is following the

same trend as California (Rejmanek and Randall 1993). 

For instance, the human population growth is expected 

to increase in the western United States overall by 45.8 

percent between 2000 and 2030 (U.S. Census Bureau,

http://www.census.gov/population/www/projections/projectionsagesex.html), with much of that almost certainly in the

PNW. Given the increase in the human population in the

area, with concomitant increases in community disturbance,

we should expect the problem to increase in the coming

years. 

TOOLS FOR CONTROL AND 

PREVENTION

As we learn more about the biology of invaders, we are 

becoming increasingly sophisticated in controlling their

spread. Earliest efforts at weed control in agricultural settings were largely achieved through tillage of the crops

because prior to 1900 there were few other tools (Timmons

2005). In the late 1800s chemical methods increased and

these methods expanded greatly in the mid-1900s when

new chemical compounds to kill weeds and new technologies to deliver the chemicals were developed (Timmons

2005). New and safer compounds are continually being

developed and other technologies are advancing. Biological

control, the introduction of natural enemies (usually from

the pest’s native range) has become increasingly sophisticated, with rigorous testing to ensure no movement to

plants of economic interest and native species. For instance,

a classical biological control program to control knotweeds

(Fallopia japonica (Houtt.) Dcne., F. sachalinensis (F. 

Schmidt Petrop.) L. P. Ronse Decraene, and F. × bohemica

(Chrtek & Chrtková) J. P. Bailey) is using four insects identified for controlling these species in Great Britain and will

be tested on about 60 species before experimental releases

will be approved for the PNW (Grevstad et al. 2007). 

Increasingly, researchers are finding that combining

methods or treating the conditions at the site are sometimes

the most effective and safe way to control invasive species.

5
General Technical Report GTR-694

For instance, many species respond to a herbicide treatment

following mowing. This allows less herbicide to be used

and provides good control results. Modifying the conditions

at the site may control invasive species without herbicides

at all. For instance, a combination of mulch and native

species plantings successfully combined to control the

shade-intolerant species, Phalaris arundinacea L. (reed

canary grass) (L. Seebacher, personal communication).

Nitrogen-fixing species often exploit nutrient-poor soils;

amending soils with nitrogen-rich biosolids appears to

reduce seedlings of nitrogen-fixing Cytisus scoparius (L.)

Link (Scotch broom) (Shaben 2007)

As the numbers of invasive species increase and the

resources to control them do not keep pace, an important

component is strategically planning control efforts. For

instance, invasive plants commonly invade lands owned by

a matrix of landowners. Controlling a species only in part

of the range, such as federal forest land, is a self-defeating

process since reinvasion from lands owned by others will

perpetuate the invasion. Increasingly Weed Management

Areas (WMA) are being formed to address these multilandowner invasions (DiTomaso et al. 2000). Weed

Management Areas include public and private stakeholders

who work collaboratively on surveying, implementing 

control work, developing monitoring plans, and educational

materials (Scott 2007). 

Even with WMA agreements increasing and new technologies, controlling invasive species is often discouraging.

Surveys find vast acreage of species and contemplating

such broad-scale control can be intimidating. One strategy

is to develop a strict control plan for species considered to

be a major threat and found in relatively small numbers

across its range. Those species should be targeted for control and possibly for complete eradication. Other species

may be widespread and only targeted for control if they

threaten to invade a new area of high conservation value

(Timmons and Owens 2001).

Another strategy for approaching weed management 

is to set definite performance goals for species or groups 

of species in a given year (Burke 2007). Such an approach

would include determining a reasonable percentage of the

population or the species range to be reduced within the

year. If set reasonably, this gives workers a clear objective

that is attainable and less demoralizing than the sense of

failure that could result from targeting the entire range.

MEETING THE CHALLENGE

While control of existing invaders is essential to minimize 

harm to wild and other lands, there is growing acceptance

that the most efficient way to manage invasions is to prevent the establishment of new species and new populations

of species already present. In the latter case, WMA agreements can increase focus on species and facilitate communication about, and action on, new species. Educational

work can increase public recognition of problem species,

increasing the potential for new populations to be detected

at an early and manageable stage of invasion. Models are

being improved that might serve to guide land managers to

likely sites of invasion by specific problem species (Jones

et al. 2007).

The United States Department of Agriculture (USDA)

regulates the introduction of new species into the country.

While the Plant Protection Act of 2000 regulates which

species are considered to be noxious in the country, resulting in quarantine, relatively few species are listed and most

listed are already present in the country. Currently, there is

little authority to restrict the entry of species based on their

invasive potential, although those known to host pathogens

or insects may be restricted. The USDA has proposed

adding “plants for planting” to the policy regulating plant

pests, which is known as Quarantine 37 or Q-37. This

reflects a change in how species now enter the United

States. Previously, planted material was introduced to the

country mostly as propagating stock, with the introduction

of 100 or fewer plants or seeds, so the entry of plants was

fairly slow. Today, however, offshore nurseries are increasingly shipping plants ready for the consumer into the 

country. The plants are now only actionable if insects or

pathogens are found. Under the proposed changes in Q-37

species would be allowed in unless they were considered

“Not Authorized for Import Pending Risk Analysis” or

NAPRA. Species that fit the definition of a noxious weed

under the Plant Protection Act of 2000 or the International

Plant Protection Convention and are not known to occur in

6
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

the United States would be considered to be NAPRA and

therefore subject to screening (Tschanz and Lehtonen

2005). 

While the USDA and Canada’s Ministry of Agriculture

and Lands appropriately have the responsibility of identifying new potential pests at their country’s borders, it is

inevitable that species will become invasive despite their

best efforts. Therefore, detection of and response to new

species must be done on a more regional and local level.

Dealing with the invasive plant problem requires partnerships to quickly find new invasive species, share information on control, and develop effective educational tools.

Throughout the United States non-profits, often labeled as

“Exotic Pest Plant Councils” (EPPC) or “Invasive Plant

Councils” (IPC) are being formed to facilitate these partnerships. The first Exotic Pest Plant Council was formed in

Florida more than 20 years ago to encourage communication between land management agencies working on invasive plants. In 1992 California expanded on this idea by

making membership open to all agencies and individuals

interested in this emerging environmental issue. Currently

there are such networks throughout mostly the eastern part

of the U.S. (fig. 2). These networks have joined together to

form a national oversight group, the National Organization

of Exotic Pest Plant Councils (http://www.naeppc.org).

In the mid 1990s there was an effort to start the PNW

EPPC. A board was formed, by-laws written and approved,

501 (c)(3) exemption status was granted by the U.S.

Internal Revenue Service, and there were a few newsletters,

meetings, and activities. However, by 1998 it was no longer

active, mostly due to a small initial founder membership

that moved on to other jobs and projects (L. Whiteaker, 

personal communication). 

Across the country the EPPC/IPCs have done a number of things that have facilitated work on plant invasions 

in their region. Here are a few examples of what a PNW

network might be able to do:

• Form working groups on particular taxa

• Write grant proposals and administer funds to

those working on a project

7

Figure 2—Regions of the United States currently with cooperative pest plant councils or coalitions. Reprinted with permission from

the National Association of Exotic Pest Plant Councils. 
General Technical Report GTR-694

• Hold regular conferences

• Produce carefully evaluated lists of plants of 

concern in the region (note: this was done for

Washington and Oregon by the PNW EPPC in 

1997 but needs major revision)

• Work with industries, such as the nursery industry, 

to prevent invasions through industry pathways

• Produce publications and educational materials 

that can be distributed through the network to a 

wide audience

• Foster communication 

• Provide an early detection warning system

Much of the organizational work for the PNW-EPPC 

(by-laws, status with the United States Internal Revenue

Service, etc.) was done in the earlier effort. To restart the

network would require forming a board of directors/leadership council, working on developing membership, deciding

priorities for the first few years, and searching for funding

to increase stability. Other decisions would include deciding

if states should have individual organizations that collaborate as a regional network or if there should be a single

regional network, and what the region would encompass.

The Pacific Northwest still has vast acreage of mostly

unspoiled wildlands. With increasing development it is 

critical that we take action to protect our resources. While

we must continue to effectively and creatively control the

invasions of current invasive species, we need better tools

to prevent the spread of current species into new areas and

the introduction of new species into the region. Better 

communication and coordination throughout the region will

help achieve these goals. The conference that resulted in

these proceedings was a good first step. The question now

becomes: how do we continue what the conference began?

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Agriculture, Forest Service, Pacific Northwest

Research Station: 63–64.

USDA Forest Service. 2006. Cooperating across boundaries: partnerships to conserve open space in rural

America. FS-861. 49 p.

Wray, J.; Anderson, M.K. 2003. Restoring Indian-set 

fires to prairie ecosystems on the Olympic Peninsula.

Ecological Restoration. 21(4): 296–301.

9
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EARLY

DETECTION 

AND RAPID

RESPONSE

Robert geranium (Geranium robertianum) (Katie Barndt)
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ABSTRACT

The National Park Service Inventory & Monitoring Program, in collaboration with the U.S. Geological Survey Status and 

Trends Program, compiled a handbook to provide guidance and insight to parks and other natural areas engaged in developing early detection monitoring protocols for invasive plants. While several rapid response frameworks exist, there is no consistent or comprehensive guidance informing the active detection of non-native plants early in the invasion process. There

are many approaches and often little consensus regarding how to develop efficient early detection monitoring; however,

because of the pervasiveness of this issue and the fact that invasive species are not place bound (i.e., they freely cross ecological boundaries, states, nations, and continents), we have tried to provide guidance to encourage a consistent approach to

monitoring. We hope this will result in an efficient, effective tool for combating the considerable threat of invasive plants to

native ecosystems and biodiversity in national parks and elsewhere. We recognize that the task of detecting invasive species

when populations are small is complex and will require a flexible approach to meet varying objectives, financial situations,

and ecological conditions of individual parks and natural areas.

KEYWORDS: Early detection, invasive species, monitoring, modeling, sample design, protocols.

1 Susan O’Neil was a Natural Resources Specialist, Inventory & Monitoring Program, National Park Service, 909 1st Ave, Seattle, WA 98104. Now with

the Woodland Park Zoo, Seattle, WA. Please contact Penelope Latham for correspondence.

2 Bradley Welch is the Invasive Species Monitoring Coordinator, Inventory & Monitoring Program and Invasive Species Branch, Fort Collins, CO, 80525.

3 Penelope Latham is the Pacific West Regional Coordinator, Inventory & Monitoring Program, National Park Service, 909 1st Ave, Seattle, WA 98104.

4 Daniel Sarr is the Klamath Network Coordinator, Inventory & Monitoring Program, 1250 Siskiyou Blvd, Ashland, OR, 97520.

13

INTRODUCTION

In 1996, the Director of the National Park Service (NPS) 

distributed the first NPS plan for managing non-native,

invasive plants with six key strategies identified including

preventing invasions and conducting inventory and monitoring of non-native plants. In 2000, a detailed action plan

was drafted, with a revision in 2006, which re-emphasized

the role of prevention and early detection/rapid response.

The Inventory & Monitoring (I&M) Program, which 

was established in 1998 as part of the Natural Resource

Challenge National Parks Omnibus Management Act,

organized 270 national parks into 32 networks to inventory

resources and conduct long-term monitoring of key indicators, used to monitor park resources. Invasive species

continue to rank high among I&M networks for long-term

monitoring. 

Early detection was selected as a primary focus for

invasive species monitoring because, along with rapid

response, it is a key strategy for successful management of

invasive species. Eradication efforts are most successful on

small infestations. By tracking new species and new infestations, parks may begin to understand the strategies and

invasion patterns which will allow for improved management actions. Rejmanek and Pitcairn (2002) determined that

eradication success is likely when the infestation is less

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

INVASIVE SPECIES EARLY DETECTION PROTOCOL

DEVELOPMENT IN THE NATIONAL PARKS: 

INTEGRATING ALL THE PIECES

Susan O’Neil1, Brad Welch2, Penelope Latham3, and Daniel Sarr4
General Technical Report GTR-694

than 1 hectare (ha), and that success of eradication is

unlikely when the infestation is over 1000 ha. Too often

early detection is conducted passively in parks. Managers

rely on erratic reports from visitors, maintenance staff, and

backcountry rangers as the source of information to trigger

management action. The nature of this approach requires

spontaneous decisions to be made about allocation of 

staff time and other resources, directing them away from

current projects toward unexpected and unconfirmed issues.

Alternatively, active detection methods may be employed

such that managers respond rapidly to predictable, confirmed reports in a timely and cost-effective manner. To this

end, NPS and the United States Geological Survey (USGS)

Status and Trends Program set out to create a handbook to

assist managers in developing an early detection program.

The document provides guidance for natural resource managers wishing to detect invasive plants early through an

active, directed monitoring program.

ORGANIZATION AND CONTENTS OF THE

HANDBOOK

Chapters 1-3 (Section 1) introduce the text, summarize 

dominant invasion theories relevant to this text, and outline

the key components required to implement a successful

early detection monitoring program. Chapters 4-10 (Section

14

Figure 1—Elements of an invasive species early detection monitoring program in National Parks.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

2) address each of the early detection steps in detail including setting goals and objectives, acquiring appropriate information, choosing analytical processes, conducting evaluation

and assessment, and implementing detection methods.

Chapters 11-15 (Section 3) provide applications of early

detection principles within the context of the Klamath, San

Francisco, and Heartland NPS I&M Networks. Two chapters

present information from the Klamath Network: Chapter 

11 shares insights with respect to early detection protocol

development across multiple parks, and Chapter 12 offers 

an approach to probabilistic predictive modeling of invasive

plants with detailed examples from Lava Beds National

Monument. The San Francisco Network presents a small

park example using volunteers. The Heartland Network

highlights a protocol for integrating early detection and

long-term trends monitoring across large and small parks.

An example using remotely sensed and geographically referenced data for predicting the risk of occurrence for target

species in Big Bend National Park is presented in Chapter

13. Subsequent materials include citations, a glossary,

detailed reports from participating researchers, and a protocol template which meets NPS I&M program standards.

The protocol template will be useful for those who may be

required to document their specific early detection procedures. Cited materials and sources of additional information

appear in the bibliography and are grouped by chapter.

The handbook also has a Quick Start guide to direct

readers to specific chapters and text relevant to their needs.

Each chapter was written by a U.S. Geological Survey

(USGS) researcher(s) or NPS managers/researchers.

Decision trees and flow charts assist the reader in deciding what methods to choose and when to employ them in

several chapters. The various steps (and thus chapters) in

Section 2 are meant to follow a conceptual model developed by the I&M program that encapsulates the idealized

components of an early detection program (Fig. 1). A park

or network may decide to implement only a few of the relevant components. The handbook is written in a modular

format to accommodate use of individual chapters. It may

also be approached in a linear fashion, as a sequence of

steps leading to a comprehensive approach to early detection.

This handbook is intended to be a living document

which will be updated regularly via an electronic internet

copy as new materials become available. In particular, we

anticipate specific protocols developed by individual I&M

networks will be linked to this document to provide readers

with a range of specific implementation examples.

Although this document has been designed with the NPS

I&M networks and parks in mind, it undoubtedly will have

broad application for natural areas professionals everywhere wanting to improve invasive plant management

strategies.

LITERATURE CITED

Rejmánek, M., and M.J. Pitcairn. 2002. When is eradication of exotic pest plants a realistic goal? Pages

249–253 in C.R. Veitch and M.N. Clout, editors.

Turning the Tide: the Eradication of Invasive Species.

IUCN SSC Invasive Species Specialist Group. IUCN,

Gland, Switzerland and Cambridge, UK.

15
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ABSTRACT

Prevention and Early Detection & Rapid Response (EDRR) practices are the most effective strategies for managing the 

invasive plant species threat over the long-term and at large-scales. When new invasive plant species are immediately

detected and identified, and rapid responses are taken to contain and eradicate those new infestations, environmental 

and economic damages and subsequent impacts can be significantly mitigated. 

EDRR programs can take different forms depending on the scale of the project site. An EDRR program can be constructed and implemented at both the site and at larger state/regional scales. The up and coming EDRR network for The

Nature Conservancy in Oregon is utilizing the site approach at several locations throughout the state. The Oregon Chapter 

of the Conservancy is planning to learn from other existing EDRR programs from around the country.

The Nature Conservancy’s Maryland/Washington D.C. Chapter Weed Watchers/Weed Busters program uses volunteer

efforts to locate new infestations, which are then reported to Conservancy staff. Once the invasive species is prioritized and

a control strategy is created, volunteers then carry out the rapid response effort. As of 2005, over 50 volunteers had worked

on 19 new invasive species at five preserves. For more information on this program visit http://tncweeds.ucdavis.edu/

outreach.html. 

A larger scale EDRR project in New England, called IPANE, which stands for Invasive Plant Atlas of New England, 

is a great example of a large scale EDRR network. IPANE is a regional effort to combat invasive plant species that also 

utilizes volunteer efforts. IPANE also benefits from paid staff thanks to funding that allows the program to work on a larger

scale. The website for IPANE contains invasive plant information, volunteer directions, distribution maps, and much more.

It can be found at http://nbii-nin.ciesin.columbia.edu/ipane/index.htm. 

EDRR work with the Oregon Chapter of The Nature Conservancy will begin in the fall of 2006. EDRR Coordinator

Tania Siemens can be reached at tsiemens@tnc.org. 

KEYWORDS: Early detection, rapid response, invasive.

1 Kyle Strauss is the Americorps Conservation Team Coordinator, The Nature Conservancy, Portland, OR, kstrauss@tnc.org.

17

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

DEVELOPING EARLY DETECTION NETWORKS 

TO ABATE THE INVASIVE PLANT SPECIES THREAT

Kyle Strauss1
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ABSTRACT

The Plant Protection Act defines a noxious weed and provides a list of species that are prohibited or restricted from entering 

the United States. Under the provisions of the Plant Protection Act and Federal Seed Act there is zero tolerance for entry of

designated Federal Noxious Weeds (FNW). Responsibility for interception, identification and exclusion of these species at

our borders is shared between the Department of Homeland Security, Customs and Border Protection (DHS-CBP) and the

U.S. Department of Agriculture, Animal and Plant Health Inspection Service (USDA-APHIS). CBP Agriculture Specialists

make interceptions from marine/air cargo and baggage at ports and border crossings. Identification is provided by the

Botanist Identifiers of USDA-APHIS and the National Identification Service. During 2005 and 2006, FNW seeds were 

intercepted at the Port of Seattle, SeaTac International Airport and the Canadian border. The incidence and frequency of

interceptions varied with location. At the Port of Puget Sound, from September 2005 through August 2006, a total of 799

botanical interceptions were processed. Of these, 44 or 5.5 percent of the total number of plant interceptions were FNW. The 

most frequently intercepted FNW seeds were Imperata cylindrica (L.) P. Beauv. (cogongrass), Pennisetum polystachion (L.)

Schult. (mission grass), and Tridax procumbens L. (coat buttons). At SeaTac Airport the most frequent interceptions were

Ipomoea aquatica Forssk. (water spinach) and Heracleum mantegazzianum Sommier & Levier (giant hogweed). At the

Canadian border, Blaine, the most frequent interceptions were of Asphodelus fistulosus L. (onionweed), a frequent contaminant of cumin, and Oryza sp. (red rice). Shipments containing a FNW seed are refused entry, unless the seeds can 

be removed or separated and/or devitalized. Interceptions of FNW seeds from passenger baggage for consumption and

propa-gation are confiscated and destroyed.

KEYWORDS: Import, noxious weed, commodity, risk pathway.

1 Margaret Smither-Kopperl is a Botanist and Identifier, USDA-APHIS-PPQ, 835 S. 192nd St., #1600, Seatac, WA 98148. 

19

INTRODUCTION

The responsibility for preventing the introductions of 

Federal Noxious Weeds (FNW) is shared jointly by two

government agencies: United States Department of

Agriculture – Animal and Plant Health Inspection Service –

Plant Protection and Quarantine (USDA-APHIS-PPQ) and 

Department of Homeland Security – Customs and Border

Protection (DHS-CBP). Prior to the formation of DHS the

responsibility was held solely by USDA. The separation of

responsibilities is found in a Memorandum of Agreement

(DHS 2003). Amongst the responsibilities of USDAAPHIS-PPQ are to: a) inspect propagative material entering

the U.S.; b) identify intercepted pests; c) recommend treatment; and d) if applicable, apply or monitor treatment. The

responsibilities of DHS-CBP include: a) inspection of international aircraft, vessels, and vehicles and passengers; b)

inspection of air and marine cargo; and c) the procedure to

refer intercepted pest species to PPQ for identification and

treatment recommendations.

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

THE FIRST LINE OF DEFENSE: INTERCEPTIONS 

OF FEDERAL NOXIOUS WEED SEEDS IN WASHINGTON

Margaret Smither-Kopperl1
General Technical Report GTR-694

The authority for USDA-APHIS to regulate noxious

weeds is provided by the Plant Protection Act of 2000,

which supersedes previous regulations. Under the Act a

noxious weed is defined quite broadly as “any plant or plant

product that can directly or indirectly injure or cause damage to crops (including nursery stock or plant products),

livestock, poultry or other interests of agriculture, irrigation,

navigation, the natural resources of the United States, the

public health, or the environment.” In addition, the Federal

Seed Act regulates interstate and foreign commerce in

seeds, and addresses “noxious weed seeds” that may be

present in agricultural or vegetable seed (USDA 1998).

Noxious Weeds may be designated as either “Federal” or

“State” Noxious Weeds. USDA-APHIS has authority to

regulate only FNW.

The list of FNW includes weeds not currently known

to be present, or with limited distribution within the U.S.,

that have been determined to pose a high risk to agriculture

and the environment (USDA 2006). The list is constantly

reviewed and updated as the need arises. There is zero tolerance for seeds of FNW; if a seed is found, entry of the

associated commodity is prohibited. There are exceptions

for mitigating the risk of noxious weed contamination in

cargo. These can be used only if the commodity can be

20

Figure 1—Interceptions of Federal Noxious Weed Seeds in Washington from September 1, 2005 to August 31, 2006.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

freed from the seed contaminants and the contaminants can

be devitalized by an appropriate treatment; if grinding is

appropriate, this method may be used. For any treatment of

an imported commodity, the importer must be in agreement

and pays the costs. This can result in a significant cost to

the importer.

USDA-APHIS-PPQ does allow movement of FNW

into the United States, or between States, under specific 

circumstances such as research. Movement is allowed under

permit (PPQ form 526) only where explicit conditions can

be met. 

Within Washington State, USDA-APHIS-PPQ inspects

propagative plant material under permit at the Seattle Plant

Inspection Station. CBP Agriculture Specialists submit

interceptions from three locations: a) Maritime cargo,

inspected at the Port of Puget Sound, both Seattle and

Tacoma; b) SeaTac Airport, including Air Cargo and international passenger baggage; c) Blaine, the land border with

Canada. Identification of plant material is conducted at the

Seattle Plant Inspection Station and plant material may also

be sent to the USDA-APHIS-PPQ National Identification

Service Botany Identifiers in Beltsville, MD. 

INTERCEPTIONS OF FEDERAL NOXIOUS 

WEED SEEDS

The incidence and frequency of interceptions varies with 

location. Federal Noxious Weed seeds were intercepted at

the Port of Puget Sound, SeaTac Airport and the Canadian

Border during 2005 and 2006 (fig. 1). 

At the Port of Puget Sound, from September 2005

through August 2006, a total of 799 plant interceptions

were processed. Of these, 44 or 5.5 percent of the total

number of plant interceptions were FNW. These were primarily wind dispersed seeds in a variety of cargo types. 

The most frequently intercepted FNW seeds were Imperata

cylindrica (L.) P. Beauv. (cogongrass), Pennisetum polystachion (L.) Schult. (mission grass), and Tridax procumbens

L. (coat buttons) (fig. 1). These seeds were found in shipments from East Asia, Australia and New Zealand, and the

Mediterranean region. The seeds are wind dispersed and

found in a variety of cargo types (table 1). Seeds of

Cuscuta spp. (dodder) a parasitic plant were intercepted as 

a seed contaminant in Guizottia abyssinica (niger) seed.

Those FNW seeds intercepted in passenger baggage are

commonly for personal consumption. At SeaTac Airport the

21

Table 1—Interceptions of Federal Noxious Weeds in Washington during July 2006

Country of 

Port Location Organism Shipment Origin Commodity

Port of Puget Sound Imperata cylindrica Vietnam Pottery

cogongrass Australia Machinery

China Household goods

Philippines Vehicle 

Japan Used tires

Port of Puget Sound Pennisetum polystachion Vietnam Pottery

mission grass Turkey Marble

Port of Puget Sound Saccharum spontaneum Turkey Marble

wild sugarcane

Port of Puget Sound Tridax procumbens Vietnam Pottery

coat buttons Thailand Military vehicles

Seattle Plant Alternanthera sessilis Singapore Aquatic plants

Inspection Station sessile joyweed

Seattle Passenger Ipomoea aquatica Cambodia Baggage

water spinach

Blaine Asphodeus fistulosus Canada Cumin seed

onionweed

Blaine Oryza sp. Canada Rice - Oryza sativa

red rice
General Technical Report GTR-694

most frequent interceptions were Ipomoea aquatica Forssk.

(water spinach), and Heracleum mantegazzianum Sommier

& Levier (giant hogweed). At the Canadian border, Blaine,

the most frequent interceptions were of Asphodelus fistulosus L. (onionweed), a frequent contaminant of cumin, and

Oryza sp. (red rice), a contaminant of rice. Cuscuta spp.

(dodder) were also intercepted as a component of Chinese

herbal medicine.

RISK PATHWAYS AND ENTRY

PREVENTION

The most common FNW in marine cargo were of wind 

dispersed seeds found in shipments of products such as pottery, marble and granite. These products may remain open

to the environment prior to shipping, which allows the

seeds to settle and contaminate the cargo. All FNW seeds

intercepted in marine cargo at the Port of Puget Sound are

prohibited. The cost of destruction or treatment is borne by

the importer and this acts as a significant deterrent. It is in

their interests to ensure that their shipments are clean and

free of FNW.

Passenger baggage is a significant risk pathway for

entry of FNW. At SeaTac Airport the most frequent interceptions were Ipomoea aquatica Forssk. (water spinach)

carried by passengers from East Asia, including China,

Cambodia and Vietnam. The seeds were intended for propagation and consumption. Heracleum mantegazzianum

Sommier & Levier (giant hogweed) seeds were intercepted

in baggage of passengers from the Middle East, particularly Iran, where the seeds are used as a spice. At Blaine,

Asphodelus fistulosus L. is a frequent contaminant of cumin

originating in India. The large number of travelers entering

the U.S. from Asia makes passenger baggage a major risk

pathway for entry of FNW. When these seeds are intercepted in baggage they are confiscated and destroyed. 

The combined actions of DHS-CBP to inspect cargo

and intercept potential weed pests, and USDA-APHIS-PPQ

to inspect propagative material and to identify interceptions

and recommend treatment, results in the exclusion of significant numbers of FNW from the State of Washington. At

other air- and seaports the same activities are being undertaken to support the mission to detect, identify and prohibit

entry of FNW into the United States. 

ACKNOWLEDGMENTS

I wish to thank Mae Ikawa, Barbara Chambers and Marla 

Cazier-Mosely (USDA-APHIS-PPQ) and Susan Spinella

(DHS-CBP) for their support. Also thanks to Rodney Young

and Gayle van de Kerckhove for critically reviewing the

manuscript.

LITERATURE CITED

Department of Homeland Security, U.S. Department of 

Agriculture, Animal and Plant Health Inspection

Service. 2003. Memorandum of Agreement (MOA)

Articles 2 & 3. Separation of Function. [online] URL:

http://www.aphis.usda.gov/ppq/moa-dhs.html

U.S. Department of Agriculture, Animal and Plant 

Health Inspection Service, Plant Protection and

Quarantine. 2000. Plant Protection Act. [online] URL:

http://www.aphis.usda.gov/ppq/weeds/PPAText.pdf 

U.S. Department of Agriculture, Animal and Plant 

Health Inspection Service, Plant Protection and

Quarantine. 2006. Federal Noxious Weed List (as of

June 30, 2006). [online] URL: http://www.aphis.usda.

gov/ppq/weeds/weedlist2006.pdf

U.S. Department of Agriculture, Marketing and 

Regulatory Programs, Agricultural Marketing

Service. 1998. Federal Seed Act. [online] URL:

http://www.ams.usda.gov/lsg/seed/fsa-98.pdf

22
ABSTRACT

Early detection/rapid response (EDRR) in invasive plant management is intended to identify and address invasive plant 

populations as quickly as possible. The economic and ecological costs of treatment increase with population size and effectiveness of treatment decreases. The sooner a target species is detected and treated the more likely it will be controlled.

However, analysis for EDRR provides challenges to land managers striving to meet the requirements for site-specific analysis under the National Environmental Policy Act (NEPA). An interdisciplinary team working for Olympic National Forest

developed a process that meets the site-specificity requirements of NEPA and provides flexibility to respond to current and

unpredictable new infestations that occur within the next five to fifteen years.

KEYWORDS: Invasive plant management, early detection/rapid response, National Environmental Policy Act, 

Olympic National Forest.

1 Rochelle Desser is an Environmental Coordinator, TEAMS Enterprise, USDA Forest Service, PO Box 687, O’Brien, OR, rdesser@fs.fed.us, 

phone: 541-596-2453.

23

THE DILEMMA

The National Environmental Policy Act (NEPA) and United 

States Forest Service (USFS) Implementing Regulations

(FSH 1909.15) outline the process the agency must use to

consider and disclose the site-specific impacts of land management programs and projects. Each “site” is generally

mapped, and the proposed actions for that site are described

at a fine scale. However, invasive plants are dynamic –

their rate and/or direction of spread cannot be precisely 

predicted. 

Analysis under NEPA often takes months or years to

complete. The rapid potential for change is greater with

invasive plants than other Forest Service endeavors such as

timber or recreation management. This poses challenges for

land managers attempting to treat invasive plants that

become introduced, established or identified during the time

taken to complete a site-specific analysis under traditional

NEPA procedures.

THE PREMISE 

The premise of the Olympic National Forest NEPA

approach for analysis of early detection/rapid response

(EDRR) is that similar treatments on similar sites will have

similar environmental impacts. Site-specificity is ensured

by focusing the analysis on treatments necessary to respond

to the current inventory of invasive plants. Site-specific

conditions found in the current inventory were classified

based on their sensitivity to the effects of treatments (for

example, proximity to water, wildlife habitat, places where

people gather, etc.). Project design criteria were then developed to limit the extent of treatment spatially and temporally, depending on sensitivity to impacts for each site type.

The intent was for the project design criteria to apply to

new detections located anytime during the life of the project. Thus, the effects of treatment were predictable, even if

the precise time and place the treatments will occur were

unpredictable. 

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

SUMMARY OF THE NATIONAL ENVIRONMENTAL

POLICY ACT APPROACH FOR EARLY DETECTION/ 

RAPID RESPONSE TO INVASIVE PLANTS ON 

THE OLYMPIC NATIONAL FOREST

Rochelle Desser1
General Technical Report GTR-694

An implementation planning process has also been

developed to ensure that project design criteria are appropriately applied. This approach follows guidance regarding

changes made to a proposal after a decision is documented

under NEPA.

THE PROCESS

1. Known vectors of invasive plant spread were inventoried across the entire Olympic National Forest.

Anecdotal information was added and inventory 

information was extrapolated into areas that were 

not surveyed on the ground. Road systems and other

invasive plant vectors were mapped and predictable

rates of spread were applied to estimates of acreage

needing treatment. Many factors were considered 

in the initial treatment proposal including priority,

intensity and objective of treatment.

2. The Olympic National Forest was divided into treatment areas, and site conditions throughout the treatment areas were evaluated.

3. The interdisciplinary analysis team considered the 

effects of invasive plant control methods known to 

be needed to treat the full range of situations found 

or predicted on the Forest. In the case of the 

Olympic National Forest, this included broadcast, 

spot and selective methods of herbicide application, 

as well as manual and mechanical non-herbicide 

methods (e.g., mowing, hand pulling, and cut stump

treatments).

4. Project design criteria were developed to provide 

analytical sideboards as needed. Limits were placed 

on the future selection, rate and extent of herbicide

use to reduce the potential intensity and magnitude 

of treatment impacts. The prescription criteria add 

layers of caution to invasive plant treatments to 

ensure “the cure is not worse than the disease.”

5. Buffers were established to limit the rate, application method, or selection of herbicides near surface

water bodies. Buffers were also established to protect native botanical species of local interest.

6. Restoration of treated sites (e.g., mulching, seeding, 

and planting to restore native plant communities 

and/or resist reintroduction of invasive plants) was

included as part of the prescription criteria.

7. The Environmental Impact Statement (EIS) documented the process the Forest will use to determine

appropriate treatment of new detections.

CONCLUSION

The EIS disclosed the effects of treating invasive plants 

given the full range of situations found or predicted across

the entire Olympic National Forest. The analysis was 

site-specific and provided flexibility to respond to rapidly

changing site conditions. Project design criteria compensated for uncertainties of treatment under EDRR. 

24
CONTROL

TECHNIQUES, 

BIOLOGY, AND

IMPACTS

English holly (Ilex aquifolium) (Katie Barndt)
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ABSTRACT

A classical biological control program is being developed for Japanese and giant knotweeds (Fallopia japonica (Houttuyn) 

Ronse Decraene and F. sachalinensis (F. Schmidt) Ronse Decaene) and their hybrid (F. × bohemica (Chrtek & Chrtková) 

J.P. Bailey) in North America. In classical biological control, host-specific natural enemies from the weed’s native range are

introduced with the intent of establishing a permanent population that will provide control of the weed. The steps in developing a biological control program include identifying the problem, surveys of existing natural enemies, foreign exploration

for new natural enemies (candidate biocontrol agents) from the weed’s native range, host specificity testing of candidate

agents, safety review and permitting, and if approved, release and monitoring of biocontrol agents and their impacts.

Biological control provides a highly economical and sustainable approach to managing widespread weeds. Provided that 

the proper steps are taken to ensure host-specific agents, biological control of weeds can be implemented with a high level

of safety (McEvoy 1996, Pemberton 2000).

KEYWORDS: Fallopia sachalinensis, Fallopia japonica, Fallopia × bohemica, biological control, knotweed.

1 Fritzi Grevstad is a Biological Control Specialist, Olympic Natural Resources Center, University of Washington, 2907 Pioneer Road, Long Beach, 

WA 98631, (360)642-3920, grevstad@u.washington.edu.

2 Richard Reardon is an Entomologist, Forest Health Technology Enterprise Team, USDA Forest Service, Morgantown, WV.

3 Bernd Blossey is a Professor of natural resources, Cornell University, Ithaca, NY.

4 Richard Shaw is an Entomologist, CABI Biosciences, Silwood Park, Ascot, United Kingdom.

27

INTRODUCTION

Invasive knotweeds are widely recognized as an environmental problem. They are listed among the “world’s worst

invasive species” by the World Conservation Union and are

listed as noxious in seven states including Oregon and

Washington. In these states, they are slated for control, but

generally considered too widespread for regional eradication. All three invasive knotweeds have spread most aggressively along rivers and streams, but they are capable of

occupying an alarming diversity of habitats. Knotweeds

crowd out native plants through shading and nutrient 

competition. The dense stands have no known value for

wildlife, harbor few invertebrates, and can prevent trees

from growing near the streams. Knotweeds are reported 

to cause increased erosion (Child et al. 1992) as well as

increased flooding (Welsh Development Agency 1991) and

their forceful roots and rhizomes can cause damage to road

surfaces and building foundations (Shaw and Seiger 2002).

Finally, knotweeds are a recreational nuisance, limiting

stream access for uses such as fishing and boating. 

IDENTIFICATION OF CANDIDATE 

BIOCONTROLS FOR KNOTWEED

A possible contributor to the invasiveness of knotweeds is a 

lack of existing natural enemies in the introduced range. In

2003 and 2004, we carried out surveys of existing natural

enemies of knotweed in the states of New York, Oregon,

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

DEVELOPING A BIOLOGICAL CONTROL PROGRAM 

FOR INVASIVE KNOTWEEDS (FALLOPIA SPP.)

Fritzi Grevstad1, Richard Reardon2, Bernd Blossey3, and Richard Shaw4
General Technical Report GTR-694

and Washington. Additional surveys were carried out in

Alaska in 2005 (J. MacIver, pers. comm.). More than 50

different species of herbivorous insects were found. However, very few of these were found repeatedly or were

clearly using knotweed as a host for feeding and reproduction.

Notable East Coast natural enemies include Japanese

beetle (Popillia japonica) and an unidentified pathogen. 

On the West Coast, notable herbivores include spittle bugs

(Philaenus spumarius), woolly bear caterpillars (Isia 

isabella), a leaf beetle (Galerucella nymphaeae), three

aphid species, the blue-green sharpshooter (Graphocephala

atropunctata), slugs and snails. All of these are generalist

species. None of the candidate agents from Asia were

found. No root or stem feeders were found. In all cases,

damage levels were low. 

Extensive exploration for candidate biocontrol agents

in Japan has already been carried out by scientists with

CABI Biosciences for a biological control program against

knotweeds in the United Kingdom. A total of 189 herbivores and 50 fungal pathogens were found on F. japonica

in Japan. Among the more promising candidate biocontrol

agents are a leaf-feeding chrysomelid beetle, Gallerucida

bifasciata, a sap-sucking psyllid, Aphalara itadori, and a

leafspot pathogen, Mycosphaerella sp. Climate comparisons

between regions of Japan and North America suggest that

additional exploration in northern areas of Japan may be

needed to obtain natural enemies that are suited to climates

in North America where knotweeds are most abundant. 

FUTURE DIRECTION

To ensure that the candidate biocontrol agents for knotweed 

will be safe to introduce, they will be tested for their ability

to feed and develop on native and economically important

plants in North America with an emphasis on plants related

to knotweed (family Polygonaceae). A list of test plants 

has been carefully prepared following guidelines from the

Technical Advisory Group on Biological Control of Weeds.

The list currently includes 63 species. The testing will be

carried out in a USDA-APHIS-certified quarantine facility

located at Oregon State University. Additional studies related to this project will be carried out at Cornell University,

CABI Biosciences, and Agriculture and Agri-food Canada’s

Lethbridge Research Center. Host specificity testing will

initially focus on those few candidate agents already

demonstrated to be host specific in the European program. 

Since 1987, weed biocontrol programs in the United

States require a rigorous review by the Technical Advisory

Group on Biological Control of Weeds (TAG). This panel 

is composed of members from 15 environmental agencies

from the United States plus representatives from Mexico

and Canada. The TAG reviews a “petition to import”, which

includes detailed taxonomies of the agent and target plant, 

a carefully compiled plant test list, results of host range

testing, and discussion of any potential non-target impacts.

APHIS-Plant Protection and Quarantine then uses the TAG

review and input from individual states to make the final

decision to issue the release permit. The TAG review and

permit process provide an effective way to ensure that only

safe biocontrol agents are introduced. 

The final phase of a biological control program is

implementation. For the knotweed biocontrol program,

releases will be made first into a limited number of locations in both Northeastern and Northwestern states. Initial

establishment can sometimes be difficult and so it is important to carefully plan the location and timing of releases.

The initial release sites will serve as nursery sites for further collection and redistribution once the populations build

up. It typically takes several years for the agents to become

abundant at the initial release sites. Although the agent will

spread on its own, human aided transport will greatly 

expedite the process. 

Successful weed biocontrol programs begin with localized damage to individual plants and finish with a sustained

regional reduction in the plant population. The impacts of

biocontrol on the weed population can sometimes be dramatic (>99 percent reduction), but in other cases the level

of control is subtle, or variable from site to site. The likelihood of overall success increases with the number of different agent species used. It can take up to a decade or more

for the full impact of the biocontrol program to be realized.

Quantitative measures of plant population changes over

time will be crucial to demonstrating the beneficial effects

of this biological control program.

28
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

LITERATURE CITED

Child, L.E.; de Waal, L.C.; Wade, P.M.; Palmer, J.P. 

1992. Control and management of Reynoutria species

(Knotweed). Aspects of Applied Biology. 29: 295–307.

Cofrancesco, Jr., A.F.; Shearer, J.F. 2004. Technical 

advisory group for biological control agents of 

weeds. In: Coombs, E.M; Clark, J.K.; Piper, G.L.;

Cofrancesco, Jr., A.F. eds. Biological Control of

Invasive Plants in the United States. Corvallis, OR:

Oregon State University Press.

McEvoy, P.M. 1996. Host specificity and biological pest 

control. Bioscience. 46: 401–405.

Pemberton, R.W. 2000. Predictable risk to native plants 

in weed biocontrol. Oecologia. 125: 489–494.

Shaw, R.H.; Seiger, L.A. 2002. Japanese knotweed. 

In: Blossey, B.; Hoddle, M.; Reardon, R., eds.

Biological control of invasive plants in the eastern

United States. Morgantown, West Virginia: USDA

Forest Service, Forest Health Technology Enterprise

Team.

29
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ABSTRACT

Cytisus scoparius (L.) Link (Scotch broom) is a quick-growing, leguminous shrub that, since its introduction to western 

North America, has become very invasive. Its ability to fix nitrogen gives it a competitive edge over other, non-nitrogenfixing plants when growing in nitrogen-limited soils and, as such, it can form very dense, sunlight-blocking monocultures.

Removal of the shrub is possible by cutting, mowing, burning or excavating; however, the prolific seedbank that accumulates under Scotch broom shrubs typically results in a dense re-invasion following shrub removal.

To determine if seedling recruitment can be suppressed by decreasing Scotch broom’s competitive advantage over other

plants by increasing the nitrogen levels of the soil, I have compared the efficacy of fertilization with sewage biosolids and

with ammonium nitrate. Experiments were conducted at three sites in southwestern British Columbia: one on a sandy dredge

till site and two on powerline rights-of-way. Site establishment consisted of hand removal of Scotch broom, soil tillage by

rototiller and seeding with native grass seed following treatment. Treatments consisted of biosolids, ammonium nitrate 

and an untreated control. Results from the first two years data of this multi-year study suggest that overall, fewer broom

seedlings emerged in the biosolids-treated plots than the other two plot types. If these patterns hold, increasing soil nutrients

through fertilization with sewage biosolids could be a useful tool in the control of invasive Scotch broom in non-environmentally sensitive sites.

KEYWORDS: Scotch broom, Cytisus scoparius, invasive plant control, sewage biosolids.

1 Jacqueline Shaben, MSc., is currently the Vegetation Ecologist, Vancouver office of Jacques Whitford-AXYS environmental consulting firm 

and can be contacted at jshaben@jacqueswhitford.com.

31

INTRODUCTION

Cytisus scoparius (L.) Link (Scotch broom) is a quickgrowing, leguminous shrub that is native to southern

Europe. Its photosynthetic stems enable it to grow yearround in the moderate climate of west coast North America

(Wheeler, et. al, 1979) As such, in this region, Scotch

broom can grow quite aggressively in open, coastal meadows such as rare Garry oak (Quercus garryana Dougl.)

ecosystems as well as open, disturbed sites such as rightsof-way for transmission lines. In ecologically sensitive sites,

these dense, sunlight-blocking monocultures effectively

suppress native species while broom-invaded disturbed sites

act as sources of seed and can significantly increase the dispersal ability of Scotch broom into new ecologically sensitive habitats. It is therefore important to eliminate broom

from these human-made sites using broad-scale methods.

Removal of the shrub is possible by cutting, mowing,

burning or excavating. In general, however, mechanical

removal of broom has proven to be only temporarily successful as a control method because the soil disturbance

caused by mowing, cutting and pulling leads to increased

germination of a large seed bank which ultimately results in

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

CYTISUS SCOPARIUS (SCOTCH BROOM) CONTROL

USING SEWAGE BIOSOLIDS – PRELIMINARY RESULTS

Jacqueline D. Shaben1
General Technical Report GTR-694

the rapid recovery of shrub populations. The goal of this

project is to investigate the effectiveness of a soil amendment method for controlling Scotch broom in open, 

disturbed, non-ecologically sensitive sites.

Scotch broom is from the legume family and is commonly associated with Rhyzobium bacteria that fix atmospheric nitrogen for use by the plant. This ability to fix 

nitrogen allows Scotch broom to establish in soils with low

nitrogen content and gives it a competitive advantage over

non-nitrogen-fixing plants. However, previous studies have

shown legume production is often significantly depressed

by nitrogen fertilizer additions (Lauenroth and Dodd 1979,

Huenneke et al. 1990). This is likely due to the inhibition 

of bacterial nodule formation in the presence of increased

nitrate (Carroll and Gresshoff 1983). Ongoing suppression

of Scotch broom has been demonstrated after fertilization

with biosolids (treated sewage sludge) at Discovery Park, 

a Seattle area ecological restoration site (Deutsch 1997).

Using the aforementioned studies as guidelines for this

experiment, I compared the efficacy of two different fertilizer types, ammonium nitrate and treated sewage biosolids, on the suppression of broom seedlings following the

mechanical removal of Scotch broom at three field sites in

British Columbia.

The underlying hypotheses to be tested are:

1. Whether or not soil fertilization inhibits Scotch

broom seedlings.

2. If there is a difference in the effect of two different fertilizers, ammonium nitrate and sewage

biosolids, on the suppression of Scotch broom.

METHODS

Sites

The three study sites, Iona Beach, Burnaby Mountain and 

Duncan, all in British Columbia, have a history of at least 5

years of dense broom that had flowered and produced seed,

have little to no slope and are at least 30 m away from any

lakes, streams, wells and dwellings and at least 10 m away

from any roadways.

The first site, Iona Beach (49°13.01’ N, 123°11.45’ W)

is situated at Iona Beach Regional Park, in Richmond, in an

area that has no public access. The soil is composed of sand

tailings dredged from the Fraser River. The site was covered in a dense stand of Scotch broom, with many shrubs c.

15 years and older. Other ground cover consisted of Bromus

tectorum (L.), mosses and lichens. A slight slope of <2 percent runs east to west. Twenty-one 3.5m x 3.5m plots were

established in October 2004 and were blocked along the

slope gradient with seven blocks of three plots each. Scotch

broom removal occurred in July 2004 by hand-pulling the

smaller shrubs and brush-sawing the larger plants, and then

carrying all plants off site.

The second site, Burnaby Mountain transmission line

right-of-way (49°16.13’ N, 122°53.80’ W), runs along a

northeast-to-southwest corridor between dense Douglas-fir

(Pseudotsuga menziesii (Mirb.) Franco) and broad-leaved

maple (Acer macrophyllum, Pursh) forest. The soil is

loamy, > 1m deep and was disturbed in 1999 to install a gas

pipeline. The site was covered in a dense stand of evenaged Scotch broom c. five years of age that produced flowers and seeds. Other ground cover consisted of Himalayan

blackberry (Rubus discolor Weihe and Nees) and a few

grasses. The site has a very minor slope of <2 percent. In

February 2005, entire Scotch broom plants, including roots,

were removed by hand. In April 2005, twelve 6m x 2m

plots were established and blocked along the slope gradient

in 4 blocks of 3 plots.

The third site, Duncan (48°44.02’ N, 123°42.98’W), 

is also on a transmission line right-of-way. This privately

owned property runs along an east-to-west corridor flanked

on both sides by Douglas-fir forest. The soil is loamy with

broken boulders and has been compacted by heavy machinery. The second generation Scotch broom was dense after

having been removed once before by heavy machinery.

Other ground cover consisted of mowed grasses and some

Oregon grape (Mahonia nervosa (Pursh) Nutt.). The site is

level. In May 2005, Scotch broom was removed by mowing, soil was loosened with an excavator, roots and large

rocks were removed by hand and twelve, 6m x 2m plots

were established in four blocks.

Prior to plot establishment, soil samples from all three

sites were analyzed for major nutrients (N, P, K, Ca, Mg,

Na) and trace elements (As, Cd, Cr, Co, Pb, Hg, Mo, Ni,

32
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

Se, Zn) to determine background levels prior to treatment

with fertilizer.

All plots were rototilled with a hand rototiller to

approximately 5 cm, seeded with native shrubs and grass

seed then treated with either ammonium nitrate fertilizer at

an application rate of 350 kgN/ha applied in 3 installments

or biosolids fertilizer at an application rate of 350 kgN/ha

applied in 1 installment, plus an untreated control.

Since the biosolids were relatively solid when they

were applied and remained in clumps of approximately 2-

7cm diameter, they did not spread in an even layer over the

soil and covered approximately 50 percent of the soil surface. Breakdown of the clumps occurred primarily via

weathering and individual clumps of biosolids were still

discernibly intact during monitoring. Most clumps were

coated in a layer of soil once they had been rototilled into

the soil.

The biosolids that were used in this study were composed of dewatered, pasteurized sewage sludge from the

Greater Vancouver Regional District’s (GVRD) Annacis

Island Wastewater Treatment Plant. The dewatered organic

material was recovered from residential (80 percent), industrial (10 percent) and commercial (10 percent) wastewater

sources and underwent anaerobic digestion at 55° C for 

20 to 30 days resulting in a 99.999 percent pathogen-free,

nutrient-rich fertilizer. Biosolids composition is a constant

ratio of water, organic matter, soil, nutrients and trace 

metals. 

Yearly average biosolids nutrient composition obtained

from the GVRD was used to determine the appropriate 

bio-solids application rate prior to fertilizing each site.

Application rates were determined as per the B.C. Biosolids

Best Management Guidelines (McDougall et al. 2001).

Monitoring Scotch broom seedling density

I monitored plant response from June to August of 2005 

and 2006. Each plot was divided into four equal-lengthed

subplots. A 1m x 1m quadrat was randomly placed within

each of the four subplots to census the plants distributed in

the subplot in order to determine if Scotch broom seedling

emergence and subsequent survival was affected by treatment.

RESULTS AND DISCUSSION

In both years, the plots treated with biosolids had the lowest

Scotch broom seedling density of the three treatments at all

three sites (fig. 1).

Other than the 2005 results from Iona Beach, the

ammonium nitrate plots consistently displayed the second

lowest Scotch broom seedling density, indicating that,

although nitrogen addition to the system had an inhibitory

effect on the success of broom seedlings, the effect of

biosolids, with their full suite of nutrients, was more

impressive. In 2005, the Iona Beach site displayed a much

33

Figure 1—Mean number of Scotch broom seedlings per metersquared by treatment at all three sites, sampled in (a) July 2005

and (b) July 2006. Error bars represent 1 standard error of the

mean.
General Technical Report GTR-694

greater difference between treatments than was seen at the

other two sites. The significantly greater number of Scotch

broom seedlings to have germinated at Iona Beach in 2005

is likely a result of the site’s having been established in the

fall rather than mid-spring like the other two sites, giving

the seeds several more months to germinate. Likewise, the

increase in broom density seen in the second year of monitoring at Duncan suggests that the seeds had not had the

opportunity to germinate in the first growing season (2005),

possibly because the site was established so late in the

spring. It is also possible that there was simply a larger

seedbank at Iona Beach because the site had had Scotch

broom for a longer period of time than did the other two

sites.

After two years of monitoring the three sites, preliminary results suggest that the application of biosolids at a

rate of 350kgN/ha to previously broom-infested areas, has

the overall effect of hindering broom seedling survival,

with the greatest decrease being at the sandy, Iona Beach

site. This site’s poor nutrient-holding capacity likely benefitted most from the input of nutrients and organic matter,

facilitating the growth of other plant species to compete

with the broom (Shaben 2006). It is also possible that some

of the broom seeds were simply covered by the biosolids

and were therefore hindered from germinating as a physical

result of mulching. 

CONCLUSION

Scotch broom has become a very problematic invasive 

plant in many countries around the world, the control of

which has been very time consuming and not very effective. Our goal was to test a method that not only would 

be less time-consuming and therefore less expensive than

current methods, but would also use an abundant waste

product: sewage biosolids.

The application of biosolids, at the rate of 350kgN/ha,

seems like a promising treatment for controlling Scotch

broom since the biosolids plots had fewer broom seedlings

than the other treatments. This means that, even though

Scotch broom suppression was not absolute at any of the

three sites, it will nevertheless be easier to control Scotch

broom by hand in sites with fewer seedlings.

The results presented here also indicate that the effectiveness of broom suppression using biosolids is sitedependent and it is therefore recommended to conduct 

fertilization test plots for at least two years prior to treating

an entire area.

Finally, as these results represent data that spans only

two years of monitoring following treatment and Scotch

broom is a long-lived shrub, it remains to be seen whether

or not the long-term success of broom suppression can be

fully achieved by fertilizing with sewage biosolids.

ACKNOWLEDGMENTS

I would like to gratefully acknowledge the support and 

guidance of my supervisor, Dr. Judy Myers. I would also

like to thank the staff of the Nutrifor program at the Greater

Vancouver Regional District and the folks at Sylvis

Environmental Consulting for their inspiration and guidance throughout this project. Funding was generously provided by the British Columbia Transmission Corporation,

under the direction of Gwen Shrimpton. Site access was

made available by Greater Vancouver Regional District

Parks, B.C. Hydro, Burnaby Parks and Recreation and the

Bjornson family. Much of the fieldwork was made possible

with the assistance of Kristen Stevenson. The review of this

paper by Dr. Michelle Tseng and Emily Gonzales is very

much appreciated. Many thanks go to all the other folks

who helped with this project.

LITERATURE CITED

Carroll, B.J.; Gresshoff, P.M. 1983. Nitrate inhibition 

of nodulation and nitrogen fixation in white clover

Trifolium repens. Zeitschrift Fuer Pflanzenphysiologie.

110: 77–88.

Deutsch, B.L. 1997. Controlling Scotch broom in landscape design: case studies in the Pacific Northwest

using soil amendments with biosolids. Master's thesis.

University of Washington, Seattle.

Huenneke, L.F.; Hamburg, S.P.; Koide, R.; Mooney, H.; 

Vitousek, P.M. 1990. Effects of soil resources on plant

invasion and community structure in Californian 

serpentine grassland. Ecology. 71: 478–491.

34
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

Lauenroth, W.K.; Dodd, J.L. 1979. Response of native 

grassland legumes to water and nitrogen treatments.

Journal of Range Management. 32: 292–294.

McDougall, N.R.; VanHam, M.D.; Douglas, M.J. 2001. 

Best Management Practices Guidelines for Biosolids

Land Application in British Columbia.

Shaben, J. 2006. Scotch broom (Cytisus scoparius) and 

soil nitrogen: ecological implications. MSc. University

of British Columbia, Vancouver.

Wheeler, C.T., Perry, D.A., Helgerson, O., Gordon, J.C. 

1979. Winter fixation of nitrogen in Scotch broom

(Cytisus scoparius). New Phytologist. 82(3): 697–702.

35
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ABSTRACT

Establishment of Scotch broom (Cytisus scoparius (L.) Link) seedlings was studied for two years after sowing seeds on 

organic or mineral seedbeds under various overstory retention levels (clearcut, shelterwood, or thinned stands) of 40- to 

70-year-old coast Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var menziesii) near Olympia, WA. Herbivory from

unknown sources and drought caused reductions in seedling abundance. Seedling abundance from Cohort #1 (sown

November 2003) averaged less in clearcuts (0.2 percent of seeds sown) than in shelterwoods (0.9 percent) or thinned stands

(2.0 percent). Seedling abundance was greater on organic than on mineral seedbeds in shelterwoods (2.1 versus 0.2 percent)

and thinned stands (3.5 versus 1.0 percent), but not in clearcuts (0.1 versus 0.3 percent). Seedling abundance from Cohort #2

(sown November 2004) did not differ significantly among overstory levels or seedbed types and averaged less than 1 percent. Although seedbeds in shelterwoods were slightly moister and warmer than those in thinned stands, these differences 

in growing conditions did not explain treatment effects on seedling abundance. Scotch broom established successfully 

under a wide range of overstory retention levels; however, <2 percent of seeds have produced seedlings after two years of

observation.

KEYWORDS: Germination, survival, forest understory, soil water, soil temperature.

1 Timothy B. Harrington is a Research Forester, Pacific Northwest Research Station, USDA Forest Service, 3625 93rd Ave SW, Olympia, 

WA 98512.

37

INTRODUCTION

Scotch broom is a large, non-native, leguminous shrub 

found extensively throughout 16 eastern and six western

U.S. states (USDA NRCS 2006). A native to western

Europe, the species was first documented in Washington in

1888 (Parker 2002). Vigorous seedling regeneration enables

dense stands to form quickly, increasing hazardous fuels,

excluding native plants, and altering community structure

of prairies, woodlands, and young forests. Although Scotch

broom typically invades roadsides and other disturbed soil

areas, it has the potential to grow in conditions of low light

availability (Williams 1981), such as those of a forest

understory.

In laboratory research, seed germination of Scotch

broom peaked at moderate temperatures (18–22°C), and

over 65 percent of seeds had an impervious coat with the

potential to delay germination for months or even years

(Bossard 1993). Seeds remained viable after being stored in

glass jars for up to 81 years at the Kew Gardens in England

(Turner 1933). 

Once germinated, seedlings were able to survive in 

ten percent of full sunlight, although their biomass was 

only five percent of those grown in full sunlight (Williams

1981). Scotch broom was not an abundant invader along

roads and streams in old-growth forests of Douglas-fir in

Oregon (Parendes and Jones 2000). On prairie sites of the

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

ESTABLISHMENT OF SCOTCH BROOM SEEDLINGS 

IN DOUGLAS-FIR FORESTS: EFFECTS OF OVERSTORY

RETENTION LEVEL AND SEEDBED TYPE 

Timothy B. Harrington1
General Technical Report GTR-694

Puget Sound region in Washington, Scotch broom germinated more abundantly when the cryptogamic layer (biological crust) of glacial outwash soils was intact versus

removed (Parker 2002). Vertebrate herbivory decreased biomass of Scotch broom seedlings along the northern coast 

of California (Bossard and Rejmanek 1994). At a Sierra

Nevada foothill site in California, a greater number of

seedlings established on disturbed- versus intact-soil microsites; whereas at a coastal site, treatment effects were not

significant, in part, because of confounding differences due

to seed dispersal by ants (Bossard 1991).

To investigate the potential of Scotch broom to germinate and grow in forest understories, two cohorts of seeds

were sown in Douglas-fir stands that had been thinned to

several overstory retention levels. In each stand, seeds were

sown on both mineral and organic seedbeds to determine 

if germination substrate affected seedling establishment.

Seedling abundance was observed for up to two years after

sowing seeds. Several microclimatic variables were measured periodically to potentially explain differences in

seedling abundance.

METHODS

Sites and Overstory Treatments

The research was conducted in the Overstory Density Study

of Capitol State Forest near Olympia, WA (Harrington

2006). At each of three sites supporting 40- to 70-year old

Douglas-fir, plots at least 1.9 ha in area (137 m x 137 m)

were randomly assigned various overstory retention levels.

Stands were thinned in summer 2000 to leave a relatively

uniform spacing of well-formed trees. Advanced conifer

regeneration and large hardwoods or shrubs also were cut.

Three plots at each site were selected to represent overstory

retention levels hereafter designated as clearcuts (absence

of overstory trees), shelterwoods (35–54 trees ha-1

retained), or thinned stands (84–146 trees ha-1 retained).

The research was conducted within the outer 30.5-m buffer

area of each plot to avoid disturbing ongoing genetic and

species-mixture studies underway in the interior of each

plot. Although growing conditions probably differed somewhat between the exterior and interior of each plot, the

three overstory levels selected per site represented a broad

range of light environments. 

Cohort #1

In September 2003, Scotch broom seeds were collected 

from several sites in the Olympia area. Seeds were air dried

for 60 days and stored at room temperature in a glass jar. 

A total of 18 samples of 40 seeds each were counted and

placed in separate plastic bags. At each of the three plots

per study site, understory shrub and herbaceous species

were cleared within a 2- x 2-m area. The upper organic

layer of the soil was removed and the soil was mixed to a

depth of 30 cm. Approximately 2.5 cm of decomposed litter

(O layer) was applied to a randomly located 0.25-m2 (50

cm x 50 cm) subplot within the cleared area. An adjacent

subplot of exposed mineral soil was left uncovered. One

sample of seeds was scattered on the surface of each subplot in November 2003. The number of living seedlings in

each subplot was counted monthly from April to November

2004, and in April and September 2005 and May 2006.

Cohort #2

In July 2004, seeds were collected from the same locations,

air dried, and stored as described previously for Cohort #1.

A total of 18 samples of 100 seeds each were placed in separate plastic bags. Within each of the three plots per study

site, a new 2- x 2-m area was cleared of all understory

shrubs and herbs. In November 2004, the two seedbed

types were created and seeds were sown as described for

Cohort #1. Seedling abundance of Cohort #2 was determined in April and September 2005 and May 2006. All

seedlings from each cohort were removed at the final count

to prevent spreading the species.

Microclimate

Overstory cover was measured at randomly selected points 

throughout each plot in July 2004 with a vertical densitometer (Geographic Resource Solutions, Arcata, CA).

Photosynthetically-active radiation (PAR) was measured

concurrently with an Accupar® ceptometer (Decagon

Devices, Pullman, WA) (see Harrington 2006 for details).

Volumetric soil water content at 0-12 cm depth was measured monthly from June to October 2004 and in March,

38
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

April, and September 2005 with a Hydrosense® (Campbell

Scientific, Inc., Logan, UT). Soil water measurements were

calibrated to the soils at each site (Harrington 2006) and

included readings for each of the organic and mineral

seedbeds per subplot. Soil temperature at 2-cm depth was

measured with an I-button® sensor (Dallas Semiconductor

Corp., Dallas, TX.) placed near the center of each subplot,

which logged data every two hours from April 2005 to 

May 2006. 

Statistical Analysis

All statistical analyses were performed with SAS (SAS 

Institute, Inc. 1999). The experimental design was a randomized complete block with three replications (sites) of

six treatments. The six treatments were arranged as a split

plot with overstory retention level (clearcut, shelterwood, or

thinned stands) as the main-plot treatment and seedbed type

(mineral versus organic) as the split-plot treatment. Periodic

values of seedling abundance by subplot were expressed as

proportions of the total number of seed sown and the data

were normalized with an arc-sine, square-root transformation (Sokal and Rohlf 1981). The same transformation was

applied to normalize measurements of soil water content.

Mean, minimum, and maximum daily values of soil temperature were averaged by month. Data for seedling abundance, soil water, and soil temperature were subjected to a

repeated-measures analysis of variance (ANOVA) in SAS

Procedure MIXED to determine the significance (α = 0.05)

of overstory level, seedbed type, and their interaction, while

accounting for random effects of individual subplots. The

analysis of seedling abundance was conducted separately

for each cohort on the pooled spring (April or May) and 

fall (September) measurements from the study to provide a 

relatively even spacing in time among measurements (i.e.,

an assumption of repeated-measures analysis). The same

approach was used for analysis of soil water and temperature data. Analyses were conducted with maximum likelihood estimation assuming an autoregressive covariance

structure to account for time series trends. For each

ANOVA, residuals were plotted against predicted values 

of the dependent variable to verify that residual variances

were relatively homogeneous. If an interaction was detected

among experimental factors, pairwise comparisons of leastsquares adjusted means were conducted with Bonferroni

adjusted probabilities (Sokal and Rohlf 1981). The following orthogonal contrasts were performed to partition effects

of overstory level (contrasts (1) and (2)) and its interaction

with seedbed type (contrasts (3) and (4)): 

(1) Absence versus presence of overstory trees

(clearcuts versus the mean of shelterwoods 

and thinned stands).

(2) Shelterwoods versus thinned stands.

(3) Effects of seedbed type differ between 

absence and presence of overstory trees.

(4) Effects of seedbed type differ between 

shelterwood and thinned stands.

RESULTS

Herbivory from unknown sources appeared to be the primary cause of reductions in seedling abundance over time,

especially in shelterwoods and thinned stands where leaves

of seedlings were commonly browsed in the late summer

and fall, resulting in their subsequent mortality. Drought

may have been responsible for mortality of some seedlings,

particularly in thinned stands where wilted seedlings were

observed in late summer. Seedling height after one growing

season ranged from 5 to 20 cm except for one seedling that

attained a height of 60 cm in a clearcut plot.

Average abundance of Scotch broom seedlings in

Cohort #1 was less in clearcuts than in shelterwoods and

thinned stands (orthogonal contrast (1): P = 0.016) (fig. 1).

None of the treatment-by-timing interactions were significant (P ≥ 0.163). Organic seedbeds had higher seedling

abundance than mineral seedbeds in Cohort #1, but only in

shelterwoods and thinned stands (orthogonal contrast (3): P

= 0.048) (fig. 2). Seedling abundance from Cohort #2 never

exceeded 1 percent. Although the interaction of overstory

level and timing was significant for Cohort #2 (P = 0.023),

pairwise comparisons between overstory levels within each

timing failed to detect significant differences in seedling

abundance. 

Average overstory cover was 0, 31, and 62 percent in

clearcuts, shelterwoods, and thinned stands, respectively,

39
General Technical Report GTR-694

and average PAR was 100, 62, and 34 percent of full sunlight, respectively (Harrington 2006). Average soil water

content was greater in shelterwoods than in thinned stands

(orthogonal contrast (2): P = 0.023) (fig. 3a). Soil water

content did not differ significantly (P = 0.358) between

organic and mineral seedbeds, and none of the treatmentby-timing interactions was significant (P ≥ 0.095). Mean

daily soil temperature was 1.5°C greater in shelterwoods

40

Figure 1—Average abundance of two cohorts of Scotch broom seedlings during two growing

seasons after sowing seeds in various overstory retention levels of Douglas-fir. Cohort #1 was

sown in November 2003 (standard error = 0.657) and cohort #2 was sown in November 2004

(standard error = 0.302).

Figure 2—Average abundance of Scotch broom seedlings from Cohort #1 as affected by the

interaction of Douglas-fir overstory retention level and seedbed type (standard error = 0.602).
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

than in thinned stands (orthogonal contrast (2): P = 0.028)

(fig. 3b). Average values of soil water and mean daily soil

temperature in clearcuts were intermediate to those in shelterwoods and thinned stands. Minimum daily soil temperature in September 2005 was lower in clearcuts (7.8°C) than

in shelterwoods (9.9°C) or thinned stands (9.9°C) (data not

shown). Maximum daily soil temperature did not vary significantly with overstory level or its interaction with timing

(P ≥ 0.244). 

DISCUSSION AND CONCLUSIONS

Abundance of Scotch broom seedlings in Cohort #1 

increased with level of overstory retention in Douglas-fir

stands, and it was greater on organic than on mineral

seedbeds. Only 1 percent or fewer of the seeds sown in

Cohort #2 produced a seedling, and seedling abundance

failed to differ among treatments. Although seedbeds in

shelterwoods were slightly moister and warmer than those

in thinned stands, these differences in growing conditions

did not explain treatment effects on seedling abundance.

The combined effects of overstory shade and an organic

seedbed probably created “safe” sites that facilitated germination of Scotch broom. For example, shade from overstory

trees reduced evaporative demand in the understory, and

organic seedbeds may have had greater variation in microtopography and improved moisture-holding characteristics

than mineral seedbeds. Once germinated, however, the

seedlings became susceptible to predation and drought. 

The current status and ultimate fate of un-germinated

seeds from the two cohorts remain unknown. Although

greater visibility of seeds in clearcuts or on mineral

seedbeds may have contributed to increased predation

41

Figure 3—Effects of Douglas-fir overstory retention level on average (a) soil water content

(standard error = 0.872) and (b) mean daily soil temperature (standard error = 0.307) during

2004–2006.
General Technical Report GTR-694

(Bossard 1991), viable seeds of Scotch broom probably

remain on the experimental seedbeds because of their

potential for delayed germination. Bossard (1990) estimated

that, from a given cohort of seeds, 98 percent would be

viable, 5 percent would be lost to seed predation by insects,

40 percent would germinate in the current year, 25 percent

would germinate in the second year, and the remaining 28

percent would germinate in future years.

Results of this study suggest that moderate levels of

overstory retention provide more favorable conditions for

germination and short-term survival of Scotch broom

seedlings than clearcuts, possibly through drought amelioration. However, up to two years after sowing two separate

cohorts of seeds, ≤2 percent have produced seedlings. Thus,

it is likely that some seeds remain and may yet germinate.

This uncertainty regarding continued viability of the sown

seeds makes it difficult to predict which environment is

most suitable for germination and survival of Scotch

broom. Continue monitoring of the experiment will be

required to address this longer-term question.

ACKNOWLEDGMENTS

I am grateful to Grace Douglass, Joe Kraft, and Diana 

Livada for assistance with laboratory and field measurements and to Washington State Department of Natural

Resources for use of study sites and installation of overstory treatments. Thanks also to Warren Devine, Peter

Gould, Dave Peter, Shibu Jose, and Lauren Urgenson for

manuscript reviews.

LITERATURE CITED

Bossard, C.C. 1990. Secrets of an ecological interloper: 

ecological studies on Cytisus scoparius (Scotch broom)

in California. Ph.D. dissertation, University of

California, Davis.

Bossard, C.C. 1991. The role of habitat disturbance, seed 

predation, and ant dispersal on establishment of the

exotic shrub Cytisus scoparius in California. The

American Midland Naturalist. 126: 1–13.

Bossard, C.C. 1993. Seed germination in the exotic shrub 

Cytisus scoparius (Scotch broom) in California.

Madroño. 40: 47–61.

Bossard, C.C.; Rejmanek, M. 1994. Herbivory, growth, 

seed production, and resprouting of an exotic invasive

shrub, Cytisus scoparius. Biological Conservation.

67: 193–200.

Harrington, T.B. 2006. Five-year growth responses of 

Douglas-fir, western hemlock, and western red cedar

seedlings to manipulated levels of overstory and understory competition. Canadian Journal of Forest

Research. 36: 2439–2453.

Parendes, L.A.; Jones, J.A. 2000. Role of light availability 

and dispersal in exotic plant invasion along roads and

streams in the H.J. Andrews Experimental Forest,

Oregon. Conservation Biology. 14: 64–75.

Parker, I.M. 2002. Safe site and seed limitation in Cytisus 

scoparius (Scotch broom): invasibility, disturbance,

and the role of cryptogams in a glacial outwash prairie.

Biological Invasions. 3: 323–332.

SAS Institute, Inc. 1999. SAS/STAT User’s Guide. Vol. 2. 

Version 8. Statistical Analysis Systems Institute, Inc.,

Cary, NC.

Sokal, R.R.; Rohlf, J.F. 1981. Biometry. 2nd ed. New 

York: W.H. Freeman.

Turner, J.H. 1933. The viability of seeds. Kew Bulletin. 

1933: 257–269.

USDA Natural Resouces Conservation Service. 2006. 

PLANTS database. [online] URL: http://plants.usda.

gov

Williams, P.A. 1981. Aspects of the ecology of broom 

(Cytisus scoparius) in Canterbury, New Zealand. 

New Zealand Journal of Botany. 19: 31–43.

42
ABSTRACT

In the western United States and Canada, there are about fourteen species of non-native hawkweeds (Hieracium spp.) 

belonging to two subgenera. Hawkweeds are fibrous-rooted, perennial herbs growing from a stout rhizome. Collectively,

they possess many characteristics that allow a species to become invasive: perennial life cycle, apomictic reproduction (seed

production without pollen), high seed production and germinability, broad ecological amplitude, long distance seed dispersal, regeneration from root fragments, root buds, rhizomes and stolons, rapid generation time (ca. 63 days), and broad latitudinal range. In addition, several hawkweed species, particularly H. aurantiacum, orange hawkweed, are popular ornamental

species, further increasing spread. Invasive hawkweeds infest a range of habitats in the Pacific Northwest, predominantly

occurring in open fields, mountain meadows, clearings in forest zones, permanent pastures, and other modified habitats

where the soil is well drained, coarse-textured, and moderately low in organic matter. Management of hawkweed-invaded

sites has relied mostly on selective herbicides, which are effective in suppressing hawkweeds but reinvasion occurs unless

other plant species fill the gaps left by hawkweed removal. Hawkweeds are thought to persist in these sites because they

capture nitrogen in nutrient-poor soils, thus limiting nutrients available to competing plants. Fertilizers and soil fertility management have been used to effectively control hawkweeds in some areas, especially in new hawkweed infestations or where

hawkweed density is relatively low. Long-term management of hawkweed needs to emphasize altering conditions in the

plant community to favor grasses and desirable forbs following initial control efforts. Alternative control solutions, including

biological control, are being investigated. 

KEYWORDS: Hawkweed, Hieracium, invasive plant, weed ecology.

1 Linda M. Wilson is an Invasive Plant Ecologist, Department of Plant, Soil and Entomological Sciences, University of Idaho, P.O. Box 442339, Moscow,

ID 83844-2339. 

43

INTRODUCTION

Invasive hawkweeds (Hieracium spp.) comprise a complex 

of about 14 species occurring throughout the Pacific

Northwest (PNW). The name Hieracium comes from the

Greek ‘hierax,’ meaning hawk; allegedly keen-sighted

hawks of yore ate the sap of the brightly colored plants to

sharpen their eyesight. Rapid spread of hawkweeds and 

distribution across many habitats has been possible because

much of the land in British Columbia, coastal and northeastern Washington, northern Idaho, and northwestern

Montana is considered susceptible to invasion by these

aggressive weeds. 

TAXONOMY

Hawkweeds are in the tribe Lactuceae of the family 

Asteraceae, having all strap-shaped (ligulate) flowers and 

a milky latex in stems and leaves. The genus Hieracium

is divided into three subgenera. Subgenus Chionoracium

(formerly subgenus Stenotheca) represents the ± 20 native

species in North America. In the PNW, subgenus Hieracium,

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

BIOLOGY AND MANAGEMENT OF INVASIVE HAWKWEEDS 

(HIERACIUM SPP.) IN THE PACIFIC NORTHWEST

Linda M. Wilson1
General Technical Report GTR-694

which includes both North American and European components, comprises two native species and six species from

central and eastern Europe, including smooth hawkweed,

common hawkweed, European hawkweed, polar hawkweed, spotted hawkweed, and wall hawkweed (table 1).

Subgenus Pilosella, entirely European in origin, represents most of the invasive species in the PNW. The eight

known invasive species in subgenus Pilosella include meadow hawkweed, orange hawkweed, mouse-ear hawkweed,

whiplash hawkweed, kingdevil hawkweed, queendevil

hawkweed, and tall hawkweed (table 1). A new species,

yellowdevil hawkweed, was identified from southeastern

British Columbia in 2001. This was the first report of this

species in North America (Wilson et al. 2006). A recent

molecular phylogeny of Hieracium in North America indicates a clear separation between subgenera as well as separation between native and invasive species (Gaskin and

Wilson, in press). Preliminary molecular studies on the

genetic diversity of orange hawkweed has revealed a single

clonal line common to all populations examined to date

(Lila Fishman, pers. comm.). 

DESCRIPTION OF INVASIVE 

HAWKWEEDS

Hawkweeds are fibrous-rooted, perennial herbs growing 

from a stout rhizome. Plants reproduce by seeds and vegetatively by stolons, rhizomes, and adventitious root buds.

The small, dandelion-like heads are borne singly at the top

of a long, hairy to hairless stem, or in compact, rounded or

loose, elongated panicle-like clusters. All but one invasive

species have yellow flowers (likewise, all but one native

species have yellow flowers). Seed production is primarily

asexual through apomixis (the production of seeds without

pollen), although occasional sexual reproduction, involving

outcrossing and hybridization, is believed to occur. Hawkweeds are distinguished largely on a few key morphological

characters, including leaf, stem and phyllary (involucral

bract) pubescence. Hairs, both type and abundance, are

important characters used to distinguish hawkweed species.

Three types of hairs are common: long simple hairs; dark,

glandular hairs; and small, star-shaped (stellate) hairs. All

invasive hawkweeds are polyploid (n=9) and typically 

44

Table 1—Invasive, non-native hawkweed species in western North Americaa

Scientific name

Subgenus Pilosella Common name Distribution

Hieracium caespitosum Dumort. meadow hawkweed AB, BC; ID, MT, OR, WA, WY

(= H. pratense Tausch.)

Hieracium aurantiacum L. orange hawkweed AB, BC; AK, ID, MT, OR, WA, WY

Hieracium pilosella L. mouse ear hawkweed BC; OR, WA

Hieracium flagellare Willd. whiplash hawkweed BC; WY

Hieracium floribundum Wimm. kingdevil hawkweed BC; ID, MT, OR, WA

& Grab.

Hieracium praealtum Vill.

(= H. bauhini Schult.) queendevil hawkweed BC 

Hieracium piloselloides Vill. tall hawkweed BC; MT

Hieracium glomeratum Froel. yellowdevil hawkweed BC; ID, WA

Subgenus Hieracium

Hieracium laevigatum Willd. smooth hawkweed BC; WA

Hieracium lachenalii Gmel. common hawkweed BC; ID, WA

(= H. vulgatum Fries.)

Hieracium sabaudum L. European hawkweed BC; WA

Hieracium atratum Fries. polar hawkweed WA

Hieracium maculatum Sm. spotted hawkweed BC; WA

Hieracium murorum L. wall hawkweed BC; AK

a Including Alaska, Alberta, British Columbia, Idaho, Montana, Oregon, Washington, and Wyoming
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

asexual, in contrast to native species which are entirely

diploid and sexual.

Invasive hawkweeds commonly occur as populations

of intermediate types throughout the PNW, making identification of invasive hawkweed species difficult. Abundant

variation in plant characteristics due to apomixis and perhaps occasional hybridization, environmental and site influences, and natural variation (polymorphism) has resulted in

the description of thousands of species, subspecies and

types worldwide. Identification of invasive and native

hawkweeds can be difficult. To aid land managers in identification of invasive hawkweeds, a diagnostic key was

developed (Wilson 2006).

INVASION SUCCESS

Hawkweeds invasive in the Pacific Northwest and 

Intermountain West are pre-adapted to many coastal and

inland habitats that are climatically similar to those in their

native European ranges. Hawkweeds possess many characteristics that allow a species to become invasive: perennial

life cycle, apomictic reproduction, high seed production and

germinability, broad ecological amplitude, long distance

seed dispersal, regeneration from root fragments, root buds,

rhizomes and stolons, rapid generation time (ca. 63 days),

and broad latitudinal range. In addition, several hawkweed

species, particularly orange hawkweed, are popular ornamentals. They have been and continue to be spread by

intentional and accidental human activities. 

Invasive hawkweeds infest similar habitats in

Washington, Idaho, Oregon, Montana and British

Columbia. Found predominantly in open fields, mountain

meadows and clearings in forest zones, hawkweeds also

infest permanent pastures, timber harvest units, abandoned

farmland and other modified habitats where the soil is well

drained, coarse-textured, and moderately low in organic

matter (Wilson et al. 1997).

Throughout most of the PNW, hawkweeds generally

grow at elevations ranging from 725 m (2400 feet) to over

1700 m (5600 feet). They occur more commonly at lower

elevations above 51°N latitude. None of the invasive hawkweeds are found in the natural grasslands or shrub-steppe of

the PNW, nor are they invasive in the dry habitats of southcentral British Columbia and central Washington. 

MANAGEMENT

Management of hawkweed-invaded sites has had mixed 

results. Controlling hawkweed has relied mostly on selective herbicides. Herbicides are effective in suppressing

hawkweeds but reinvasion occurs unless other plant species

fill the gaps left by hawkweed removal. Control and management of meadow hawkweed has been complicated by

the plant’s ability to persist following chemical and cultural

control inputs. 

Herbicides have been widely used to manage hawkweeds. An experiment was established in 2005 near Santa,

Idaho, to evaluate control of meadow hawkweed (H. caespitosum) after foliar applications of aminopyralid, clopyralid, and triclopyr in each of three seasons: spring (bolting

stage), summer (flowering stage), and fall (senescence)

(Wilson et al. 2006). Each treatment varied by the percent

mortality of meadow hawkweed rosettes, and prevented

seed production one and two months after treatment. The

greatest amount of control was achieved using aminopyralid at 7 oz / acre applied at the rosette stage. 

Hawkweeds are thought to persist in nutrient-poor soils

because they capture limited nutrients, possible through

their association with soil mycorrhizal fungi. Fertilizers and

soil fertility management have been used to effectively control hawkweeds in some areas, especially new hawkweed

infestations or where hawkweed density is relatively low.

The Invasive Hawkweed Consortium comprises numerous stakeholder groups with interest in the biology, ecology,

and management of invasive hawkweeds. Members include

the Idaho Department of Agriculture, Idaho Department of

Lands, University of Idaho, Hawkweed Action Committee,

Rimrock Hawkweed Cooperative, USDI Bureau of Land

Management, British Columbia Ministry of Forests and

Range, British Columbia Ministry of Agriculture and

Lands, Washington State Noxious Weed Control Board,

Montana Noxious Weed Trust Fund, Kootenai County

Weed Control Board, Stevens County Weed Control Board,

USDA Forest Service, Pend Oreille County Weed Board,

45
General Technical Report GTR-694

Palouse Cooperative Weed Management Area, Selkirk

Cooperative Weed Management Area, Panhandle Lakes

Cooperative Weed Management Area, Dow AgroSciences,

Nez Perce BioControl Center, Potlatch Corporation, and

Crown Pacific. The goals of the Invasive Hawkweed

Consortium are primarily to develop a classical biological

control program, but also to increase awareness through

workshops, field tours, and extension outreach activities. 

In addition to biological control, the Consortium supports

research in invasive hawkweed genetics, and herbicide and

fertilizer field studies. Although biological control of invasive hawkweed is not currently available, investigations 

are underway to determine if several insect species from

hawkweed’s native range in Europe are suitable agents

(Grosskopf 2005). 

CONCLUSION

The occurrence of several species of invasive hawkweeds 

in the Pacific Northwest poses additional challenges to

managers. Managers will need to be able to distinguish

non-native, invasive hawkweeds from the native hawkweed

that occur in the region before they can develop and implement effective management programs. Established infestations of hawkweeds are fast spreading and successful control will require aggressive suppression or containment programs, including early detection and eradication programs.

Regardless of the control method used for initial removal 

of hawkweed, long-term management of hawkweeds must

emphasize altering conditions in the plant community to

favor the establishment of desirable vegetation. 

LITERATURE CITED

Gaskin, J.F.; Wilson, L.M. 2006. Phylogenetic relationships among native and naturalized Hieracium

(Asteraceae) in Canada and the United States based on

plastid DNA sequences. Systematic Botany (in press). 

Grosskopf, G. 2005. Biological Control of Hawkweeds 

(Hieracium spp.). CABI Bioscience Annual Report.

Delémont, Switzerland.

Wilson, L.M. 2006. Key to Identification of Invasive and 

Native Hawkweeds (Hieracium spp.) in the Pacific

Northwest. B.C. Ministry of Forests and Range, Forest

Practices Branch, Kamloops, B.C. [online] URL:

http://www.for.gov.bc.ca/hfp/invasive.

Wilson, L.M.; McCaffrey, J.P.; Quimby, Jr, P.C.; 

Birdsall, J.L. 1997. Hawkweeds in the Northwestern

United States. Rangelands. 19(4): 18–23.

Wilson, L.M.; Fehrer, J.; Bräutigam, S.; Grosskopf, G. 

2006. A new invasive hawkweed, Hieracium glomeratum (Lactuceae, Asteraceae), in the Pacific Northwest.

Canadian Journal of Botany. 84: 133–142. 

Wilson, L.M.; Prather, T.; Wallace, J.; Lass, L. 2006. 

Meadow hawkweed control using aminopyralid and

other selective herbicides in an abandoned pasture near

Santa, ID. Western Society of Weed Science, Progress

Rep: 5 p.

46
ABSTRACT

To what extent do biotic interactions affect plant competition and fitness? This question relates to the general problem of 

understanding invasions by some exotic plants. Without the benefit of any evolutionary period that would allow for local

adaptation to environmental conditions, some exotic plants are able to enter a plant community and outcompete the natives.

The Novel Weapons Hypothesis, developed by Ray Callaway and his colleagues, explains this phenomenon in terms of biotic, plant-to-plant interactions that favor the novel invader. Developed from earlier ideas about allelopathy, the hypothesis

appears to explain the phenomenal success of spotted knapweed, Centaurea stoebe, in western North America. Spotted

knapweed roots have been reported to exude a chemical, (-)-catechin, that is thought to be a novel weapon suppressing the

growth of naïve neighbors in the native plant community. Questions were raised last year, however, by a report of an unsuccessful attempt to repeat some of the research on (-)-catechin. Our interest in spotted knapweed as a model system for plant

invasions is also based on biotic interactions. But, in our case, we are hypothesizing that fungal endophytes are influencing

plant competition, as well as other plant behaviors. There is precedent, at least by analogy, for this hypothesis. John

Klironomos showed that soil microbiota may promote plant invasions because invasive species avoid or fail to elicit negative feedback with soil pathogens. This negative feedback reduces the competitiveness and abundance of many native plants.

More specifically, knapweed’s interactions with native plants are affected by soil fungi, and soil microbes in its native range

inhibit knapweed, as predicted by Klironomos.

KEYWORDS: Centaurea stoebe, spotted knapweed, endophytes, competition.

1 George Newcombe is Associate Professor of Forest Pathology/ Plant Symbiosis, University of Idaho, georgen@uidaho.edu, (208) 885-5289.

2 Center for Research on Invasive Species and Small Populations [CRISSP], University of Idaho.

3 College of Natural Resources, University of Idaho.

4 College of Agricultural and Life Sciences, University of Idaho.

47

ROLE OF ENDOPHYTES

Soil microbes are notoriously diverse (Curtis et al. 2002, 

Fierer and Jackson 2006); some may be ‘residents,’ whereas others may be ‘transients’ that disappear from the soil

community as their specific plant substrate degrades. It is

not yet clear whether fungal endophytes of aerial plant parts

are generally soil residents or transients; the latter seems

more likely. Apart from a brief soil phase, even non-systemic endophytes would thus spend most of their poorly

understood life cycle in their host plants. In any event,

endophytes are associated with a growing list of ecological

roles: thermotolerance of plants growing in geothermal

soils (Redman et al. 2002); community biodiversity (Clay

and Holah 1999); enhancement of plant growth (Ernst et al.

2003). Specific endophytes may play specific roles, and

many plants host very diverse arrays of endophytes (Ganley

et al. 2004; Ganley and Newcombe 2006).

Endophytes in introduced plants like knapweeds must

either have been co-introduced in seeds of their host, or

have ‘jumped’ from other plants in the invaded range of

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

FUNGAL ENDOPHYTES IN SPOTTED KNAPWEED: 

DO THEY AFFECT ITS INVASIVENESS?

George Newcombe1,

2,3, Anil K. Raghavendra2,3, Alexey Shipunov2,3, Cort Anderson2,3,

Hongjian Ding2,

4, Sanford Eigenbrode2,4, Timothy Prather2,4, and Mark Schwarzlaender2,4
General Technical Report GTR-694

their host. Because we are interested in the nativity or origin

of fungi, we have been sampling fungal endophytes from

knapweed in both its native range in Europe, and in its

invaded range in North America. From 2004 to 2006, we

have obtained isolation frequencies from seeds of the native

range in Europe varying from 13 to 100 percent; some sites

in the Pacific Northwestern portion of the invaded range

have yielded relatively high isolation frequencies also, but

more often samples of 100, surface-sterilized seeds have

yielded no endophytes at all. The reasons for the variation

in isolation frequency may emerge in analyses that we will

be performing with our in-progress, extensive, 2006 survey.

So far, most of our endophytes from knapweed seed

have tended to belong to a few fungal genera: Alternaria,

Fusarium, and Botrytis. There are also rarer occurrences 

of endophytes belonging to Ulocladium, Cladosporium,

Aureobasidium, Epicoccum, Phoma, and Nemania. Within

many of these genera, we are also discovering diversity in

ITS haplotypes; for Alternaria, various Alt a1 haplotypes

are emerging. Some haplotypes have been found in knapweed in both of its ranges, and these are suggestive of 

co-introduction. Differences in community composition

between native and invaded ranges also appear likely, 

and these differences may affect behavior, including 

invasiveness.

IDENTIFYING SOURCE AND MOVEMENT 

OF ENDOPHYTES

Greenhouse experiments have been performed with seedlings that were either E+ [endophyte-infected] or E- [endophyte-free]. Flowers of both E+ and E- plants, after 

pollination with bee abdomens, yielded E- seed. Apparently,

endophytes are not systemic in spotted knapweed. An endophyte cannot be vertically transmitted in seed of a knapweed plant unless it has infected that plant at some point. 

In the greenhouse, infection events are unlikely as foliage 

is never wet for the period of time that most fungi require

for infection.

Re-isolation experiments in the greenhouse have

revealed that if flowers are pollinated and then inoculated

48 hours later, the inoculated endophyte can be re-isolated

from the surface-sterilized, mature seed at a rate that varies

with the endophyte. In other words, infection can take place

during flowering, but it is likely that it can also occur

before flowering.

Some endophytes produced sesquiterpenoid volatiles;

these same isolates repelled seedhead weevils, Larinus 

minutus, when inoculated into pollinated flowers 12 hours

prior to weevil introduction. Many knapweed endophytes

significantly reduced germination of seed of Idaho fescue,

Festuca idahoensis. Significantly fewer knapweed endophytes reduced germination of seed of knapweed itself. 

One Fusarium isolate caused significant knapweed seedling

mortality.

In model competition experiments that were patterned

after Callaway’s groundbreaking work (Ridenour and

Callaway 2001), endophytes in knapweed affected Idaho

fescue plants grown in the same pots. E+ knapweed plants

were significantly bigger (i.e., greater dry biomass) than

their E- counterparts. The opposite was true of fescue: E+

plants were significantly smaller than the E-. This result, in

particular, is difficult to reconcile with the emerging differences in isolation frequency in the two ranges. Where spotted knapweed is native and non-invasive, endophytes occur

at high frequency. Where spotted knapweed is non-native

and invasive, endophytes are frequently absent. Resolution

of this conundrum will likely involve distinctions among

specific endophytes, their distributions, isolations from

leaves and roots in addition to seeds, and possibly other

factors.

In conclusion, our experimental research to date suggests that endophytes influence the invasiveness of spotted

knapweed by affecting competition and relationships with

herbivorous insects. But endophyte diversity in spotted

knapweed is substantial, and effects on competition and

other biotic relationships may very well depend on specific

endophytes. Our understanding of the distributions and life

cycles of specific endophytes in knapweed’s native and

invaded ranges is still fragmentary. Explaining invasiveness

in simple terms has proven challenging for ecologists (Blair

et al. 2006), but no hypothesis to date has included the contributions of endophytes.

48
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

LITERATURE CITED

Bais, H.P.; Vepachedu, R.; Gilroy, S.; Callaway, R.M.; 

Vivanco, J.M. 2003. Allelopathy and exotic plant invasion: from molecules and genes to species interactions.

Science. 301: 1377–1380.

Blair, A.C.; Nissen, S.J.; Brunk, G.R.; Hufbauer, R.A. 

2006. A lack of evidence for an ecological role of 

the putative allelochemical (±)-catechin in spotted

knapweed invasion success. Journal of Chemical

Ecology. 32: 2327–2331.

Blair, A.C.; Hanson, B.D.; Brunk, G.R.; Marrs, R.A.; 

Westra, P.; Nissen, S.J.; Hufbauer, R.A. 2005. New

techniques and findings in the study of a candidate

allelochemical implicated in invasion success. Ecology

Letters. 8: 1039–1047.

Callaway, R.; Thelen, G.C.; Barth, S.; Ramsey, P.W.; 

Gannon, J.E. 2004a. Soil fungi alter interactions

between the invader Centaurea maculosa and North

American natives. Ecology. 85: 1062–1071.

Callaway, R.M.; Thelen, G.C.; Rodriguez, A.; Holben, 

W.E. 2004b. Soil biota and exotic plant invasion.

Nature. 427: 731–733.

Clay, K.; Holah, J. 1999. Fungal endophyte symbiosis and 

plant diversity in successional fields. Science. 

285: 1742–1744.

Curtis, T.P.; Sloan, W.T.; Scannell, J.W. 2002. Estimating 

prokaryotic diversity and its limits. Proceedings of the

National Academy of Sciences. 99: 10494–10499.

Ernst, M.; Mendgen, K.W.; Wirsel, S.G.R. 2003. 

Endophytic fungal mutualists: seed-borne

Stagonospora spp. enhance reed biomass production 

in axenic microcosms. Molecular Plant-Microbe

Interactions. 16: 580–587.

Fierer, N.; Jackson, R.B. 2006. The diversity and biogeography of soil bacterial communities. Proceedings

of the National Academy of Sciences. 103: 626–631.

Ganley, R.J.; Newcombe, G. 2006. Fungal endophytes 

in seeds and needles of Pinus monticola. Mycological

Research. 110: 318–327.

Ganley, R.J.; Brunsfeld, S.J.; Newcombe, G. 2004. A

community of unknown, endophytic fungi in western

white pine. Proceedings of the National Academy of

Sciences. 101: 10107–10112.

Klironomos, J.N. 2002. Feedback with soil biota 

contributes to plant rarity and invasiveness in communities. Nature. 417: 67–70.

Redman, R.S.; Seehan, K.B.; Stout, R.G.; Rodriquez, 

R.J.; Henson, J.M. 2002. Thermotolerance generated

by plant/fungal symbiosis. Science. 298: 1581.

Ridenour, W.M.; Callaway, R. 2001. The relative importance of allelopathy in interference: the effects of an

invasive weed on a native bunchgrass. Oecologia. 

126: 444–450.

49
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ABSTRACT

The Integrated Weed Control Project (IWCP) of Washington State University Extension has been implementing biocontrol 

of weeds since 1999. In this time we have noted several substantial successes after the biocontrol releases. Larinus minutus

Gyllenhal (Coleoptera: Curculionidae), a seed-feeding beetle, has reduced Centaurea diffusa Lam. (diffuse knapweed) by

not only reducing the seed production, but outbreak populations of the adults have defoliated and killed many plants.

Before- and after-treatment pictures demonstrate reduced populations within five years. Similarly, Lythrum salicaria L. 

(purple loosestrife) has been dramatically reduced with the use of the foliage-feeding beetles, Galerucella calmariensis (L.)

and G. pusilla (Duft.) (Coleoptera: Chrysomelidae). Infestations throughout much of Washington have been reduced to 

non-damaging levels in as little as four years after the initial release. Recently, Mecinus janthinus Germar. (Coleoptera:

Curculionidae), a stem-mining beetle for the control of Linaria dalmatica (L.) Mill. (Dalmatian toadflax), is demonstrating

similar results in areas of eastern Washington. Although successes such as these are encouraging, many challenges still exist.

The distribution of biocontrol agents for a variety of weed species and their impact in reducing invasive plant populations

are critical to weed management success. IWCP aims to meet these challenges by continuing biocontrol redistribution, 

site-specific evaluation and planning, and insect and plant monitoring. IWCP serves county, state, federal, tribal and private

land-managers by providing expertise, on-site evaluations, educational seminars and demonstrations, and biological control

agents at no cost.

Project Objectives

• Educate land-managers on rapid response for new invaders and integrated weed control methods.

• Encourage and increase biological control agent use for large and scattered infestations.

• Support innovative methods in weed control.

• Improve coordination of weed boards, USDA Forest Service Ranger Districts, Native American Tribes and other

land-management agencies and land owners in Washington and surrounding states.

Ongoing Work and Future Plans

• Continue implementing biocontrol across the state.

• Continue educating land-managers about biocontrol.

1 Jennifer Andreas is the Program Coordinator, Western Washington Integrated Weed Control Project, 919 SW Grady Way, Ste. 120, Renton, 

WA 98057, jennifer.andreas@metrokc.gov, phone: (206) 205-3135, fax: (206) 296-0952.

2 Washington State University, King County Extension.

3 Washington State University, Ferry County Extension.

4 Washington State University, Chelan County Extension.

51

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

SUCCESSFUL BIOLOGICAL CONTROL OF INVASIVE

PLANT SPECIES IN WASHINGTON

Jennifer Andreas1, Tara J. Zimmerman2, Daniel L. Fagerlie3,

Brad W. Gaolach2, Dale K. Whaley4, and Tyler W. Wilson3
General Technical Report GTR-694

• Monitor biocontrol agents and weed infestations to assess the efficacy of the practice.

• Further expand early detection / rapid response efforts.

• Continue developing educational pamphlets and workshops.

• Continue pesticide recertification courses.

• Encourage and assist land-managers with appropriate pasture management techniques and revegetation information.

• Provide expertise and on-site evaluations and recommendations to land-managers.

• Act as a resource to land-managers for weed control information and equipment (e.g., weed wrenches).

KEYWORDS: Integrated weed control, biological control, early detection-rapid response, IWCP. 

52
ABSTRACT

Garden loosestrife (Lysimachia vulgaris) is a rhizomatous perennial native to Eurasia that is widely distributed across the 

northern United States and southern Canada. It thrives in wetland and riparian areas. Considered naturalized in areas like

New England, it may be that the cold winters are the only factor keeping it from becoming invasive. Garden loosestrife is

particularly worrisome for several reasons. It is difficult to control, and according to Cusick (1986), it will form easily overlooked thickets of non-flowering young plants before appearing in flower as a fully established infestation. Also, although

several western states have mild climates that are conducive to serious infestations of garden loosestrife, only Washington

State has it listed as a noxious weed. In King County, Washington, there are a number of large garden loosestrife populations

that may give us some indication of its real potential for spread in the West and the challenges in controlling it. The King

County Noxious Weed Control Program is currently working with government agencies, community groups and private

landowners to control garden loosestrife. Our control strategies vary according to the extent of the infestation and land 

ownership in each area.

KEYWORDS: Garden loosestrife, Lysimachia vulgaris, King County, wetlands.

1 Katie Sauter Messick is an Aquatic Weed Specialist, King County Noxious Weed Control Program, 201 S. Jackson St., Suite 600, Seattle, WA 98104.

2 Drew Kerr is the Assistant Field Operations Manager, San Francisco Estuary Invasive Spartina Project.

53

HOW TO RECOGNIZE GARDEN 

LOOSESTRIFE

Garden loosestrife is a tall, emergent perennial in the primrose family with a large terminal panicle of showy, bright

yellow flowers in July and August (fig. 1). The leaves are

ovate and softly hairy, generally in whorls of three or four,

and are usually dotted with small orange or black glands.

The stems are also hairy, and in a large stand five percent or

more can be fasciated (i.e., flattened). In King County, we

have noted the red rhizomes reaching more than ten feet

into adjacent waters. 

Garden loosestrife is extremely aggressive and forms

monocultures, much like its common namesake, purple

loosestrife (Lythrum salicaria). In fact, it outcompetes purple loosestrife, as well as aggressive natives like spirea

(Spirea douglasii) and cattails (Typha latifolia). The result

is a depauperate shoreline plant community, clogged waterways and diminished wildlife habitat.

WHERE TO FIND GARDEN LOOSESTRIFE

The first records of garden loosestrife in King County 

are from Juanita Bay on Lake Washington in 1978. By

1990, it had spread along shorelines throughout the Lake

Washington and Sammamish systems. Populations in Lake

Burien, to the south of Seattle, and in Rutherford Slough

near Fall City in the Snoqualmie River basin were discovered in the late 1990s, possibly originating from garden

plantings. Most recently, this program has found pioneering

stands dotting the banks of the Snoqualmie River downstream (north) of Rutherford Slough (fig. 2). 

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

GARDEN LOOSESTRIFE (LYSIMACHIA VULGARIS), 

A SPREADING THREAT IN WESTERN WATERWAYS

Katie Sauter Messick1 and Drew Kerr2
General Technical Report GTR-694

54

Figure 1—Garden loosestrife along the Sammamish River.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

CASE STUDY: CONTROL AROUND LAKE

SAMMAMISH

An estimated 25 percent of the Lake Sammamish shoreline 

is infested with garden loosestrife, covering some 20 total

miles. The worst hit spots are natural areas, most particularly the large public parks at the north and south ends of the

lake. While the public agencies are dealing with the extensive, inaccessible populations in those areas, King County’s

strategy with private landowners on Lake Sammamish has

been one of voluntary control. In 2006 we sent 1,000 letters

to property owners informing them of the situation and asking them to control the plants on their land, and five percent

responded to the letter. Using continued direct mailings,

workshops and other forms of public education, we hope to

increase the percent of property owners voluntarily controlling the garden loosestrife as we gain control of the plant on

public land.

CASE STUDY: CONTROL IN 

RUTHERFORD SLOUGH

The discovery of garden loosestrife in Rutherford Slough 

presented a different problem, since this was the only area

in a large and highly functional river basin to be infested.

Our program worked with the property owners to hire 

contractors to apply herbicides to the populations starting 

in 2002 (fig. 3a). The most effective herbicide tried has 

been a 1.5 percent solution of triclopyr TEA (Renovate3TM),

which has significantly reduced the population over much

of the Slough as of 2006 (fig. 3b). A two percent solution of

55

Figure 2—Garden loosestrife distribution in King County.
General Technical Report GTR-694

56

Figure 3—Garden loosestrife infestation on Rutherford Slough (a) before (2002) and (b) four years after herbicide treatment

(2006).

A

B
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

glyphosate (e.g. AquaMasterTM or RodeoTM) is also effective,

but the reduction in all plants can lead to greater germination of the seed bank. Unfortun-ately, in 2004 garden

loosestrife was found in the mainstem Snoqualmie River,

and we are now forming a strategy to contain the further

spread of this weed.

NEXT STEPS

1. Continue to survey all known areas of infestation 

to get a solid picture of the garden loosestrife population in King County.

2. Work with homeowners in the large lake systems 

to get voluntary control.

3. Work toward eradication of garden loosestrife in the

Snoqualmie River basin.

4. Apply principles of “early detection, rapid response”

to any new infestations.

5. GET THE WORD OUT: HAVE YOU SEEN THIS

WEED?

LITERATURE CITED

Cusick, A.W. 1986. Distributional and taxonomic notes 

on the vascular flora of West Virginia. Castanea. 

51: 56–65.

Washington Department of Ecology. 2006. Garden 

Loosestrife: A Problem Wetland Plant in Washington.

[online] URL: http://www.ecy.wa.gov/programs/

wq/plants/weeds/aqua007.html. August 2006.

57
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ABSTRACT

Garlic mustard, Alliaria petiolata (M. Bieb.) Cavara and Grande, is an extremely invasive biennial plant that is poised to 

significantly impact the Pacific Northwest’s biodiversity unless urgent action is taken to curtail it. Garlic mustard has

demonstrated the ability to thrive in sensitive forested ecosystems. An aggressive IPM approach is recommended which

should include a combination of methods, most importantly, monitoring and timely control to prevent seeding. It is imperative that all property stakeholders allocate a portion of their time and resources each year for garlic mustard control and

monitoring. The state of Washington has the unique advantage of having it listed as a Class A noxious weed which mandates

eradication as a landowner responsibility. Garlic mustard populations in King County are still limited enough that there is a

chance that its spread can be restricted in the short term and that they could be eventually eradicated. The majority of known

sites have been prevented from seeding in the last few years and, as a result, spread has been contained. A coordinated

regional approach will be necessary to achieve long-term eradication of garlic mustard in the Pacific Northwest.

KEYWORDS: Noxious weeds, garlic mustard, invasive plants, Pacific Northwest, Washington.

1 Karen J. Peterson is a Noxious Weed Specialist, King County Noxious Weed Control Program, 201 S. Jackson St., Suite 600, Seattle, WA.

59

INTRODUCTION

Garlic mustard has already caused widespread environmental damage on the East Coast and the Midwest (Welk et al.

2002). Garlic mustard’s extreme adaptability makes it highly likely to become a similar problem on the West Coast. It

produces significant amounts of seed (up to 500 seeds per

plant) which are moderately long lived in the soil (up to 10

years). It rapidly colonizes exposed, disturbed sites, but will

also invade relatively undisturbed forested sites, displacing

native vegetation (Nuzzo 1999). When cut back, it will

often continue to re-sprout and flower. It has recently been

shown that garlic mustard produces a toxin that interferes

with the ability of trees to take in necessary nutrients

(Stinson et al. 2006). 

Recent modeling (Welk et al. 2002) shows the existing

and potential distribution of garlic mustard in North

America (figs. 1a-b). The extensive potential distribution 

of garlic mustard, combined with the significant degree of

potential impact suggests garlic mustard presents a major

threat to the biodiversity of forested ecosystems in the

Pacific Northwest.

Garlic Mustard Control in King County

Garlic mustard was first discovered in King County in 

1999. At this stage, the infestations were likely in a lag

phase of establishment. The area of garlic mustard found

has increased since that time, although the area controlled

has also increased (fig. 2). The increased infestation area

can be attributed to a combination of improved and more

thorough survey techniques and to actual spread of garlic

mustard beyond the original range. Most of these new sites

are small, consisting of private properties. From 2004 to

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

GARLIC MUSTARD CONTROL: IS SUCCESS A POSSIBILITY? 

STRATEGY AND POTENTIAL IMPACT

Karen J. Peterson1
General Technical Report GTR-694

60

Figure 1—Populations of garlic mustard in North America in (a) 2002 and (b) potential future populations of 

garlic mustard in North America if not controlled (Welk et al. 2002).

B

A
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

2006, close to 100 percent of known garlic mustard infestations have been controlled and seeding prevented. The

exceptions may be in some forested urban parks, where it is

possible that a small number of plants have escaped detection in the undergrowth.

A comprehensive survey and control regimen has been

implemented that involves large numbers of volunteers and

parks gardeners searching for and pulling out flowering

garlic mustard. Significant support and resources have been

provided by the Seattle Parks Department who are responsible for a number of large infestations. In known infestation

sites, this approach is successfully achieving reduced garlic

mustard density, an elimination of seeding, and recruitment

of native plants. It is becoming more difficult to find new

large patches of garlic mustard. 

Successful eradication of garlic mustard requires

increased education of the public. This should engage their

support in the following ways: 1) reporting new infestations, 2) working to control them, and 3) monitoring results.

Eradication of garlic mustard is a landowner responsibility

under the Washington State Noxious Weed Control Law.

Enforcement action against landowners who fail to cooperate in the control of garlic mustard on their land will also 

be implemented now that a clear community standard of

control has been established.

An important part of the long-term control strategy will

involve cooperation and coordination with other federal,

state and local agencies fighting this problem in the Pacific

Northwest. There are several known garlic mustard sites in

the Western states. Sharing information and developing an

overarching regional strategic approach to control will be

essential for long-term success. 

CONCLUSION

With comprehensive, aggressive control and collaboration 

with other local, state and federal agencies as well as private citizens, we do have the chance to reduce and eventually eradicate garlic mustard in the Pacific Northwest. This

61

Figure 2—Area of garlic mustard found and controlled from 2001 to 2006 in King County, WA.
General Technical Report GTR-694

will avoid potentially significant environmental impacts to

the region.

LITERATURE CITED

Nuzzo, V. 1999. Invasion pattern of herb garlic mustard 

(Alliaria petiolata) in high quality forests. Biological

Invasions. 1(2-3): 169–179.

Stinson, K.A.; Campbell, S. A.; Powell, J.R.; Wolfe, 

B.E.; Callaway, R.M.; Thelen, G.C.; Hallett, S.G.;

Prati, D.; Klironomos, J.N. 2006. Invasive plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms. PLoS Biology. 4(5):

e140.

Welk, E.; Schubert, K.; Hoffman, M.H. 2002. Present 

and potential distribution of invasive garlic mustard

(Alliaria petiolata) in North America. Diversity and

Distributions. 8: 219–233.

62
ABSTRACT

Giant (Polygonum sachalinense), Japanese (P. cuspidatum), and bohemian (P. × bohemicum) knotweeds are non-native 

plant invaders of riparian corridors throughout the United States and Europe. Knotweed invasion is suspected to alter critical

riparian processes including forest and understory regeneration, streambank stability, soil nutrient cycling and allochthonous

litter inputs. Currently, there is limited quantitative evidence of the level or significance of these impacts. We investigated

the effects of giant knotweed invasion on: 1) the composition and diversity of forest understory communities, and 2) the

quantity and nutrient quality of riparian leaf litter inputs into streams. Field data were collected in summer and fall 2004 

at Grandy Creek, a tributary of the Skagit River, Washington, densely colonized by giant knotweed.

There was a negative correlation between giant knotweed invasion and the species richness and abundance of native

understory herbs, shrubs, and juvenile trees. In addition to the displacement of native vegetation, data suggest that knotweed

invasion alters stream nutrient subsidies from riparian litterfall. We observed an average 70 percent reduction of leaf litter

from native species in knotweed invaded sites. The carbon-to-nitrogen ratio (C:N) of senesced knotweed leaves was 52:1, 

a value 38 to 58 percent higher than dominant native riparian species. Analysis of nutrient re-absorbance from senescing

leaves revealed that knotweed reabsorbed 75.5 percent of its foliar nitrogen prior to litterfall. In contrast, native species

reabsorbed 4.8 to 33 percent.

Knotweed’s displacement of understory herbs, shrubs and juvenile trees can have detrimental and long-lasting effects on

riparian forest structure and function. Loss of juvenile trees in the understory leads to a decline in overstory tree density and

canopy cover over time and can alter the successional trajectory of riparian forests. Deciduous and coniferous trees are foundation species within Pacific Northwest riparian forests. Empirical evidence from the region demonstrates that loss of riparian trees has detrimental effects on the bank stability, hydrology, nutrient loading, micro-habitat conditions and aquatic biota

of adjacent lotic systems.

Litterfall from riparian vegetation comprises a primary source of nutrients and energy in forested streams and backwater

channels. High C:N, as in knotweed litter, is generally associated with low resource quality and slower rates of nutrient

release through decomposition. Thus, knotweed leaf litter may provide a poorer quality food resource for aquatic consumers

than native riparian species. Efficient autumnal re-absorbance of foliar nitrogen suggests that knotweed transports a majority

of nitrogen resources down into its own rhizome system for reuse during subsequent growing seasons. In contrast, native

species contribute a higher percent of their nutrient resources to riparian soils and aquatic environments through their leaf

litter. 

1 Lauren Urgenson is a doctoral candidate, University of Washington, College of Forest Resources, UW Botanic Gardens, Box 354115, 

Seattle, WA 98195-4115, lsu@u.washington.edu.

2 Sarah H. Reichard is an Associate Professor, University of Washington Botanic Gardens, Box 354115, Seattle, WA 98195, 

reichard@u.washington.edu.

63

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

THE ECOLOGICAL CONSEQUENCES OF GIANT KNOTWEED

INVASION INTO RIPARIAN FORESTS

Lauren Urgenson1 and Sarah H. Reichard2
General Technical Report GTR-694

By displacing native vegetation and altering the species composition and nutrient quality of riparian litterfall, knotweed

invasion may have a detrimental impact on the structure and productivity of riparian forests and adjacent aquatic food webs. 

KEYWORDS: Giant knotweed, Polygonum sachalinense, Polygonum cuspidatum, Polygonum × bohemicum, invasion

impacts, riparian vegetation.

64
MANAGEMENT

APPROACHES

Orange hawkweed (Hieracium aurantiacum) (Lisa K. Scott)
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ABSTRACT

Government invasive species control programs can create significant public value in terms of economic, environmental, 

recreational and public health benefits. These benefits are however constrained by: 1) inconsistencies between jurisdictions,

and 2) lack of effective administration within many jurisdictions. As well as being ecological opportunists, invasive species

also can be viewed as exploiting administrative and jurisdictional weaknesses and opportunities. Increasingly, land management agencies are developing strategies to focus their activities to achieve defined invasive species program missions and

goals. Effectiveness of these strategies will be increased through their alignment with overarching regional and national

strategic frameworks to achieve a more collaborative, coordinated and accountable effort. Equally important, although less

strongly emphasized, strategic management is necessary over time to successfully implement these strategies, and account

for increases in public value. In King County, Washington, strategic management is used to deliver increased public value

from noxious weed control. Working towards the mission of “minimizing noxious weed impact on the environment, recreation, public health and economic resources,” the program measures performance using indicators for specific operational

goals. These goals and some 2005 performance indicator results are: 1) eradicating existing infestations and preventing new

infestations of Class A (highest priority) noxious weeds - 60 percent of area found to date eradicated, and 2) controlling designated Class B (second-tier priority) noxious weeds to below thresholds of significant impact - 78 percent of area found in

2005 controlled. A variety of management tools are used to improve these indicators over time. These emphasize alignment

of staff accountability with program goals, and increasing their empowerment and participation in program decision-making.

Strategic management of public-sector invasive species programs is critical to enhancing their effectiveness. By better aligning the strategies of the various government entities engaged in invasive species management and more effectively implementing them, the public value created by these public programs can be significantly increased.

KEYWORDS: Invasive species, noxious weeds, management, government, public.

1 Steven Burke is a Program Manager, King County Noxious Weed Control Program, 201 South Jackson St., Suite 600, Seattle, WA 98104, 

Steve-j.burke@metrokc.gov.

67

WHY PUBLIC INVESTMENT?

Government action is essential for effective management 

of many invasive species. Landowners acting in their own

self-interest will not always efficiently reduce the costs 

and impacts created by invasive species introduction and

spread. This is a classic case of market failure where insufficient action by some landowners, jurisdictions and 

communities can result in massive externalized costs to 

others (Perrings et al. 2005).

Minimizing the impact of invasive species to the economy, public health and the environment is clearly a public

good (McNeely 2004). Public programs therefore play an

important role in a comprehensive and effective approach 

to managing them. These programs can create significant

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

STRATEGIC MANAGEMENT OF PUBLIC 

INVASIVE SPECIES PROGRAMS

Steven J. A. Burke1
General Technical Report GTR-694

public value and are justified when that value greatly

exceeds their costs to citizens.

WEAKNESSES OF THE CURRENT 

PUBLIC-SECTOR EFFORT

Unfortunately, the lack of effective administration of government invasive species control programs in the United

States currently constrains the public value they can produce. It also provides fodder for those who desire less government involvement in invasive species management. 

In particular, benefits from the management of invasive

species are being significantly constrained by: 1) inconsistencies between jurisdictions, and 2) lack of effective

administration within many jurisdictions. As well as being

ecological opportunists, invasive species also can be viewed

as exploiting administrative and jurisdictional weaknesses

and opportunities.

There has been significant recent effort to address these

challenges. Increasingly, land management agencies and

organizations are developing invasive species strategies.

These identify their goals, objectives, activities and measures of success for their programs.

Some examples (in decreasing scale and increasing

level of detail) include:

• Global Strategy for Addressing the Problem of

Alien Invasive species (IUCN Global Invasive

Species Programme)

• National Invasive Species Management Plan

(National Invasive Species Council)

• Pulling Together Strategy – FICMNEW

National Strategy for Invasive Plant 

Management

• USDA Forest Service National Strategy and

Implementation Plan

• Washington Invasive Species Council – 

proposed strategy

• Oregon Invasive Species Action Plan 

• Idaho Strategic Plan for Managing Noxious 

Weeds

• Washington State Noxious Weed Management

Plan

• King County Noxious Weed Control Program 

– Strategic Plan

Nonetheless the current overall national invasive 

species management effort is still unfocused and poorly

coordinated, despite significant strategic planning effort and

considerable funding (in excess of one billion dollars annually) by the Federal Government (GAO 2005). A recent 

survey of a range of federal, state and local agency staff

involved in weed management found that they believed

lack of consistent and adequate funding was a significant

barrier to improved invasive species management outcomes

(GAO 2005). It is clear, however, that lack of coherent 

and accountable national strategy is a more fundamental

problem.

Making progress in reducing the impacts of invasive

species will require two critical improvements in the development and implementation of these strategies: 1) strategic

alignment and 2) improved strategy implementation.

STRATEGIC ALIGNMENT

Strategic alignment of invasive species programs can be 

defined as increasing the effectiveness of invasive species

strategies through their alignment with overarching regional

and national strategic frameworks to achieve a more collaborative, coordinated and accountable effort. 

Invasive species do not respect jurisdictional boundaries or authorities. Poorly coordinated actions from individual jurisdictions will be far less effective than harmonized approaches and they possibly will be doomed to failure in the long run (Doelle 2003). More importantly, poor

invasive species management in just one jurisdiction can be

the weak link which undermines the effectiveness of others.

The prevention and control of invasive species is a weakest-link public good (Perrings et al. 2002) where the ability

to provide public good is in the hands of the least supportive and cooperative jurisdictions and stakeholders.

Increasing comprehensiveness is a major challenge 

to invasive species management in the United States.

American natural resource managers must deal with highly

fragmented jurisdictions which in no way correspond to the

natural systems managed. While every state has some form

68
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

of invasive species legislation, their scope and implementation vary widely. 

Successful invasive plant management strategies must

therefore be comprehensive and engage or compel even the

most unenthusiastic stakeholders. Currently there is no

comprehensive approach at the national level and in many

states. Some examples of the weakest jurisdictional links

are: 1) in Washington State, Douglas County does not have

a noxious weed board or program, and 2) nationally, the

State of Mississippi also does not have an effectively 

operating state noxious weed law.

Another major challenge that needs to be overcome is

how to achieve a degree of consistency regarding the objectives and methods between agencies that operate in bureaucratic silos. A land management agency that simply concerns itself with the control of invasive species on its lands

in isolation, without regard to neighboring landowners and

pathways of spread, would be a classic example of this.

The effectiveness of invasive species management

strategies will increase with their comprehensiveness and

level of adoption of application and the level of scale 

(temporal and spatial) as described in Figure 1.

One promising approach is to develop and encourage

networks based on shared strategy that achieve collaboration and accountability across bureaucratic boundaries.

Increased cooperation and coordination is essential for the

long-term effective management of invasive species (GAO

2005). At the regional, national and international level,

these collaborative institutional frameworks can address

gaps, weaknesses and inconsistencies of bureaucratic silos

(Doelle 2003, Behn 2006). 

A lot of work has already been done developing overarching strategic frameworks at the international and

national level. These, however, tend to be symbolic documents of good intent rather than effective operational strategies. They primarily give lip-service recognition to the need

for broader coordination but offer little accountability to the

overarching objectives. These frameworks must be reinforced by a hierarchy of accountability mechanisms and

made more comprehensive in order to work. 

Existing multi-jurisdictional strategies such as the

National Invasive Species Management Plan involve few 

or no binding performance management or accountability

mechanisms to drive the achievement of the overarching

objectives. An accountability hierarchy must be established

to support these strategic frameworks, encourage collaboration and establish performance management mechanisms.

For example, the excellent Forest Service Invasive Species

Strategy would be more effective if it were strongly imbedded in an accountable national approach.

One way to achieve such accountability would be to

link funding to participation in overarching strategies. This

could be done by: 1) providing dedicated new funding or

incentives to participating jurisdictions or organizations

based on performance, or 2) making commitment to overarching strategies binding in some way, perhaps by requiring

participation and accountability as a pre-requisite to eligibility for certain state or federal funds. 

Some elements of linking federal funding to performance are a feature of the currently unappropriated Noxious

Weed Control and Eradication Act (2004). This funding

69

Figure 1—The relationship of strategy comprehensiveness, level

of adoption and scale to its effectiveness.
General Technical Report GTR-694

would be tied to grants and agreements to weed management entities working in a collaborative fashion.

Another way to drive increased collaboration and

accountability is to identify a peak agency responsible for

this. A National Center for Invasive Species Management

has been proposed (ESA 2006). This would expand the role

of the National Invasive Species Council to facilitate policy

coordination and implementation of the National Invasive

Species Management Plan. The National Invasive Species

Management Plan is a good strategic framework that would

be made far more effective if its stakeholders were accountable to a lead agency linked to federal funding initiatives.

Finally, Cooperative Weed Management Areas

(CWMAs) can also play a valuable role in increasing larger-scale collaboration and comprehensiveness of invasive

plant control. The growth and vitality of the CWMA concept is very encouraging.

IMPROVED STRATEGY IMPLEMENTATION

Improved strategy implementation is employing strategic 

management over time to successfully implement programs

and account for increases in public value. 

Strategic management “helps organizations define their

visions and core purposes – the outcome goals that are most

important to them and aim their entire systems at fulfilling

them. This is accomplished by changing the organization’s

purpose, accountability incentives, power structure and 

culture” (Osborne and Plastrik 2000).

Strategic management processes are well described 

and understood. They include defining missions, goals, 

outcomes and outputs; devising strategies to achieve them;

engaging and motivating stakeholders; defining performance measures; negotiating performance agreements with

implementing organizations; and monitoring and evaluating

results. Incentives are created to improve performance and

drive the organization towards its goal.

70

Figure 2—Control and eradication of Class A noxious weeds in King County 1999 – 2006.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

For example, in King County, Washington, strategic

management is being used to deliver increased public value

from noxious weed control. Here, a comprehensive approach

to noxious weed management is being implemented across

the county. This includes coordinating and working with 38

separate incorporated cities, at least four state and two federal land management agencies, two tribes as well as, of

course, the 1.8 million citizens of the county (U.S. Census

Bureau 2005).

Working towards the mission of “minimizing the

impact of noxious weeds on the environment, recreation,

public health and economic resources of King County,” the

program measures performance using indicators for specific

operational goals. These goals and 2005 results for some

relevant performance indicators are:

a) Eradication of existing infestations and preventing

new infestations of Class A (highest priority) 

noxious weeds – 60 percent of noxious weed area

found to date eradicated (fig. 2).

b) Controlling designated Class B (widespread and 

2nd tier priority) noxious weeds to below thresholds 

of significant impact - 78 percent of designated 

Class B infestations found in 2005 controlled.

Some strategic management tools the program is 

employing to drive improvement of these indicators over

time are:

• Clearly developed mission goals and performance measures

• Aligning individual accountability with 

program goals

• Team review of individual activities and 

outputs

• Identifying and developing team culture and 

values

• Contracting out a significant proportion of 

operational tasks to lower cost providers

• Staff empowerment and participation in 

program decision-making

CONCLUSION

Invasive species management is a significant public administration challenge. Invasive species spread by effectively

exploiting ecological opportunities, and administrative and

jurisdicational weakness. There is considerable potential to

provide significantly increased public value from investment in invasive species control programs. Large scale,

comprehensive, collaborative strategies and subsequent

strategic management are necessary for their long-term 

success. 

ACKNOWLEDGMENTS

This paper benefited from helpful reviews from Sasha 

Shaw, Steve McGonigal and Lisa Richmond. I acknowledge

the opportunity provided by Mary Jo Bane to audit her

course “Strategic Management for Public Purposes” at the

Harvard University, Kennedy School of Government. This

experience provided valuable perspective and insight which

was important in the development of this paper.

LITERATURE CITED

Behn, R.D. 2006. Stovepipe bureaucracies live. Bob 

Behn’s Public Management Report. 3(6).

Doelle, M. 2003. The quiet invasion: legal and policy 

responses to aquatic invasive species in North America.

The International Journal of Marine and Coastal Law.

18(2): 261–294. 

Ecological Society of America [ESA]. 2006. Position 

paper of the Ecological Society of America. Biological

invasions: recommendations for U.S. policy and management. March 2006.

Government Accounting Office [GAO]. 2005. United 

States Government Accounting Office. Invasive

Species – Cooperation and Coordination are Important

for Effective Management of Invasive Weeds. GAO05-1026T.

Osborne, D.; Plastrik, P. 2000. The reinventors 

fieldbook: tools for transforming your government.

New York: Jossey Bass.

McNeely, J.A. 2004. Control of the spread of invasive 

species as a global public good. United Nations

Development Programme/ Office of Development

Studies Background Paper.

71
General Technical Report GTR-694

Perrings, C.; Williamson M.; Barbier, E.B.; Delfino, D.; 

Dalmazzone, S.; Shogren, J.; Simmons, P.;

Watkinson, A. 2002. Biological invasion risks and the

public good: and economic perspective. Conservation

Ecology 6(1) 1. [online] URL: http://www.consecol.org

/vol6/iss1/art1/

Perrings, C.; Dehnen Schmutz, K.; Touza, T.; 

Wiliamson, M. 2005. How to manage biological 

invasions under globalization. Trends in Ecology and

Evolution. 19(5): 212–215.

U.S. Census Bureau 2005. [online] URL: http://quickfacts

census.gov/qfd/states/53/53033.html

72
ABSTRACT

The South Okanagan-Similkameen Invasive Plant Society (SOSIPS) identified a need to invest substantial efforts on weed 

management (inventory, treatment, monitoring and research) within the region of three wildfires that occurred in the South

Okanagan in 2003. The three wildfires – Okanagan Mountain Fires, Vaseux and Anarchist (Osoyoos) – encompassed an estimated 34,500 hectares of shrub-steppe and forested landscapes. Wildfire and associated soil disturbances create an ideal

seedbed for invasive plant establishment; consequently, effective management is an even greater concern in fire-impacted

areas. There is also limited published data on the predicted fire response of invasive plants occurring in the Southern

Interior, although most species are anticipated to rapidly spread as a consequence of the fire and related soil disturbances. A

five-year study focusing on the impact of the wildfire and associated soil disturbances on the invasive species was initiated

in 2004 for each of the fire-impacted sites. A ‘task team’ of collaborating stakeholders was established for each site, including representatives from regional, provincial and federal governments, industry, conservation groups, ranchers and private

landowners. The task teams cooperatively developed and implemented an effective approach to invasive plant management.

Detailed inventories were completed, treatment was conducted (herbicide, physical, biological and cultural control), permanent vegetation plots and photo points were established and predictive models were developed for selected species. The risk

assessment models will help guide future management in the Okanagan and potentially can be expanded to other areas of

the Southern Interior.

KEYWORDS: Wildfire, invasive plant management, inventory, monitoring, treatment, risk assessment model.

1 Lisa K. Scott is the Invasive Plant Program Coordinator, South Okanagan-Similkameen Invasive Plant Society, C/O 8703 Palmer Pl., 

Summerland, BC V0H 1Z2 Canada.

73

INTRODUCTION

The summer of 2003 was an unprecedented year for wild

fires in British Columbia (BC). Abnormally hot, dry weather resulted in over 2,500 wildfire starts, most of which

occurred in the BC Interior. The fires also set an all-time

record high for the number of urban-wildland interface

fires. In the Okanagan alone, over 334 homes and many

businesses were lost. Total costs for the year were estimated

at $700 million (Canadian dollars). 

The Okanagan Valley was impacted by three wildfires

– Okanagan Mountain, Vaseux and Anarchist (Osoyoos).

The Okanagan Mountain fire was the largest of the three,

consuming 30,000 hectares of ponderosa pine parkland,

Douglas-fir forest, Engelmann spruce and other forested

habitats. The South Okanagan-Similkameen Invasive Plant

Society had already identified this site as a priority treatment area in 2000, primarily due to the extensive infestation of the provincially noxious weed, tansy ragwort

(Senecio jacobaea). The Vaseux fire area was also previously identified as a priority area for the South Okanagan

Region. This fire encompassed 3300 hectares of antelopebrush shrub-steppe, a provincially endangered ecosystem,

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

INVASIVE PLANT MANAGEMENT FOLLOWING THE 2003 

OKANAGAN VALLEY WILDFIRES, BRITISH COLUMBIA

Lisa K. Scott1
General Technical Report GTR-694

ponderosa pine parkland and Douglas-fir forest. These habitats have been invaded by sulphur cinquefoil (Potentilla

recta), a regionally noxious weed. The Anarchist wildfire

was the smallest of the three wildfires, consuming 1200

hectares of sagebrush shrub-steppe, ponderosa pine parkland and Douglas-fir forest. Prior to the fire, the area was

not identified as a priority site for invasive plant management. 

Following the wildfires, the necessity for effective

weed management (inventory, treatment, monitoring and

research) within the confines of these three wildfires was

more critical. Invasive plants thrive in fire-impacted sites.

Wildfires release soil nutrients, expose ground surfaces,

reduce shade and stimulate seed germination (through

breakage of dormancy). These factors assist the regeneration of both native plants and invasive species. Unfortunately, due to their competitive advantages, invasive

species are often the first to colonize fire-impacted sites.

Once established, invasive species are hard to control,

extremely competitive and quick to spread. As a result, 

the ability of native and desirable plants to re-establish is

limited. 

Fire-impacted areas are further prone to the establishment of invasive plants through post-fire soil disturbances.

Creation of fireguards (or firebreaks), salvage logging,

mushroom picking and off-road recreation all help to

expose soils and spread invasive plant fragments and seeds.

Consequently, effective management of invasive plants is

an even greater concern in fire-impacted areas. As a result,

the South Okanagan-Similkameen Invasive Plant Society

identified the three Okanagan fire-impacted sites as a top

priority for invasive plant management. 

PROJECT COORDINATION

In early 2004, a ‘task team’ of collaborating stakeholders 

was established for each of the three wildfire sites. Each

team included representatives from regional, provincial and

federal governments; industry; conservation groups; ranchers; and private landowners. The task teams met biannually,

including field trips, and cooperatively developed and

implemented an effective approach to invasive plant management for each of the wildfire locations. This coordinated

effort increased the effectiveness of results and allowed for

the most efficient use of limited funding through joint funding proposals. 

INTEGRATED MANAGEMENT

Inventory and Mapping

Inventories were conducted using a Trimble® Pro XRS 

Differential GPS with a TSC-1 data logger in accordance

with the Ministry of Forests Invasive Alien Plant Program

Standards. Invasive plant infestations were logged as point

features with an associated distribution code. Areas of

exposed soils resulting from fire suppression activities and

high burn severity were also mapped due to their vulnerability to weed invasion. Post processing occurred in Arc

View v3.2a to create geographical ‘shape files’ containing

polygons identifying plant species and distribution code. 

Comprehensive inventory and mapping of invasive

plants within the three project areas was essential to

improving the efficiency of management as well as establishing a base-line inventory for monitoring and evaluation. 

The following invasive plants were focal species

detected within the three fire-impacted project areas:

• Tansy ragwort (Senecio jacobaea)a

• Orange hawkweed (Hieracium aurantiacum)a

• Sulphur cinquefoil (Potentilla recta)

• Diffuse knapweed (Centaurea diffusa)

• St. John’s-wort (Hypericum perforatum)

• Canada thistle (Cirsium arvense)

• Bull thistle (Cirsium vulgare)

• Hound’s-tongue (Cynoglossum officinale)

• Oxeye daisy (Chrysanthemum leucanthemum)a

• Dalmatian toadflax (Linaria genistifolia spp. 

dalmatica)

• Plumeless x nodding thistle hybrid (Carduus sp.)a

• Nodding thistle (Carduus nutans)b

• Common burdock (Arctium minus)

a Okanagan Mountain Inventory only

b Anarchist Mountain Project Inventory only 

74
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

Treatment

The inventory and mapping component of the project was 

used to generate planning maps that guided invasive plant

management activities. Treatment activities included an

integrated approach, involving seeding of native grasses,

herbicide spot-treatment, hand cutting, and release of biological control agents. Select areas of all three project sites

were seeded with an agronomic seed mixture developed

from input by each task team. Seeding was used as a preventative tool to help establish a competitive ground cover

to reduce weed establishment. Certified spray contractors

conducted all herbicide treatments. Chemical control

focused on areas of seed pick up and dispersal for target

weed species. Additional physical control was conducted by

the BC Conservation Corps contracted through The Nature

Trust of BC for tansy ragwort at the Okanagan Mountain

site. Biological control agent releases were conducted by

SOSIPS with insects provided by the BC Ministry of

Forests and Range or the BC Ministry of Agriculture and

Lands.

Monitoring and Research

General observations made during field assessments 

showed a positive response to treatments within each of the

fire sites. General observations of seeding indicated a high

germination rate of most agronomic species. Visual inspections of herbicide treated areas showed a high level of 

control (90-95 percent) for all sites. 

In 2004, vegetation plots (Daubenmire plots - 20 x 50

cm) and photo points were established at each of the fire

sites. Plot transects were selected in areas that support various noxious weeds or where weeds were detected on the

periphery of the transect. Each plot was surveyed for percent cover and abundance for each plant species present.

Subsequent data from these plots will be recorded at least

once during the five-year term of the project, and ideally 

at ten years post-fire. 

The weed inventory data from 2004 was also used in

the development of predictive (spatial) models for tansy

ragwort and sulphur cinquefoil. Variables used to develop

the models included crown closure, forest type (dominant

timber type), fire intensity mapping (completed by the BC

Ministry of Environment) and elevation. The result of the

modeling process was the production of risk assessment

models for each species. The models have since been

applied to the remaining Okanagan landscape and have

assisted in guiding management of invasive species. 

Education and Outreach

Field days, workshops and community stewardship events 

were essential to overall success of the projects. Of particular value was the landowner contact conducted in the

Anarchist Fire area, as the fire directly affected an estimated thirty private landowners. Two educational field days

were conducted for private landowners living on Anarchist

Mountain. The events featured an invasive plant presentation, focusing on weed species abundant in the community.

At Okanagan Mountain, a full day field tour of the fireimpacted landscape was held exactly two-years post-fire.

Participants toured three high profile locations, chosen to

highlight site concerns, containment priorities, treatment

successes and seeding response. This event served to

increase the awareness of tansy ragwort and other invasive

plants, provide an example of successful multi-stakeholder

coordination and highlight treatment efforts. 

FUTURE OUTLOOK

The South Okanagan-Similkameen Invasive Plant Society 

will continue to work closely with partners to achieve ongoing collaboration for invasive plant management. Each of

the three project areas remains a high priority for SOSIPS

and the task team partners. Continued invasive plant management over the next three years is essential to protect the

natural resources within these areas as well as the significant investments of 2004-2006, and years prior. A monitoring and evaluation regime will be continued to determine

the efficiency of treatment and adjust management practices

as required. Education/outreach/stewardship programs with

landowners/managers, tenure holders and recreations group

will also continue. SOSIPS will endeavor to work with as

many of these sectors as possible, to raise awareness of

invasive plants and garner additional support. Additional

research opportunities and partnerships will be investigated.

75
General Technical Report GTR-694

ACKNOWLEDGMENTS

Many people contributed to the successful completion of 

the past three years of post-fire weed management activities

within the 2003 Okanagan Wildfire projects. These individuals represent a wide variety of affiliations and backgrounds. 

The primary funding body for all three-project sites

was provided through the Inter-Ministry Invasive Plant

Committee for the provincial government. Other financial

contributions were received from Forests for Tomorrow,

Agriculture Environmental Partnership Initiative, The

Nature Trust of BC (BC Conservation Corps and Human

Resources Development Canada Funding), BC Ministry of

Transportation, BC Ministry of Environment, BC Ministry

of Forests and Range, BC Ministry of Agriculture and

Lands, Terasen Gas, BC Transmission Corporation, and

FortisBC.

76
ABSTRACT

USDA Forest Service risk assessments were used to characterize risks to wildlife from use of dicamba, glyphosate, triclopyr, 

and 2,4-D herbicides. Ten different exposure scenarios in the risk assessments quantified potential acute doses of these herbicides to mammals and birds. Estimated doses of dicamba and glyphosate exceeded toxicity thresholds only at maximum

application rates. Triclopyr exceeded toxicity thresholds in two scenarios, and 2,4-D in eight scenarios, at typical application

rates. Vegetation-eating and insect-eating birds and mammals were at most risk. Estimated doses to predatory birds consuming small mammals or fish did not exceed toxicity thresholds for any herbicide analyzed. Estimated doses sometimes

exceeded doses reported to cause mortality or adverse effects to reproduction or internal organs. Analysis was conducted 

at a broad scale, so estimated doses are unlikely to result in most field applications. However, results suggest that large area

broadcast or aerial spray applications of some herbicides could expose wildlife with small foraging areas to unacceptable

risks. Awareness of potential risks to wildlife allows invasive plant managers to design projects to minimize or avoid these

risks. Results of the analysis were used to help develop management standards for use of herbicides on National Forests in

Oregon and Washington. On these National Forests, dicamba and 2,4-D are not approved for use on invasive plants and

broadcast spray of triclopyr is prohibited. Spot spray and selective applications of triclopyr are permitted.

KEYWORDS: Herbicides, toxicology, wildlife, risk assessment.

1 Shawna L. Bautista is a Wildlife Biologist, USDA Forest Service, Pacific Northwest Region, PO Box 3623, Portland, OR 97208.

2 Results discussed here are based on information available prior to 2005. Herbicide risk assessment is a rapidly changing science and information 

currently available may lead to different conclusions regarding risk to birds and mammals.

77

INTRODUCTION

The USDA-Forest Service Pacific Northwest Region 

(Oregon and Washington) recently reviewed and updated

management direction for its invasive plant program.

Herbicide risk assessments conducted prior to 2005 were

used to characterize the relative potential risks to birds,

mammals and other wildlife from use of any of twelve herbicides used to treat invasive plants. Results2 of the analysis

for acute exposures to dicamba, glyphosate, triclopyr, and

2,4-D are notable and reported here. Chronic exposure and

risk were also analyzed in the assessments (see USDA

Forest Service 2005, Appendix P) but are not discussed

here.

The analysis indicated that high doses of these herbicides have the potential to affect internal organs, reproduction, or health, or increase risk of mortality for insect- and

grass-eating mammals and birds. Project planners considering use of herbicides to control invasive plants may not be

aware of these potential risks because they have not been

widely reported. Increased awareness can help applicators

reduce herbicide risks to birds and mammals in the field.

The analysis was used to support the development of

regional standards and practices to minimize or eliminate

risk to free ranging wildlife from herbicide use on National

Forests in Oregon and Washington. Risks were compared 

to the need for effective invasive plant control across the

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

A SUMMARY OF ACUTE RISK OF FOUR COMMON 

HERBICIDES TO BIRDS AND MAMMALS

Shawna L. Bautista1
General Technical Report GTR-694

region and factored into the Regional Forester’s Record of

Decision (USDA 2005b) regarding use of these herbicides

on National Forests in Oregon and Washington. The broad

scale of the decisions dictated a broad, general scale of

analysis. Analysis conducted at a project-level scale could

yield different results in terms of risk to birds and 

mammals.

METHODS

The USDA Forest Service used methods recommended by 

the National Research Council of the National Academy 

of Sciences to conduct the ecological risk assessment and

characterize risk to wildlife (National Research Council

1983, SERA 2001).

Toxicity information, including data submitted to the

United States Environmental Protection Agency (EPA) for

registration, was used to establish a dose-response threshold

(toxicity threshold) for types of wildlife found on National

Forests in Oregon and Washington. When sufficient data

existed, the toxicity threshold was the “no-observableadverse-effect-level” (NOAEL) rather than a lethal dose

measure, such as an LD50.

3 The NOAEL from the most

sensitive effect from the most sensitive test species was

used to establish the threshold (e.g., for the herbicide

glyphosate, the NOAEL threshold was diarrhea in rabbits;

see table 1).

The acute threshold values for dicamba, glyphosate,

and triclopyr were taken from the respective risk assessments without modification (SERA 2003a, SERA 2003b,

SERA 2004). The risk assessment for 2,4-D did not identify

a specific NOAEL for mammals, so dose-severity values in

the risk assessment (SERA 1998, p. 3–52) were used to

determine the likelihood of an adverse effect. Also, the 2,4-

D risk assessment contains very little information specific

to birds, so data cited in Weed Science Society of America

(2002), were used as the threshold value. Two forms of triclopyr (ester and acid) have somewhat different toxicities 

to birds and different threshold values were used for each,

as identified in the risk assessment.

The analysis for the invasive plant program used ten

exposure scenarios to quantify acute doses to wildlife from

herbicide exposure. Exposure scenarios varied by exposure

mechanism (e.g., feeding, direct spray), animal type and

body size (SERA 2001). For example, one scenario is a

deer-sized mammal consuming vegetation sprayed with herbicide, with the animal consuming solely the contaminated

vegetation. While such a scenario may not be likely to

occur on the National Forests, it provided a consistent

worst-case basis for comparing degree of risk associated

with the different herbicides. 

Doses were estimated for applications at the average

rate used in Forest Service herbicide applications (typical)

and at the highest rates included in the risk assessments

(highest) (table 2). The highest rates included in the risk

assessments are rarely used in Forest Service applications,

but had been reported in the past and were included in the

analysis.

For each of the ten scenarios, the acute dose an animal

could receive was compared to toxicity information from

laboratory studies. Doses above the toxicity threshold were

considered to pose a risk of adverse effects. Exposure scenarios produced a range of estimated doses, listed as lower,

central, and upper estimates in the risk assessments.

Generally, lower and central estimates of dose would not

exceed toxicity thresholds, but the upper range of estimated

doses would exceed the thresholds. Consistent with a

“worst case” analysis approach, results reported here

include instances in which the threshold was exceeded only

in the upper range of estimated doses.

The number of scenarios in which an estimated dose

exceeded a threshold was recorded as a general measure

potential risk to birds and mammals for each herbicide.

When estimated acute doses exceeded these threshold values, the actual estimated dose was compared with available

toxicity data to determine the nature or magnitude of potential adverse effects.

RESULTS

At the typical application rates, none of the acute scenarios 

presented unacceptable risks to wildlife when using dicamba or glyphosate (table 3). At the typical rate, triclopyr

78

3 LD50 is the dose of a toxin that will kill 50 percent of test organisms

within a designated period of time. A lower LD50 indicates a more toxic

compound.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

exposures exceeded thresholds in two scenarios, both of

which assume that 100 percent of a bird’s diet (either grass

or insects) has been directly sprayed. Exposure to 2,4-D 

at the typical rate resulted in eight of ten acute exposure

scenarios where estimated dose exceeded the toxicity

threshold.

At the highest application rates, triclopyr exposures

exceeded toxicity thresholds in five acute scenarios.

Dicamba exposures exceeded toxicity thresholds in four

scenarios at the highest application rate. Glyphosate exposures exceeded toxicity thresholds in three scenarios at the

highest application rate. Exposures to 2,4-D exceeded toxicity thresholds in eight scenarios at the highest application

rates.

79

Table 1—Acute toxicity threshold values from laboratory studies used to determine acute risk for the invasive

plant program in the Pacific Northwest Region of the USDA Forest Service

Effect Noted at 

higher dose 

Herbicide Endpoint Dose Species (usually LOAELa)

Mammals

Dicamba NOAEL 45 mg/kg Rat Decreased pup growth at 105-135 

mg/kg

Glyphosate NOAEL 175 mg/kg Rabbit Diarrhea, and some mortality to dams, 

at 350 mg/kg

Triclopyr NOAEL 100 mg/kg Rat Malformed fetuses at 300 mg/kg

2,4-D “no effects likely” 0.1 mg/kg various mammals Increase thyroid weight and/or 

decreased testicular weight at doses 

greater than 0.1 and less than 1.0

Birds

Dicamba NOAEL 92 mg/kg Mallard Decreased hatchability and survival of 

young at 184 mg/kg

Glyphosate NOAEL 562 mg/kg Mallard & Quail No effects at highest dose

Triclopyr ester 0.1 LD50 38.8 mg/kg Quail 50 percent mortality at 388 

(BEE) mg/kgb

Triclopyr acid 0.1 LD50 53.5 mg/kg Quail 50 percent mortality at 535 

(TEA) mg/kgb

2,4-D 0.1 LD50 56.2 mg/kg Mallard & Quail 50 percent mortality at 562 mg/kg

a LOAEL – lowest observable adverse effect level

b There is some confusion in the risk assessment as to whether the 535 mg/kg for TEA and the 388 mg/kg for BEE are interpreted as LC50’s or NOAELs. 

However, the 1998 RED for triclopyr (EPA 1998) lists as LC50’s for birds as 5357 ppm (p. 73) and 3884 ppm (p. 74), which can be converted to mg/kg

based on birds consuming 10 percent of their body weight per day.

Source: SERA 1998, SERA 2003a, SERA 2003b, SERA 2004, Weed Science Society of America 2002.

Table 2 - Application rates used in calculating doses to birds and mammals

Herbicide Typical application 

rate reported in risk Highest application rate

assessment (lb ae/ac) reported in risk

(lb ae/aca) assessment

Dicamba 0.3 2.0

Glyphosate 2.0 7.0

Triclopyr 1.0 10.0

2,4-D 1.0 2.0

a pounds of acid equivalent per acre
General Technical Report GTR-694

For most scenarios, doses exceeded the NOAEL

threshold, but did not necessarily cause a specific adverse

effect on any wildlife groups. Some scenarios resulted in

theoretical exposures that reached known levels of adverse

effects, including potential mortality.

For dicamba, a large mammal or bird consuming only

treated vegetation would receive a dose that was approximately equivalent to doses that caused adverse effects to

reproduction. Doses to small mammals and birds consuming only contaminated insects for a day exceeded the level

for adverse effects to reproduction. Thus, for both birds and

mammals, adverse effects on offspring are plausible, in the

context of these conservative scenarios, for exposures associated with maximum application rates (SERA 2004).

Glyphosate applied at the highest application rate could

expose large herbivorous mammals feeding exclusively in

the treatment area to a dose that has caused mortality in

tests involving rabbits. While this estimated dose is well

below the reported LD50 for mammals (2,000 mg/kg), mortality in some animals is plausible; other lesser effects (e.g.,

diarrhea) are likely.

The typical application rate of triclopyr resulted in a

dose to herbivorous and insectivorous birds that exceeded

the threshold value, but did not reach a lethal dose.

However, at the highest application rate, the upper range 

of estimated doses to birds exceeded the acute lethal dose

measure (LD50). The maximum application rate of triclopyr

resulted in doses to large herbivorous and small insectivorous mammals that exceeded the level reported to cause

malformed fetuses in rats (Bryson 1994 as cited in SERA

2003b).

Estimated doses to mammals of 2,4-D at the typical

and highest rates equaled or exceeded doses reported, in

laboratory studies, to cause effects such as mild signs of

systemic toxicity, increased thyroid weight, decreased testicular weight, decreased weight gain, sub-clinical pathology to kidney and liver, or damage to other internal organs.

Doses of 2,4-D to birds at typical and highest rates

exceeded 0.1 of the acute LD50, but did not exceed the

LD50. No direct mortality to birds is likely; the potential 

for lesser effects is unknown.

None of these four herbicides exceeded any toxicity

thresholds for fish-eating birds (e.g., bald eagles, ospreys)

or raptors that eat small mammals (e.g., northern spotted

owls, great gray owls) at typical or highest application

rates.

80

Table 3—Acute exposure scenarios and application rates in which estimated 

doses exceeded toxicity thresholds. (✱ - threshold exceeded at typical and highest 

application rates; ♦ - threshold exceeded at highest application rates only; 

-- estimated doses were below threshold)

Scenario/Animal Dicamba Glyphosate Triclopyr 2,4-D

Direct spray, small mammal -- -- ♦ ✱

Consume contaminated vegetation

small mammal -- -- -- ✱

large mammal ♦♦ ♦ ✱

large bird ♦ -- ✱ ✱

Consume contaminated water

small mammal -- -- -- ✱

Consume contaminated insects

small mammal ♦♦ ♦ ✱

small bird ♦♦ ✱ ✱

Consume contaminated prey

carnivore (consumes small mammal) -- -- -- ✱

predatory bird (consumes small mammal) -- -- -- --

predatory bird (consumes fish) -- -- -- --
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

CONCLUSION

Herbicide risk assessments used exposure scenarios to 

quantify doses to wild birds and mammals. In some scenarios, the estimated doses received by mammals and/or birds

exceeded those known to cause adverse effects. The scenarios used are conservative, by design, to produce a “worst

case” analysis. The exposure scenarios do not account for

mitigating factors, such as animal foraging behavior, action

of the herbicide, seasonal presence, timing and methods of

application, and strict adherence to label advisories. For

example, a deer is unlikely to feed exclusively within an

area immediately after treatment, nor would an insectivorous bird forage exclusively on insects that had been inadvertently sprayed with herbicide. Therefore, the estimated

doses from the scenarios would not necessarily translate to

noticeable effects to individuals or populations in the field.

Field studies evaluating the effects of herbicide use on freeranging wildlife have attributed changes in populations to

habitat alteration rather than direct toxic effects (e.g.,

Johnson and Hansen 1969, Lautenschlager 1993,

McMurray et al.1993, Sullivan 1990).

On National Forests in the Pacific Northwest, herbicides are unlikely to be applied to treat invasive plants over

an area large enough to encompass the entire day’s forage

for an herbivorous or insectivorous bird or mammal.

However, the scenarios cannot be dismissed. For instance,

if a broadcast treatment of dicamba or triclopyr were

applied to a large area aerially or by ground equipment, a

bird could encounter and feed upon enough contaminated

food to receive a potentially harmful dose. Similarly, broadcast applications of triclopyr or dicamba could pose a risk

to herbivorous animals if the application area was large

enough to encompass their foraging area (e.g., a meadow).

The USDA Forest Service Pacific Northwest Regional

Forester used this information to guide herbicide use for

invasive plant management (USDA Forest Service 2005b).

All applications of 2,4-D and dicamba were prohibited,

along with aerial and broadcast application of triclopyr

(selective applications of triclopyr, which far reduce the

potential for exposure to birds and mammals, were permitted). The Regional Forester documented her rationale for

these decisions based on the relative cost, effectiveness, and

need for and risks associated with each of the herbicides

(ibid.). The Regional Forester’s 2005 Record of Decision

and supporting documentation are available at: http://www.

fs.fed.us/r6/weeds/

The risk assessments may be further used to guide

implementation of invasive plant treatment projects by

helping land managers avoid scenarios that could result in

harmful exposures to wildlife. 

ACKNOWLEDGMENTS

A draft of this paper was reviewed by:

Pat Durkin of Syracuse Environmental Research Associates, 

Inc.

Dave Bakke, Pesticide Use Coordinator, USDA Forest 

Service, Region 5

Catherine G. Parks, USDA Forest Service Pacific 

Northwest Research Station

Doug Johnson, California Invasive Plant Council

Their comments were considered in this published version. 

Rochelle Desser, Environmental Coordinator, TEAMS 

Enterprise, provided editorial expertise. 

LITERATURE CITED

Note: All of the Forest Service Risk Assessments are 

available on-line at http://www.fs.fed.us/foresthealth/ 

pesticide/risk.shtml

Johnson, D.R.; Hansen, R.M. 1969. Effects of range 

treatment with 2,4-D on rodent populations. Journal of

Wildlife Management. 33: 1125–32.

Lautenschlager, R.A. 1993. Response of wildlife to forest 

herbicide applications in northern coniferous ecosystems. Canadian Journal of Forest Resources. 

23: 2286–99.

McMurray, S.T.; Lochmiller, R.L.; Boggs, J.F.; Leslie, 

Jr., D.M.; Engle, D.M. 1993. Woodrat population

dynamics following modification of resource 

availability. American Midland Naturalist. 

129: 248–56.

81
General Technical Report GTR-694

National Research Council [NRC]. 1983. Risk 

Assessment in the federal government: managing the

process. National Research Council. Washington, D.C.:

National Academy Press. 115 p.

Syracuse Environmental Research Associates, Inc. 

[SERA]. 1998. 2,4-Dichlorophenoxyacetic acid formulations – human health and ecological risk assessment

final report. Fayetteville, NY: SERA; Report TR 95-21-

09-01d. [Irregular pagination].

SERA. 2001. Preparation of environmental documentation 

and risk assessments. Unpublished report, SERA MD2001-01a. 81 pp. 

SERA. 2003a. Glyphosate- human health and ecological 

risk assessment final report. Fayetteville, NY: SERA;

Report TR 02-43-09-04a. [Irregular pagination].

SERA. 2003b. Triclopyr - revised human health and 

ecological risk assessment final report. Fayetteville,

NY: SERA; Report TR 02-43-13-03b. [Irregular 

pagination].

SERA. 2004. Dicamba - human health and ecological 

risk assessment final report. Fayetteville, NY: SERA;

Report TR 04-43-17-06d. [Irregular pagination].

Sullivan, T.P. 1990. Demographic responses of small 

mammal populations to an herbicide application in

coastal coniferous forest: population density and

resiliency. Canadian Journal of Zoology. 68: 874–883.

USDA Forest Service. 2005a. Pacific Northwest Region, 

Invasive Plant Program final environmental impact

statement. Portland, OR: USDA Forest Service, Pacific

Northwest Region. 329+ p.

USDA Forest Service. 2005b. Pacific Northwest Region 

Invasive Plant Program record of decision. Portland,

OR: USDA Forest Service, Pacific Northwest Region.

R6-NR-FHP-PR-02-05. 39 p. + appendices.

Weed Science Society of America. 2002. Herbicide 

handbook. 8th edition. W.K. Vencill (ed.). Lawrence,

KS: Weed Science Society of America. 493 p.

82
ABSTRACT

Milk thistle, Silybum marianum (L.) Gaertner, can be an aggressive pasture and rangeland weed. Milk thistle is native to 

southern Europe, the Mediterranean region and Northern Africa. Its current distribution includes most temperate areas of the

world. In the western United States, it is believed to have been introduced in cattle feed (Bean 1985). In Washington State,

the largest occurrence of milk thistle is found on the Enumclaw plateau in southeastern King County. Milk thistle can form

dense stands and reduce forage availability by shading pasture grasses (Roché 1991). Milk thistle is a nitrate accumulator

and can cause nitrate poisoning if ingested by grazing animals. In Washington State, milk thistle is a Class A noxious weed

and eradication is required by state law (RCW 17.10).

KEYWORDS: Milk thistle, Silybum marianum, eradication, aminopyralid, Washington State.

1 Dennis Chambreau and Patricia A. MacLaren are Noxious Weed Control Specialists, King County Noxious Weed Control Program, Water and Land

Resources Division, Department of Natural Resources and Parks, 200 South Jackson Street, Suite 600, Seattle, WA 98104-3855.

83

INTRODUCTION

Milk thistle is a winter annual or biennial, germinating 

throughout the year in western Washington State. It is 

mainly confined to high fertility soils and frequently establishes on disturbed areas with higher than normal soil nitrogen levels (Bean 1985). In King County milk thistle was 

introduced into the dairy farming area near the town of

Enumclaw and has spread in these heavily fertilized pastures. These soils have a very high nitrogen content due to

the constant application of liquid manure from the waste

lagoons of the dairies. These pastures are continually disturbed by cows and equipment. Seed dispersal is the only

means by which milk thistle spreads. Means of dispersal

include water, mud, hay, vehicles, machinery and animals. 

Milk thistle can be identified from other thistles by its

distinctive white veins on dark green leaves. The broad,

lobed leaves can grow up to 20 inches long and ten inches

wide. The leaf margins are tipped with spines up to 0.5

inches long. Each stem ends in a solitary purple flower

head approximately two inches in diameter. Milk thistle

flower heads are unique in having broad leathery bracts

tipped with spines up to two inches in length. In our region

flowers are produced from late April through October. Each

terminal head of the plant produces approximately 100

seeds and ten to 50 heads are produced per plant (Bean

1985). Because the seeds are heavy (20 mg), and have a

deciduous pappus, most seeds fall near the parent plant.

Milk thistle seed has the potential to remain viable in the

soil for up to nine years. Seeds germinate from late summer

through winter, whenever temperature (32° to 86° F) and

moisture conditions are favorable. Plants overwinter as

rosettes which may reach three feet in diameter (Roché

1991).

ERADICATION EFFORTS IN KING 

COUNTY

In King County, approximately seven acres are infested in 

an area of eight square miles. Eradicating these infestations

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

GOT MILK THISTLE?

AN ADAPTIVE MANAGEMENT APPROACH TO 

ERADICATING MILK THISTLE ON DAIRIES IN 

KING COUNTY, WASHINGTON STATE

Dennis Chambreau and Patricia A. MacLaren1
General Technical Report GTR-694

is the management goal as mandated by state weed control

law. Eradication is difficult for a plant species as well established as milk thistle in King County and requires persistence and dedication. King County Noxious Weed Control

Program partners with landowners through education, technical and control assistance, surveying and monitoring. Our

goal is to stop seed production, exhaust the seed bank and

prevent new infestations and introductions.

The first plants in the Enumclaw area were discovered

on the roadside in 2001. This was thought to be an isolated

infestation, but a large parent infestation was found in 2003.

Beginning in 2003, the infestations were surveyed,

landowners were provided with 2,4-D herbicide and results

monitored. These efforts were not sufficiently reducing the

weed population, and in 2006 an adaptive management

strategy utilizing strong partnerships with landowners and

new tools on the market was initiated. This is an integrated

management approach that utilizes chemical, cultural and

manual techniques.

Three areas with germinating seedlings and established

rosettes were treated with clopyralid in October 2005 to test

the efficacy of fall applications of this herbicide. It is hoped

that the residual activity from the clopyralid will help control germinating seedlings. In April 2006, known infestations were surveyed and 23 landowners were provided 2,4-

D. As an adaptive management trial on the largest infestation, five acres were treated with aminopyralid herbicide at

a rate of five ounces per acre. Aminopyralid does not have

grazing restrictions for lactating animals. The aminopyralid

was slow-acting and the soil residual effects are not yet

known. In May, plants missed by the initial applications

were surveyed and treated with a tank mix of aminopyralid

plus 2,4-D to achieve more rapid herbicide activity. In June,

when plants had bolted and flower heads were visible, 

surveys of the entire infested area were conducted for survivors and new infestations. When plants were found,

viable seed heads were removed and bagged, and the plants

were dug out or treated with aminopyralid plus 2,4-D. In

October, aminopyralid will be applied to rosettes and

seedlings at all infested sites. 

In conclusion, with a strategy of exhaustive surveys

and meticulous control methods to prevent flowering and

seed production, milk thistle on the Enumclaw plateau can

be eradicated as the seed bank becomes exhausted. 

ACKNOWLEDGMENTS

The authors wish to thank the landowners and the King 

County Noxious Weed Control Board for their support of

this long term, time intensive effort.

LITERATURE CITED

Bean, C. 1985. The Nature Conservancy element 

stewardship abstract for Silybum marianum. Arlington,

VA. 10 p.

Roché, C. 1991. Milk thistle (Silybum marianum (L.) 

Gaertn.). Pacific Northwest Extension Publication 

382. 2 p.

84
ABSTRACT

The >91,000-acre Cedar River Municipal Watershed is managed to supply Seattle’s drinking water while restoring fish 

and wildlife habitat, natural ecosystem processes, and native biological diversity. Invasive plants threaten all three of these

restoration goals. Currently no herbicides are used within the watershed, so we are exploring alternative methods to control

three invasive species. In 2004 we used geotextile fabric to cover several large Bohemian knotweed (Polygonum ×

bohemicum) (J. Chrtek & A. Chrtková) patches in a wetland. We monitored the project every three weeks during the growing season, crushing any growth under the fabric and pulling all small starts outside the fabric. After the first growing 

season, there was no growth under the fabric. After three seasons, the pulling, along with growth of competing native vegetation, has greatly reduced the knotweed outside the fabric. Tansy ragwort (Senecio jacobaea) occurs along many of the

roads in the watershed. We pulled all plants annually from 2002 through 2006 and are monitoring this method’s effectiveness and cost efficiency. In fall of 2006 we will establish four biocontrol areas in high density patches, where tansy flea 

beetles (Longitarsus jacobaeae) will be released and results monitored. Scotch broom (Cytisus scoparius) grows along

roads and in other disturbed areas in the watershed. In 2004 we began hand-cutting Scotch broom at the ground surface 

during or shortly after flowering. There was little or no regrowth when the plants were large and competing native vegetation present, as well as in sites where the plants only recently established and the seed bank was minimal.

KEYWORDS: Knotweed, tansy ragwort, Scotch broom, herbicides.

1 Sally Nickelson is a Wildlife Biologist and Watershed Ecologist, Seattle Public Utilities, Cedar River Watershed, 19901 Cedar Falls Rd. S.E., 

North Bend, WA 98045, Sally.Nickelson@seattle.gov.

85

INTRODUCTION

The Cedar River Municipal Watershed, located approximately 30 miles east of the city of Seattle, supplies 70 percent of Seattle’s drinking water (over 100 million gallons

per day). It encompasses >91,000 acres, with elevations

ranging from 538 to 5,447 feet above sea level (fig. 1). It 

is predominately forested and was logged extensively from

the 1880s through the mid-1990s, with only about 14,000

acres of old-growth forest remaining. One of the legacies of

the extensive logging is over 600 miles of road within the

watershed. Habitat types in addition to forest include lakes,

ponds, wetlands, meadows, and talus slopes. 

The city manages the watershed under a 50-year

Habitat Conservation Plan (HCP), signed in 2000. The 

primary goals of the HCP are to restore natural ecosystem

processes, functions, and fish and wildlife habitat, as well

as maintain and foster native biodiversity. No logging for

commercial purposes will be conducted, although we will

thin some dense young forest stands to facilitate older 

forest habitat conditions. In addition, we will create snags,

downed wood, and canopy gaps, and plant native species to

restore and enhance wildlife habitat. As part of the HCP,

over 200 miles of road will be decommissioned by 2020.

Numerous invasive plants are present in the watershed,

including tansy ragwort, Bohemian knotweed, Scotch

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

CONTROLLING INVASIVE PLANTS WITHOUT HERBICIDES,

CEDAR RIVER MUNICIPAL WATERSHED

Sally Nickelson1, Heidy Barnett, David Chapin, Bill Richards, and Dwayne Paige
General Technical Report GTR-694

broom, yellow or meadow hawkweed (Hieracium caespitosum = H. pratense), orange hawkweed (Hieracium aurantiacum), Himalayan blackberry (Rubus discolor), evergreen

blackberry (Rubus laciniatus), and Eurasian watermilfoil

(Myriophyllum spicatum). Currently no herbicides are used

within the watershed. Control methods we have employed

include pulling, cutting, deflowering, and shading with 

geotextile fabric. In the future we will use biocontrol for

some of the invasive species. Although we are monitoring

the success of our invasive plant control program, our

efforts are intended to maximize control within a limited

budget rather than provide answers to research questions.

A KNOTWEED CONTROL PROJECT 

WITHIN A WETLAND

Bohemian knotweed poses one of the most significant 

ecological threats to riparian and wetland habitats in the

municipal watershed. It can rapidly dominate riparian areas,

forming dense monocultures that are little used by native

wildlife species. In addition to simplifying habitat structure,

knotweed may alter the habitat of adjacent streams because

of the different chemistry, timing, and quantity of leaf litter

input. This could potentially alter the invertebrate community and consequently affect the native fish populations,

including threatened salmonids.

In the early 1900s a logging road (Road 16) was constructed through the Rock Creek wetland, a large wetland

complex in the western, lower-elevation portion of the

watershed. This road compromised wetland hydrology and

habitat connectivity for many decades. In 2002 a road

deconstruction project was initiated to restore the natural

hydrology within the wetland. Over 1,200 feet of Road 16

was heavily infested with Bohemian knotweed; therefore,

we combined a knotweed eradication project with the road

deconstruction (fig. 2). Goals of the project were to restore

the natural hydrology, eliminate the knotweed infestation,

reestablish the native plant community, and restore natural

ecological functioning. In the fall of 2002 we deconstructed

the road using heavy equipment, removing all culverts and

86

Figure 1—Location of Cedar River Municipal Watershed.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

some road material. This included excavating the majority

of the large rhizomes of the knotweed and transporting

them off site to an old gravel pit, where they were buried

six to ten feet deep with the road material.

As expected, knotweed along the deconstructed road

bed grew back from the small root and stem fragments that

remained after the heavy equipment phase of the project

was complete. The infested area was approximately 1.25

times that prior to the road excavation because the fragments had been spread by the equipment. The majority of

the small plants were along the road edges adjacent to the

forest, in the same locations as the original infestation. In

2003 we hosted two volunteer events (in May and July) and

all the small knotweed plants were pulled. It was apparent,

however, that this level of effort would be insufficient to

control the infestation. Prior to this project we had tested

the efficacy of using geotextile fabric (road cloth) to control

the knotweed, with promising initial results. In the spring 

of 2004 we installed >31,000 square feet of fabric in an

attempt to starve the small remaining roots of sunlight. We

placed the fabric very loosely over the heavily infested

areas, which allowed some growth under the cloth. We

found that if the cloth is stretched tightly, the plants easily

puncture the cloth. But if it is left loose, this permits some

plant growth but keeps the cloth intact. This allows us 

to maintain the cloth by simply walking over the fabric,

stomping the new growth and breaking the stems. We covered the heavily infested areas along the road edges, leaving

the center of the old roadbed uncovered to function as a

wildlife travel corridor (fig. 3). A variety of large mammals, including elk (Cervus elaphus), deer (Odocoileus

hemionus), black bear (Ursinus americanus), cougar (Felis

concolor), and bobcat (Lynx rufus) use the wetland and

travel frequently along the old roadbed. We wanted to keep

the animals off the fabric as much as possible, to minimize

tearing of the fabric. 

We monitored the project every two weeks during the

remainder of the 2004 growing season. Some growth (~two

87

Figure 2—Map of the Road 16 deconstruction and knotweed control project.
General Technical Report GTR-694

feet) occurred under the fabric early in the summer. During

each visit we stomped down all growth under the fabric and

pulled all plants from the center of the road and around the

edges of the fabric. We used three types of fabric: a heavy

black plastic (6 ml polyethylene sheeting) and two types 

of geotextile fabric, woven (product LP200 obtained from

Layfield Plastics) and non-woven (6 oz weight, product

LP6 from Layfield Plastics). We found the heavy plastic

tore very easily, and several pieces had to be replaced by

the more durable geotextile fabric. The woven geotextile

fabric allowed less light penetration, and consequently had

less growth under it. However, it was very stiff, difficult to

work with, and shed small plastic filaments from the edges.

The non-woven fabric was the best overall for durability,

ease of use, and efficacy of control. A slightly heavier grade

of non-woven fabric (LP7), which provides more shade,

will likely be the best for overall performance and ease of

use.

In 2005 and 2006 we monitored the project every three

weeks during the growing season (May to October). There

was no growth under intact fabric, although some canes

grew through holes made by wildlife. We did not conduct

growth measurements, but observed that both the number

and size of plants outside the fabric decreased both as the

growing season progressed, and from 2005 to 2006. In May

2006, we removed the fabric from three small patches and

no growth of knotweed was observed during the remainder

of the growing season. In 2007 we will remove fabric from

larger areas and continue to monitor every three weeks. We

anticipate we will need to continue monitoring the site for 

88

Figure 3—Photograph of the knotweed covering at the Road 16 project.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

a minimum of five more years to ensure that we have completely eradicated the knotweed. In 2006 the gravel pit

where the old rhizomes from the project had been buried

now had growth over approximately half of the pit. So

burying the rhizomes under a large amount of fill was not

sufficient to kill them.

CONTROLLING TANSY RAGWORT 

ALONG ROADS AND RIVERS

Tansy ragwort occurs extensively along roads and some 

rivers in the watershed, and may spread into recently disturbed areas within the forest (e.g., areas of windthrow after

a major wind storm). This plant can be toxic to ungulates

and several insect species, and control is legally required 

in King County. In 1999 we began to survey the extent of

the infestation and initiate some limited control. Little work

was completed in 2000 or 2001 due to funding and staffing

shortages. In 2002 we initiated a tansy ragwort control project. Project goals included complying with noxious weed

laws, mapping the distribution and density of tansy ragwort within the watershed, tracking population trends and

responses to control efforts, identifying the most costeffective control methods, and recommending management

actions to decrease the spread of tansy ragwort. Since 2002

we have annually counted, mapped, and pulled all tansy

ragwort plants (fig. 4). We cut off all flowers and disposed

of them in a sealed bag, leaving the stems and roots to desiccate in the road bed. The highest densities have consistently been along the most heavily traveled roads at lower

elevations in the western portion of the watershed (fig. 5).

At higher elevations in the eastern portion of the watershed

there are only two high-density patches, with the remaining

isolated patches usually consisting of four or less plants.

No control of tansy ragwort along a specific road segment from 1999 to 2003 resulted in a large increase in both

area covered and density of plants (figs. 6a–b). Pulling all

tansy plants along this road from 2003 through 2006 resulted in far fewer high-density segments (fig. 6c). We plan to

continue pulling all low-density areas, but we will be creating biocontrol areas in the high-density locations by releasing tansy flea beetles under the guidance of the Washington

State University Extension biocontrol coordinator for western Washington. We will clip the flowers in these areas to

prevent seed spread, but allow the plants to become perennial to serve as hosts for the beetle. We anticipate that in

approximately five years the beetle populations will build

89

Figure 4—Summary of tansy ragwort survey and control efforts, 1999 – 2006.
General Technical Report GTR-694

to a level where we will no longer need to do major control

efforts.

SCOTCH BROOM CONTROL PROJECTS

Scotch broom out-competes native vegetation in several 

sunny, dry areas in the watershed. It can invade disturbed

areas within the forest (e.g., windthrow) and provides poor

forage for most wildlife species. In 2004 we began a Scotch

broom control project in the watershed. Project goals were

to map all Scotch broom locations within the watershed, 

try several different removal techniques, eliminate Scotch

broom from the higher elevation eastern portion of the

watershed where populations are small, and eliminate it

from key habitats in the western watershed. Initially we

pulled the larger plants with a weed wrench, but found that

the soil disturbance appeared to stimulate the seed bank,

resulting in a large number of small plants the following

year. We found that cutting the larger plants at the ground

surface during flowering killed most plants, and in areas

with competing native vegetation little growth from seed

occurred. In two areas that had only recently been invaded,

we found that cutting the larger plants and pulling the small

plants eliminated the infestations within two years.

CONCLUSIONS

Four years of attempting to control three invasive species 

without the use of herbicides has had mixed results.

Relatively small patches of Bohemian knotweed appear 

to be greatly suppressed after three years of shading with

geotextile fabric. It remains to be seen how long this treatment will be required to kill all the rhizomes. The large

expense of purchasing, installing, and maintaining the fabric over several years prohibits the use of this treatment on

large patches. We have installed this treatment on many

small infestations that pose a high risk to riparian, wetland,

and high quality forest habitat in the watershed, and results

continue to be promising. 

90

Figure 5—2006 Tansy ragwort distribution and density in the Cedar River Municipal Watershed.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

91

Figure 6—Tansy ragwort distribution and density along selected road 

segments in (a) 1999, (b) 2003, and (c) 2006.

A

B

C
General Technical Report GTR-694

Pulling tansy ragwort prevents seed production, but is

not a cost-effective solution for large patches. We anticipate

that once tansy flea beetle populations build to a sufficient

level, they will provide better and more cost-effective 

control over time. Finally, cutting older Scotch broom

plants at the ground surface during flowering does appear

to kill a significant percentage of the plants. The large seed

bank in areas not previously controlled is problematic, and

the long-term solution in the watershed will be to plant

native vegetation such as conifer trees that will ultimately

out-compete the Scotch broom. 

92
DISTRIBUTION 

AND MAPPING OF

INVASIVE PLANTS

Yellow iris (Iris pseudacorus) (Lisa K. Scott)
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The Mt. Baker-Snoqualmie National Forest is one of the most visited National Forests. More than 5 million people live

within a 70-mile drive. The high visitation rate and the proximity to large numbers of noxious weeds and ornamental plants

make the Forest particularly vulnerable to the establishment and spread of invasive plants. To understand the current status

and distribution of invasive plants on the Forest, we queried our botany, ecology, and noxious weed databases and GIS layers for all non-native plants. The resulting data were then analyzed by ranger district and by county. We examined the spatial

distribution of species to understand how invasive plants may have established and to determine which areas are most vulnerable. In total, 148 non-native species in 28 plant families have been documented. Of these, 34 are noxious weeds, and 

25 are garden ornamentals not currently listed as weeds. We identified the following sources of establishment and spread:

spread from interstates, highways, and power lines that cross the Forest; spread from floods; spread by animals; spread from

vehicles directly accessing the Forest; dumping of yard waste; using materials infested with invasive plant propagules; and

the deliberate planting of ornamentals. In addition, several recent land acquisitions included parcels that were already 

infested with invasive plants.

KEYWORDS: Noxious weeds, invasive plants, non-native plants, Mt. Baker-Snoqualmie National Forest, Washington

State.

1 Tracy L. Fuentes , formerly a Botanist, Mt. Baker-Snoqualmie National Forest, is now a Program Analyst, U.S. Geological Survey, Western Regional

Office for Biology, 909 1st Ave, Suite 800, Seattle, WA 98104.

2 Laura L. Potash and Ann Risvold are Botanists, Mt. Baker-Snoqualmie National Forest, 21905 64th Avenue West, Mountlake Terrace, WA 98045.

3 Kimiora Ward, formerly a Botanist, Mt. Baker-Snoqualmie National Forest, is now a Botanist, Native Seed Network, Institute for Applied Ecology, 

563 SW Jefferson Ave, Corvallis, OR 97333.

4 Robin D. Lesher and Jan A. Henderson are Ecologists, Mt. Baker-Snoqualmie National Forest, 21905 64th Avenue West, Mountlake Terrace, 

WA 98045.

95

INTRODUCTION

The Mt. Baker-Snoqualmie National Forest stretches along 

the western slope of the Cascade Mountains in Washington

State, from the Canada/United States border to the northern

border of Mt. Rainier National Park (fig. 1). More than 5

million people in Washington State (U.S. Census Bureau

2006) and British Columbia (Statistics Canada 2002) live

within a 70-mile drive of the Forest. Given the high visitation rate and the proximity to large numbers of noxious

weeds and ornamental plants, the Forest is vulnerable to the

establishment and spread of noxious weeds and escaped

ornamentals.

Not all non-native plants on the Forest are listed as

noxious weeds by federal and state agencies (USDA APHIS

2006, Washington State Noxious Weed Control Board

2006). To be listed as noxious weeds, non-native plants

must cause economic and/or environmental harm. Species

not listed as noxious weeds may include species which are

not believed to cause such harm or for which harm has not

yet been recognized.

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

NON-NATIVE PLANTS ON THE 

MT. BAKER-SNOQUALMIE NATIONAL FOREST

Tracy L. Fuentes1, Laura L. Potash2, Ann Risvold2, Kimiora Ward3,

Robin D. Lesher4, and Jan A. Henderson4
General Technical Report GTR-694

The goals of the Federal Noxious Weed Program

(USDA APHIS 2006) are to prevent the introduction of

non-native plants into the United States and to prevent the

spread of newly introduced invasive plants within the

United States. The program focus is to minimize the threat

to the nation’s agriculture and environment. Species on the

federal noxious weed list may not be introduced into the

United States or moved between the states. Such importation or movement may be allowed under specific permit

conditions.

Washington State’s Noxious Weed Board updates the

state noxious weed list each year and coordinates education

and control efforts throughout the state. The state noxious

weed list separates species into three categories, based on

their current distribution in the state, their potential for

harm, and required control measures (Washington State

Noxious Weed Control Board 2006). Class A noxious

weeds have limited distribution in the state and a high

potential for harm should they become well established.

Class B noxious weeds are limited in distribution in parts of

Washington State, but very abundant in other parts of the

96

Figure 1—Location of the Mt. Baker-Snoqualmie National Forest in Washington State.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

state. Class C noxious weeds are widespread in Washington

State. State law requires control of all Class A noxious weed

infestations, as well as Class B designates (regions of the

state where that species is not well established). County

weed boards and districts may also require control of other

Class B and Class C noxious weeds. 

As a routine part of their duties, the Forest botanists

work to prevent noxious weed infestations, to document

noxious weed sightings, and to develop noxious weed control plans (USDA Forest Service 2005). To be most effective in preventing new infestations, however, the Forest

must not only be able to detect new infestations and new

species, but also to predict where and how these species

may spread. 

Both the Forest Botany and Forest Ecology Programs

have high quality, long-term data regarding the distribution

of all vascular plants on the Forest. Because of this, we are

able to assess where non-native plants occur on the Forest.

In addition, the different data collection methods allow us 

a unique opportunity to assess how different sampling

schemes detect non-native plants. 

Specifically, we address the following questions: 

1) Which non-native vascular plants occur on the Forest? 

2) In which ranger districts and counties do they occur? 

3) Do the two sampling schemes (botany surveys and 

ecology plots) detect the same non-native species? 

4) Do the botany surveys and ecology plots detect similar

numbers of non-native plants?

METHODS

Existing datasets

The botany survey database contains all vascular plant, 

bryophyte, and lichen species recorded by a professionally

trained botanist in a specific area. These areas are usually

tied to a specific project: proposed roads, trails, or timber

sales, for example. Because project areas are usually easily

accessible by vehicle or foot, the survey areas are often

associated with roads or trails. Area surveyed ranges from

0.1–712 acres. Survey intensity ranges from complete 

surveys for small areas to intuitive controlled surveys for

larger areas (Nelson 1985). Botany survey dates range from

1990 to 2005. 

The ecology plot database contains all vascular plant,

bryophyte, and lichen species recorded by a professionally

trained plant ecologist in an ecology plot. To classify and

characterize the vegetation types of the Forest (Henderson 

et al. 1992), ecologists established 3,097 plots across the

Forest. An additional 1,169 ecology plots have also been

established to document benchmarks of species composition

and stand conditions, to monitor changes over time, to

measure stand growth and productivity, to describe conditions and successional patterns of different plant associations, and to calibrate and validate species habitat models.

Ecology plots are deliberately placed to minimize the influence from roads and trails. Plot sizes are typically 0.15–

0.20 acre, but range from 0.001 to 0.5 acres, depending on

structure and extent of the homogeneous community.

Ecology plot installation dates range from 1979 to 2005.

The noxious weed database contains all records of

reported noxious weeds on the Forest. Reporters of noxious

weeds include botanists, ecologists, county weed program

specialists, knowledgeable Forest employees, and knowledgeable members of the public.

Analysis

To understand the current status and distribution of nonnative plants on the Forest, we compiled a list of all nonnative vascular plants known to occur on the Forest, querying our botany survey, ecology plot, and noxious weed

databases for all vascular plants. We classified the resulting

list of species as native or non-native, based on the regional

flora (Hitchcock et al. 1955–69; Hitchcock and Cronquist

1973), the PLANTS database (USDA Natural Resources

Conservation Service 2006), recent work on Pacific

Northwest Hieracium spp. (Wilson 2006) and professional

opinion. Using the existing databases and GIS, we documented the presence of each non-native species by ranger

district and by county.

Species whose native or non-native status could not 

be readily determined were not included in this analysis

(i.e., Poa pratensis). Because Washington State classifies

Phalaris arundinacea L. as a noxious weed (Washington

State Noxious Weed Board 2006), we followed this usage. 

97
General Technical Report GTR-694

To compare detection rates of area-based and plotbased sampling schemes, we tallied the number of detections of noxious weeds, other non-native plants, and all

non-native plants recorded in the botany survey database

and in the ecology plot database.

Results for surveys, plots, and weed sightings in

Chelan and Kittitas Counties were not included in this

analysis. Although the Mt. Baker-Snoqualmie National

Forest manages some National Forest System lands in 

these counties, most are managed by the Okanogan and

Wenatchee National Forests. 

RESULTS

Species and Distribution

Our analysis indicates that 148 non-native plants have been 

reported to occur on the Forest (tables 1 and 2). Of these,

34 are noxious weeds (table 1), 25 are escaped garden ornamentals not currently listed as weeds (table 2), and 17 do

not appear in the regional flora (table 3, Hitchock and

Cronquist 1973). Noxious weeds and other non-native

plants occur in every county and ranger district on the

Forest (table 4).

Of the reported noxious weeds, the Forest had one

known site of a State Class A noxious weed (Heracleum

mantegazzianum Sommier & Levier), which was controlled

in 2001. In addition, 17 State Class B noxious weeds are

known to occur on the Forest, as are 13 State Class C noxious weeds. At least 1,080 acres of the Forest are infested

with noxious weeds; most of these are along highways,

roads, trails, and railroad and powerline right of ways.

Buddleja davidii Franch., Polygonum cuspidatum Sieb. &

Zucc., Polygonum sachalinense F. Schmidt ex Maxim., and

Polygonum × bohemicum (J. Chrtek & Chrtková) Zika &

Jacobson are mostly known to occur in riparian areas.

Of the 28 plant families represented, Asteraceae and

Poaceae had the highest number of non-natives (fig. 2; table

4). In addition, more noxious weeds were members of the

Asteraceae than any other plant family.

Nine noxious weeds are ubiquitous on the Forest,

occurring on all four ranger districts and in all five counties

(table 1): Cirsium arvense (L.) Scop., Cirsium vulgare

(Savi) Ten., Cytisus scoparius (L.) Link, Geranium robertianum L., Hypericum perforatum L., Hypochaeris radicata

L., Leucanthemum vulgare Lam., Polygonum × bohemicum,

and Senecio jacobaea L.

Detection

Neither a project area-specific approach (botany surveys) or 

a systematic, plot-based approach (ecology plots) detected

every single noxious weed or non-native plant known to

occur on the Forest (tables 4 and 5). The most frequently

detected non-native species on the Forest for both sampling

schemes was Mycelis muralis (L.) Dumort. (table 4). The

most frequently detected noxious weed in the botany survey

database was L. vulgare, while for the ecology plots it was

C. vulgare (table 4). 

Botany surveys detected 25 different species of noxious weeds; ecology plot installation detected 13 different

species (tables 4 and 5).

DISCUSSION

Species and Distribution

Unsurprisingly, most of the known infestations of noxious

weeds are concentrated in already disturbed areas on the

Forest. However, the actual area occupied by noxious

weeds and invasive non-natives is likely to be greater than

that of the already documented sites. Many parts of the

Forest have not been inventoried for non-native plants.

Several highways cross the Forest, and most of the infested

areas along the rights-of-way are not included in the Forest

total area.

Many species appear to be well established, given the

numerous sightings of certain species all over the Forest.

However, other species that are well established in the

Puget Sound area, but not on the Forest, appear to be invading. These new invaders should be our highest priority to

locate and to eradicate. 

Although Impatiens glandulifera Royle was a known

threat to the Forest, based on its presence in King County

(King County Noxious Weed Control Program 2005), we

had no records of it until 2006. During presentation of this

paper at the September 2006 Invasive Plant conference 

in Seattle, WA, a professional botanist in the audience

98

Continued on page 113
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

Ta

ble 1—

N

o

xio

u

s

w

e

e

d

s

o

n th

e

Mt. B

a

k

er-S

n

o

q

u

almie

N

atio

n

al F

ore

st, b

y

State

W

e

e

d

Cla

s

s (W

a

s

hin

gto

n

State

N

o

xio

u

s

W

e

e

d

C

o

ntrol

B

o

ard

2

0

0

6), R

a

n

g

er District a

n

d

C

o

u

nty. S

p

e

cie

s

m

ark

e

d

with

a

n

a

steris

k (*) are liste

d

b

y

U

S

D

A

A

P

HIS (2

0

0

6) a

s

n

o

xio

u

s

w

e

e

d

s

Family

Scientific Name Com

mon

Name

B

u

d

dlejaceae Buddleja davidii Franch. B

utterfly

b

ush

C

X

X

X

X

X

X

Asteraceae Centaurea biebersteinii D

C. Spotted

k

nap

weed

B

X

X

X

X

X

X

X

X

Asteraceae Centaurea diffusa Lam. Diffuse k

nap

weed

B

X

X

X

X

X

Asteraceae Centaurea ×

moncktonii C.E.

Britton

Meadow

k

nap

weed

B

X

X

X

Asteraceae Cirsium arvense (L.) Scop. Can

da thistle C

X

X

X

X

X

X

X

X

X

Asteraceae Cirsium vulgare (Savi) Ten. B

ull thistle C

X

X

X

X

X

X

X

X

X

A

piaceae Conium

maculatum

L. Poison

hemlock

C

Con

volv

ulaceae Convolvulus arvensis L. Field

bin

d

weed

C

X

X

Fabaceae Cytisus scoparius (L.) Lin

k Scot’s broom

B

X

X

X

X

X

X

X

X

X

A

piaceae Daucus carota L. Q

ueen

A

n

ne’s lace B

X

X

X

X

Geraniaceae Geranium robertianum

L. Herb

Robert B

X

X

X

X

X

X

X

X

X

Araliaceae Hedera helix

L. E

n

glish iv

y

C

X

X

X

A

piaceae Heracleum

mantegazzianum

Som

mier &

Levier* Giant hog

weed

A

X

X

Asteraceae Hieracium aurantiacum

L. Oran

ge haw

k

weed

B

X

X

X

X

X

X

X

X

Asteraceae Hieracium caespitosum

D

u

mort. Meadow

haw

k

weed

B

X

X

X

X

X

X

Asteraceae Hieracium lachenalii K.C. . Com

mon

haw

k

weed

C

X

X

X

X

X

G

mel

Asteraceae Hieracium laevigatum

Willd. Smooth

haw

k

weed

B

X

X

Clusiaceae Hypericum

perforatum

L. St. Joh

n’s wort C

X

X

X

X

X

X

X

X

X

Asteraceae Hypochaeris radicata L. Spotted cats’-ear B

X

X

X

X

X

X

X

X

X

Balsaminaceae Impatiens glandulifera Royle Policeman’s helmet B

X

X

Asteraceae Leucanthemum vulgare Lam. O

xeye daisy

B

X

X

X

X

X

X

X

X

X

Scrop

h

ulariaceae Linaria dalmatica (L.) P. Mill Dalmatian toadflax

B

X

X

X

X

Scrop

h

ulariaceae Linaria vulgaris P. Mill. B

utter an

d eg

gs C

X

X

Poaceaeae Phalaris arundinacea L. Reed canary

grass C

X

X

X

X

X

X

X

X

Poly

gonaceae Polygonum cuspidatum Sieb. &

Z

ucc. Japanese k

notweed

B

X

X

X

X

X

Poly

gonaceae Polygonum sachalinense F.

Sch

midt ex

Maxim. Giant k

notweed

B

X

X

99

WA Weed Class

RD Mt. Baker

Darrington RD

Skykomish RD

Snoqualmie RD

Whatcom County

Skagit County

Snohomish County

King County

Pierce County
General Technical Report GTR-694

Ta

ble 1—

N

o

xio

u

s

w

e

e

d

s

o

n th

e

Mt. B

a

k

er-S

n

o

q

u

almie

N

atio

n

al F

ore

st, b

y

State

W

e

e

d

Cla

s

s (W

a

s

hin

gto

n

State

N

o

xio

u

s

W

e

e

d

C

o

ntrol

B

o

ard

2

0

0

6), R

a

n

g

er District a

n

d

C

o

u

nty. S

p

e

cie

s

m

ark

e

d

with

a

n

a

steris

k (*) are liste

d

b

y

U

S

D

A

A

P

HIS (2

0

0

6) a

s

n

o

xio

u

s

w

e

e

d

s

(c

o

ntin

u

e

d)

Family

Scientific Name Com

mon

Name

Poly

gonaceae Polygonum

× bohemicum (J.

C

hrtek

&

C

hrtková) Zika &

Jacobson

Bohemian

k

notweed

B

X

X

X

X

X

X

X

X

X

Rosaceae Potentilla recta L. Sulfur cin

q

uefoil B

X

X

X

X

X

Rosaceae Rubus armeniacus Focke* Himalayan

blackberry

X

X

X

X

X

X

X

X

X

Rosaceae Rubus laciniatus Willd.* C

ut-leaf blackberry

X

X

X

X

X

X

X

Asteraceae Senecio jacobaea L. Tansy rag

wort B

X

X

X

X

X

X

X

X

X

Asteraceae Senecio vulgaris L. Com

mon

grou

n

dsel C

Caryop

h

yllaceae Silene latifolia Poir. ssp. alba (P.

Mill.) Greuter &

B

urdet Blad

der cam

pion

C

X

X

Asteraceae Tanacetum vulgare L. Com

mon tansy

C

X

X

X

X

X

100

WA Weed Class

Mt. Baker RD

Darrington RD

Skykomish RD

Snoqualmie RD

Whatcom County

Skagit County

Snohomish County

ing County K

ierce County P
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

101

Ta

ble

2

—

All oth

er n

o

n-n

ativ

e

pla

nts

o

n th

e

Mt. B

a

k

er-S

n

o

q

u

almie

N

atio

n

al F

ore

st, b

y

R

a

n

g

er District a

n

d

W

a

s

hin

gto

n

State

C

o

u

nty.

S

p

e

cie

s

m

ark

e

d

with

a

n

a

steris

k (*) are

e

s

c

a

p

e

d

or d

elib

erately

pla

nte

d

orn

a

m

e

ntals

Family

Scientific Name Com

mon

Name

A

piaceae Aegopodium

podagraria L. Bishop’s goutweed

X

X

Poaceae A

grostis capillaris L. Colonial bentgrass X

X

X

X

X

X

X

X

Poaceae A

grostis gigantea Roth

Redtop

X

X

X

Simarou

baceae Ailanthus altissima (P. Mill.) Tree-of-heaven

X

X

Swin

gle*

Poaceae Aira caryophyllea L. Silver hairgrass X

X

X

Lamiaceae Ajuga reptans L. Com

mon

b

u

gle X

X

Poaceae Alopecurus geniculatus L. Water foxtail X

X

Asteraceae Arctium lappa L. Greater b

urdock

X

X

X

X

X

Asteraceae Arctium

minus Lesser b

urdock

X

X

X

X

X

X

Bern

h.

Poaceae Arrhenatherum elatius (L.)

Beau

v. ex J.&

K. Presl Tall oatgrass X

X

X

X

X

X

Poaceae Avena fatua L. Wild oat X

X

Asteraceae Bellis perennis L. Law

n

daisy

X

X

X

Poaceae Bromus tectorum

L. C

heatgrass X

X

Poaceae Bromus inermis Leyss. Smooth

brome X

X

X

X

X

su

bsp. inermis

Con

volv

ulaceae Calystegia sepium (L.) R. Br. False hed

ge bin

d

weed

X

X

X

X

Brassicaceae Capsella bursa-pastoris (L.) Shep

herd's p

urse X

X

X

X

Medik.

Caryop

h

yllaceae Cerastium

fontanum

Big chick

weed

X

X

X

X

X

X

Bau

m

g. ssp. vulgare

(Hartman) Greuter &

B

urdet

Caryop

h

yllaceae Cerastium semidecandrum

L. Fivestamen chick

weed

X

X

Rosaceae Chaenomeles sp.* Flowerin

g

q

uince X

X

Liliaceae Convallaria majalis L.* Lily-of-the-valley

X

X

X

X

Rosaceae Crataegus monogyna Jacq.* O

neseed

hawthorn

X

X

X

X

Asteraceae Crepis capillaris (L.) Wallr. Smooth

haw

ksbeard

X

X

X

X

Poaceae Cynosurus cristatus L. Crested

dogstail grass X

X

Poaceae Dactylis glomerata L. Orchard

grass X

X

X

X

X

X

X

X

X

Mt. Baker RD

Darrington RD

Skykomish RD

Snoqualmie RD

Whatcom County

Skagit County

Snohomish County

ing County K

Pierce County
General Technical Report GTR-694

102

Ta

ble

2

—

All oth

er n

o

n-n

ativ

e

pla

nts

o

n th

e

Mt. B

a

k

er-S

n

o

q

u

almie

N

atio

n

al F

ore

st, b

y

R

a

n

g

er District a

n

d

W

a

s

hin

gto

n

State

C

o

u

nty.

S

p

e

cie

s

m

ark

e

d

with

a

n

a

steris

k (*) are

e

s

c

a

p

e

d

or d

elib

erately

pla

nte

d

orn

a

m

e

ntals (c

o

ntin

u

e

d)

Family

Scientific Name Com

mon

Name

Caryop

h

yllaceae Dianthus armeria L. Deptford

pin

k

X

X

Caryop

h

yllaceae Dianthus deltoides L. Maiden

pin

k

X

X

Scrop

h

ulariaceae Digitalis purpurea L.* Fox

glove X

X

X

X

X

X

X

X

X

Brassicaceae Draba verna L. Sprin

g

draba X

X

Rosaceae Duchesnea indica (A

n

dr.) In

dian straw

berry

X

X

Focke*

Poaceae Ely

mus repens (L.) Gould

Q

uack

grass X

X

X

X

X

X

Geraniaceae Erodium cicutarium (L.) Stork’s bill X

X

L'Hér. ex

Ait.

Scrop

h

ulariaceae Euphrasia stricta D. Wolff ex

E

yebrig

ht X

X

X

X

X

X

X

J.F. Leh

m

Lamiaceae Galeopsis tetrahit L. Brittlestem

X

X

X

X

hem

p

nettle

Ru

biaceae Galium odoratum (L.) Scop.* Sweetscented

X

X

bedstraw

Geraniaceae Geranium

dissectum

L. C

utleaf geraniu

m

X

X

Geraniaceae Geranium

molle L. Dovefoot geraniu

m

X

X

X

Lamiaceae Glechoma hederacea L.* Grou

n

d iv

y

X

X

X

X

X

X

Asteraceae Gnaphalium uliginosum

L. Marsh cu

d

weed

X

X

Brassicaceae Hesperis matronalis L. Dame’s rocket X

X

Poaceae Holcus lanatus L. Velvetgrass X

X

X

X

X

X

X

X

X

Liliaceae Hosta sp.* Hosta X

X

Clusiaceae Hypericum calycinum

L.* Aaron’s beard

X

X

Asteraceae Hypochaeris glabra L. Smooth catsear X

X

A

q

uifoliaceae Ilex aquifolium

L.* E

n

glish

holly

X

X

X

X

X

X

X

Asteraceae Lactuca serriola L. Prickly lettuce

Lamiaceae Lamiastrum

galeobdolon Yellow archan

gel X

X

X

(L.) E

hren

d. &

Polatschek*

Lamiaceae Lamium amplexicaule L. Hen

bit dead

nettle X

X

Asteraceae Lapsana com

munis L. Nip

plewort X

X

X

X

X

X

X

Fabaceae Lathyrus latifolius L.* Peren

nial pea X

X

Lamiaceae Lavandula angustifolia* Laven

der X

X

X

Mt. Baker RD

Darrington RD

Skykomish RD

Snoqualmie RD

Whatcom County

Skagit County

Snohomish County

ing County K

Pierce County
Ta

ble

2

—

All oth

er n

o

n-n

ativ

e

pla

nts

o

n th

e

Mt. B

a

k

er-S

n

o

q

u

almie

N

atio

n

al F

ore

st, b

y

R

a

n

g

er District a

n

d

W

a

s

hin

gto

n

State

C

o

u

nty.

S

p

e

cie

s

m

ark

e

d

with

a

n

a

steris

k (*) are

e

s

c

a

p

e

d

or d

elib

erately

pla

nte

d

orn

a

m

e

ntals (c

o

ntin

u

e

d)

Family

Scientific Name Com

mon

Name

Brassicaceae Lepidium

perfoliatum

L. Claspin

g

pep

perweed

X

Poaceae Lolium

perenne L. Peren

nial ryegrass X

X

X

X

X

Fabaceae Lotus corniculatus L. Bird’s-foot trefoil X

X

X

X

X

X

X

X

X

Malvaceae Malva moschata L.* M

usk

mallow

X

X

Malvaceae Malva parviflora L. C

heeseweed

X

X

Asteraceae Matricaria discoidea D

C. May

weed

X

X

X

X

X

X

X

Fabaceae Medicago lupulina L. Black

medic X

X

X

X

X

X

X

X

Fabaceae Melilotus officinalis (L.) Lam. W

hite sweetclover X

X

X

X

X

X

X

X

Asteraceae Mycelis muralis (L.) D

u

mort. Wall lettuce X

X

X

X

X

X

X

X

X

Asteraceae Myosotis arvensis (L.) Hill Field forget-me-not X

X

Boraginaceae Myosotis scorpioides L. True forget-me-not X

X

X

X

X

Boraginaceae Myosotis sylvatica E

hrh. ex

Woodlan

d forget-me X

X

Hoffman

n -not

Liliaceae Narcissus sp.* Daffodil X

X

Brassicaceae Nasturtium officinale Ait. f. Watercress X

X

X

Asteraceae Petasites japonicus (Siebold

&

Z

ucc.) Maxim. su

bsp. giganteus Japanese b

utterb

ur X

X

(F. Sch

midt ex

Trautv.) Kitam.*

Poaceae Phleum

pratense L. Timoth

y

X

X

X

X

X

X

X

X

X

Plantaginaceae Plantago lanceolata L. E

n

glish

plantain

X

X

X

X

X

X

X

X

X

Plantaginaceae Plantago major L. Com

mon

plantain

X

X

X

X

X

X

X

X

X

Poaceae Poa annua L. A

n

n

ual bluegrass X

X

X

X

X

X

X

X

Poaceae Poa bulbosa L. B

ulbous bluegrass X

X

Poaceae Poa compressa L. Canada bluegrass X

X

X

Poaceae Poa trivialis L. Rou

g

h

bluegrass X

X

Poaceae Polypogon monspeliensis an

n

ual rab

bitsfoot X

X

(L.) Desf grass.

Rosaceae Prunus laurocerasus L.* C

herry laurel X

X

Ran

u

nculaceae Ranunculus acris L. Tall b

uttercup

X

X

X

X

Ran

u

nculaceae Ranunculus repens L. Creepin

g

b

uttercup

X

X

X

X

X

X

X

X

X

Poly

gonaceae Rheum rhabarbarum

L.* Rh

u

barb

X

X

Ericaceae Rhododendron sp.* Rhododen

dron

X

X

X

X

X

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

103

Mt. Baker RD

Darrington RD

Skykomish RD

Snoqualmie RD

Whatcom County

Skagit County

Snohomish County

King County

ierce County P
General Technical Report GTR-694 Table 2—All other non-native plants on the Mt. Baker-Snoqualmie National Forest, by Ranger District and Washington State County. Species marked with an asterisk (*) are escaped or deliberately planted ornamentals (continued) Family Scientific Name Common Name Lamiaceae Rosmarinus officinalis L.* Rosemary X X

Polygonaceae Rumex acetosella L. Sheep sorrel X X X X X X X X XPolygonaceae Rumex crispus L. Curly dock X X X X X X X X X

Polygonaceae Rumex obtusifolius L. Bitter dock X X X X X X X X

Caryophyllaceae Sagina apetala Ard. Annual pearlwort X X

Caryophyllaceae Sagina procumbens L. Birdeye pearlwort X X X X

Poaceae Schedonorus pratensis Meadow fescue X X X X X X

(Huds.) Beauv.

Poaceae Schedonorus phoenix (Scop.) Tall fescue X X X X X X X

Holub

Asteraceae Senecio sylvaticus L. Woodland groundsel X X X X X X X

Solanaceae Solanum dulcamara L. Bitter nightshade X X X X X X XAsteraceae Sonchus asper (L.) Hill Spiny sowthistle X X X X X X

Asteraceae Sonchus oleraceus L. Common sowthistle X X

Rosaceae Sorbus aucuparia L.* European mountain X X X X X

ash

Caryophyllaceae Spergularia rubra (L.) J.& K. Red sandspurry X X

Presl

Lamiaceae Stachys byzantina K. Koch* Lamb’s ears X X

Caryophyllaceae Stellaria media (L.) Vill. Common chickweed X X X X

Boraginaceae Symphytum officinale L.* Comfrey X X X X

Asteraceae Taraxacum laevigatum (Willd.) Rock dandelion X X

DC.

Asteraceae Taraxacum officinale G.H. Dandelion X X X X X X X X X

Weber ex. Wiggers

Asteraceae Tragopogon dubius Scop. Yellow salsify X X X X

Fabaceae Trifolium arvense L. Rabbitfoot clover X X X X X

Fabaceae Trifolium campestre Schreb. Field clover X X

Fabaceae Trifolium dubium Sibthorp Suckling clover X X X X X X X X

Fabaceae Trifolium hybridum L. Alsike clover X X X X X X X X

Fabaceae Trifolium pratense L. Red clover X X X X X X X XFabaceae Trifolium repens L. White clover X X X X X X X X X

104

Mt. Baker RD

Darrington RD

Skykomish RD

Snoqualmie RD

Whatcom County

Skagit County

Snohomish County

King County

ierce County

P
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

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Scientific Name Com

mon

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Caryop

h

yllaceae Vaccaria segetalis (P. Mill.) Cow soap

wort X

X

Rauschert

Scrop

h

ulariaceae Verbascum thapsus L. Com

mon

m

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X

X

X

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X

X

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Scrop

h

ulariaceae Veronica arvensis L. Corn speed

well X

X

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Asteraceae Tragopogon dubius Scop. Yellow salsify

X

X

X

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Scrop

h

ulariaceae Veronica chamaedrys L. German

der X

X

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speed

well

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ulariaceae Veronica officinalis L. Com

mon

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X

X

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g

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weed

Scrop

h

ulariaceae Veronica persica Poir. Birdeye speed

well X

X

Fabaceae Vicia cracca L. Bird

vetch

X

X

Fabaceae Vicia sativa L. Garden

vetch

X

X

X

X

X

A

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naceae Vinca minor L.* Com

mon

X

X

X

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periwin

kle

Poaceae Vulpia bromoides (L.) S.F. Gray

Brome fescue X

X

105

Mt. Baker RD

Darrington RD

Skykomish RD

Snoqualmie RD

Whatcom County

Skagit County

Snohomish County

ing County K

Pierce County
General Technical Report GTR-694

106

Table 3—Updated nomenclature of all non-native plants known to occur on the Mt. 

Baker-Snoqualmie National Forest compared with the Pacific Northwest regional flora 

(Hitchcock and Cronquist 1973). All accepted scientific names follow the PLANTS 

database (USDA Natural Resources Conservation Service 2006) 

Accepted Scientific Name In PNW flora? Synonym Used in PNW flora

Aegopodium podagraria

Agrostis capillaris X Agrostis tenuis

Agrostis gigantea X

Ailanthus altissima X

Aira caryophyllea X

Ajuga reptans X

Alopecurus geniculatus X

Arctium lappa X

Arctium minus X

Arrhenatherum elatius X

Avena fatua X

Bellis perennis X

Bromus tectorum X

Buddleja davidii X

Calystegia sepium X Convolvulus sepium

Capsella bursa-pastoris X

Centaurea ×moncktonii X Centaurea pratensis

Centaurea biebersteinii X Centaurea maculosa

Centaurea diffusa X

Cerastium fontanum ssp. vulgare X Cerastium vulgatum

Cerastium semidecandrum X

Cirsium arvense X

Cirsium vulgare X

Conium maculatum X

Convallaria majalis 

Convolvulus arvensis X

Crataegus monogyna X C. monogyna and C. oxyacantha

Crepis capillaris X

Cynosurus cristatus X

Cytisus scoparius X

Dactylis glomerata X

Daucus carota X

Dianthus armeria X

Dianthus deltoides X

Digitalis purpurea X

Draba verna X

Duchesnea indica X

Elymus repens X Agropyron repens

Erodium cicutarium X

Euphrasia stricta X Euphrasia officinalis

Galeopsis tetrahit X

Galium odoratum X Asperula odorata

Geranium dissectum X

Geranium molle X

Geranium robertianum X

Glechoma hederacea X

Gnaphalium uliginosum X
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

107

Table 3—Updated nomenclature of all non-native plants known to occur on the Mt. 

Baker-Snoqualmie National Forest compared with the Pacific Northwest regional flora 

(Hitchcock and Cronquist 1973). All accepted scientific names follow the PLANTS 

database (USDA Natural Resources Conservation Service 2006) (continued)

Accepted Scientific Name In PNW flora? Synonym Used in PNW flora

Hedera helix X

Heracleum mantegazzianum 

Hesperis matronalis X

Hieracium aurantiacum X

Hieracium caespitosum X Hieracium pratense

Hieracium lachenalii X Hieracium vulgatum

Hieracium laevigatum

Holcus lanatus X

Hosta sp.

Hypericum calycinum

Hypericum perforatum X

Hypochaeris glabra X

Hypochaeris radicata X

Ilex aquifolium 

Impatiens glandulifera X

Lactuca serriola X

Lamiastrum galeobdolon 

Lamium amplexicaule X

Lapsana communis X

Lathyrus latifolius X

Lavandula angustifolia

Lepidium perfoliatum X

Leucanthemum vulgare X Chrysanthemum leucanthemum

Linaria dalmatica X

Linaria genistifolia X

Linaria vulgaris X

Lolium perenne X

Lotus corniculatus X

Lychnis coronaria X

Malva moschata X

Malva parviflora X

Matricaria discoidea X Matricaria matricariodes

Medicago lupulina X

Melilotus officinalis X M. officinalis and M. alba

Mycelis muralis X Lactuca muralis

Myosotis arvensis X

Myosotis scorpioides X

Narcissus sp.

Nasturtium officinale X Rorippa nasturtium-aquaticum

Petasites japonicus subsp. giganteus

Phalaris arundinacea X

Phleum pratense X

Plantago lanceolata X

Plantago major X

Poa annua X

Poa bulbosa X

Poa compressa X
General Technical Report GTR-694

108

Table 3—Updated nomenclature of all non-native plants known to occur on the Mt. 

Baker-Snoqualmie National Forest compared with the Pacific Northwest regional flora 

(Hitchcock and Cronquist 1973). All accepted scientific names follow the PLANTS 

database (USDA Natural Resources Conservation Service 2006) (continued)

Accepted Scientific Name In PNW flora? Synonym Used in PNW flora

Poa trivialis X

Polygonum cuspidatum X

Polygonum sachalinense X

Polygonum × bohemicum 

Polypogon monspeliensis X

Potentilla recta X

Prunus laurocerasus X

Ranunculus acris X

Ranunculus repens X

Rheum rhabarbarum 

Rhododendron sp.

Rosmarinus officinalis 

Rubus armeniacus X Rubus discolor

Rubus laciniatus X

Rumex acetosella X

Rumex crispus X

Rumex obtusifolius X

Sagina apetala X

Sagina procumbens X

Schedonorus pratensis X Festuca pratensis

Schedonorus phoenix X Festuca arundinacea

Senecio jacobaea X

Senecio sylvaticus X

Senecio vulgaris X

Silene latifolia ssp. alba X Lychnis alba

Silene noctiflora X

Solanum dulcamara X

Sonchus asper X

Sonchus oleraceus X

Sorbus aucuparia X

Spergula arvensis X

Spergularia rubra X

Stachys byzantina 

Stellaria media X

Symphytum officinale X

Tanacetum vulgare X

Taraxacum laevigatum X

Taraxacum officinale X

Tragopogon dubius X

Trifolium arvense X

Trifolium campestre X Trifolium procumbens

Trifolium dubium X

Trifolium hybridum X

Trifolium pratense X

Trifolium repens X

Vaccaria segetalis X

Verbascum thapsus X
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

Table 3—Updated nomenclature of all non-native plants known to occur on the Mt. 

Baker-Snoqualmie National Forest compared with the Pacific Northwest regional flora 

(Hitchcock and Cronquist 1973). All accepted scientific names follow the PLANTS 

database (USDA Natural Resources Conservation Service 2006) (continued)

Accepted Scientific Name In PNW flora? Synonym Used in PNW flora

Veronica arvensis X

Veronica chamaedrys X

Veronica officinalis X

Veronica persica X

Vicia cracca X

Vicia sativa X

Vinca minor 

Vulpia bromoides X Festuca bromoides

109

Table 4—Comparison of the ecology plot and botany survey detections of noxious weeds table 1) 

and other non-native plants (table 2) known to occur on the Mt. Baker-Snoqualmie National Forest 

Noxious # detections

Family Scientific Name Weed Ecology Botany

Apiaceae Aegopodium podagraria 0 1

Apiaceae Conium maculatum 0 1

Apiaceae Daucus carota 0 3

Apiaceae Heracleum mantegazzianum X00

Apocynaceae Vinca minor 0 3

Aquifoliaceae Ilex aquifolium 5 4

Araliaceae Hedera helix X02

Asteraceae Arctium lappa 0 4

Asteraceae Arctium minus 2 5

Asteraceae Bellis perennis 0 2

Asteraceae Centaurea biebersteinii X 0 7

Asteraceae Centaurea diffusa X 0 2

Asteraceae Centaurea × moncktonii X00

Asteraceae Cirsium arvense X 14 87

Asteraceae Cirsium vulgare X 18 72

Asteraceae Crepis capillaris 0 8

Asteraceae Gnaphalium uliginosum 0 1

Asteraceae Hieracium aurantiacum X 0 21

Asteraceae Hieracium caespitosum X02

Asteraceae Hieracium lachenalii X 0 1

Asteraceae Hypochaeris glabra 0 1

Asteraceae Hypochaeris radicata X 13 144

Asteraceae Lactuca serriola 0 0

Asteraceae Lapsana communis 1 32

Asteraceae Leucanthemum vulgare X 7 230

Asteraceae Matricaria discoidea 0 24

Asteraceae Mycelis muralis 439 422

Asteraceae Petasites japonicus subsp. giganteus 0 1

Asteraceae Senecio jacobaea X 1 22

Asteraceae Senecio sylvaticus 6 5

Asteraceae Senecio vulgaris X05
General Technical Report GTR-694

Table 4—Comparison of the ecology plot and botany survey detections of noxious weeds (table 1) 

and other non-native plants (table 2) known to occur on the Mt. Baker-Snoqualmie National Forest 

(continued) 

Noxious # detections

Family Scientific Name Weed Ecology Botany

Asteraceae Sonchus asper 0 4

Asteraceae Sonchus oleraceus 0 3

Asteraceae Tanacetum vulgare X 1 66

Asteraceae Taraxacum laevigatum 0 1

Asteraceae Taraxacum officinale 3 115

Asteraceae Tragopogon dubius 0 10

Balsaminaceae Impatiens glandulifera X00

Boraginaceae Myosotis arvensis 0 1

Boraginaceae Myosotis scorpioides 0 2

Boraginaceae Symphytum officianale 0 1

Brassicaceae Capsella bursa-pastoris 0 6

Brassicaceae Draba verna 0 1

Brassicaceae Hesperis matronalis 0 1

Brassicaceae Lepidium perfoliatum 0 1

Brassicaceae Nasturtium officinale 0 1

Buddlejaceae Buddleja davidii X 0 1

Caryophyllaceae Cerastium fontanum ssp. vulgare 0 14

Caryophyllaceae Cerastium semidecandrum 0 1

Caryophyllaceae Dianthus armeria 0 2

Caryophyllaceae Dianthus deltoides 0 1

Caryophyllaceae Lychnis coronaria 0 1

Caryophyllaceae Sagina apetala 0 1

Caryophyllaceae Sagina procumbens 0 8

Caryophyllaceae Silene latifolia ssp. alba X15

Caryophyllaceae Silene noctiflora 0 2

Caryophyllaceae Spergula arvensis 0 1

Caryophyllaceae Spergularia rubra 0 7

Caryophyllaceae Stellaria media 1 6

Caryophyllaceae Vaccaria segetalis 0 1

Clusiaceae Hypericum calycinum 0 1

Clusiaceae Hypericum perforatum X 7 176

Convolvulaceae Calystegia sepium 0 1

Convolvulaceae Convolvulus arvensis X01

Ericaceae Rhododendron sp. 0 1

Fabaceae Cytisus scoparius X 0 35

Fabaceae Lathyrus latifolius 0 4

Fabaceae Lotus corniculatus 1 57

Fabaceae Lotus pinnatus 0 1

Fabaceae Medicago lupulina 0 15

Fabaceae Melilotus officinalis 0 10

Fabaceae Trifolium arvense 0 2

Fabaceae Trifolium campestre 0

Fabaceae Trifolium dubium 0 6

Fabaceae Trifolium hybridum 0 9

Fabaceae Trifolium pratense 0 79

Fabaceae Trifolium repens 10 173

Fabaceae Vicia cracca 1 0

110
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

Table 4—Comparison of the ecology plot and botany survey detections of noxious weeds (table 1) 

and other non-native plants (table 2) known to occur on the Mt. Baker-Snoqualmie National Forest 

(continued) 

Noxious # detections

Family Scientific Name Weed Ecology Botany

Fabaceae Vicia sativa 1 3

Geraniaceae Erodium cicutarium 0 1

Geraniaceae Geranium dissectum 0 1

Geraniaceae Geranium molle 0 2

Geraniaceae Geranium robertianum X 1 69

Lamiaceae Ajuga reptans 0 2

Lamiaceae Galeopsis tetrahit 1 7

Lamiaceae Glechoma hederacea 0 3

Lamiaceae Lamiastrum galeobdolon 0 1

Lamiaceae Lamium amplexicaule 1 1

Lamiaceae Lamium maculatum 1 0

Lamiaceae Lavandula angustifolia 0 1

Lamiaceae Rosmarinus officinalis 0 1

Lamiaceae Stachys byzantina 0 0

Liliaceae Convallaria majalis 0 2

Liliaceae Hosta sp. 0 1

Liliaceae Narcissus sp. 0 1

Malvaceae Malva moschata 0 0

Malvaceae Malva parviflora 0 2

Plantaginaceae Plantago lanceolata 1 153

Plantaginaceae Plantago major 0 191

Poaceae Agrostis capillaris 7 9

Poaceae Agrostis gigantea 0 4

Poaceae Aira caryophyllea 2 1

Poaceae Alopecurus geniculatus 0 5

Poaceae Arrhenatherum elatius 0 8

Poaceae Avena fatua 0 1

Poaceae Bromus tectorum 0 0

Poaceae Cynosurus cristatus 0 1

Poaceae Dactylis glomerata 12 195

Poaceae Elymus repens 0 6

Poaceae Holcus lanatus 12 41

Poaceae Lolium perenne 0 9

Poaceae Phalaris arundinacea X 0 107

Poaceae Phleum pratense 13 105

Poaceae Poa annua 0 23

Poaceae Poa bulbosa 0 1

Poaceae Poa compressa 0 2

Poaceae Poa trivialis 1 1

Poaceae Polypogon monspeliensis 0 1

Poaceae Schedonorus pratensis 1 3

Poaceae Schedonorus phoenix 3 12

Poaceae Vulpia bromoides 0 1

Polygonaceae Polygonum cuspidatum X03

Polygonaceae Polygonum sachalinense X02

Polygonaceae Polygonum × bohemicum X00

Polygonaceae Rheum rhabarbarum 0 0

111
General Technical Report GTR-694

Table 4—Comparison of the ecology plot and botany survey detections of noxious weeds (table 1) 

and other non-native plants (table 2) known to occur on the Mt. Baker-Snoqualmie National Forest 

(continued)

Noxious # detections

Family Scientific Name Weed Ecology Botany

Polygonaceae Rumex acetosella 8 106

Polygonaceae Rumex crispus 0 59

Polygonaceae Rumex obtusifolius 2 24

Ranunculaceae Ranunculus acris 0 15

Ranunculaceae Ranunculus repens 11 246

Rosaceae Crataegus monogyna 0 1

Rosaceae Duchesnea indica 0 1

Rosaceae Potentilla recta X01

Rosaceae Prunus laurocerasus 0 1

Rosaceae Rubus armeniacus X 2 35

Rosaceae Rubus laciniatus X 9 65

Rosaceae Sorbus aucuparia 1 8

Rubiaceae Galium odoratum 0 1

Scrophulariaceae Digitalis purpurea 29 160

Scrophulariaceae Euphrasia stricta 0 26

Scrophulariaceae Linaria dalmatica X 0 4

Scrophulariaceae Linaria genistifolia 0 1

Scrophulariaceae Linaria vulgaris 0 1

Scrophulariaceae Verbascum thapsus 2 26

Scrophulariaceae Veronica arvensis 2 4

Scrophulariaceae Veronica chamaedrys 11 11

Scrophulariaceae Veronica officinalis 0 70

Scrophulariaceae Veronica persica 0 2

Simaroubaceae Ailanthus altissima 0 1

Solanaceae Solanum dulcamara 3 4

112

Table 5—Summary of detection rates of non-native vascular plants and comparison of botany 

surveys and ecology plots

Ecology Plots Botany Surveys

# of Detections Noxious Weeds 74 1165

Other Non-native Plants 594 2299

All Non-Native Plants 668 3464

# Species Detected Noxious Weeds 13 25

Other Non-native Plants 30 112

All Non-Native Plants 43 139

Percent of Species Detected All Known Noxious Weeds on Forest 38 74

All Other Known Non-native Plants 26 98

All Known Non-native Plants 29 94
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

reviewed our list of noxious weeds known to occur on the

Forest, wondered at the omission of I. glandulifera, and

then informed us that he had recently collected it on the

Darrington Ranger District, Snohomish County (David

Giblin, University of Washington Herbarium, pers. comm).

Although we knew that noxious weeds and other nonnative plants are present in all four ranger districts and five

counties, we did not expect so many ornamental plants not

listed as noxious weeds to be present on the Forest (table 2).

Until September 2005, none of us had ever seen

Lamiastrum galeobdolon (L.) Ehrend. & Polatschek, a 

shade tolerant ground cover on the Forest, until one of us

(Fuentes) discovered it near the Denny Creek trailhead on

the Snoqualmie Ranger District (King County). We later 

discovered that it was rapidly spreading in Seattle’s city

parks (Wendy Descamp, University of Washington Botanical

Gardens, pers. comm) and that the King County Noxious

Weed Board was proposing that it be listed as a state noxious weed (Steven Burke, King County Noxious Weed

Board, pers. comm). We have since documented it on three

more sites on the Forest.

Limiting disturbance and maintaining canopy cover to

provide shade are part of the Forest’s prevention strategy

(USDA Forest Service 2005). However, this assumes that

invasive plants cannot invade intact plant communities and

are shade intolerant. Because sites of L. galeobdolon and G.

robertianum are in deep to dappled shade, it would appear

that much more of the Forest is vulnerable to invasion by

such shade tolerant species.

Sources of establishment and spread

Because noxious weeds on the Forest tend to be associated 

with highways and roads, trails, railroad tracks, and powerline corridors, we infer that: 1) noxious weeds and other

non-native plants spread themselves along these constantly

disturbed areas, and that 2) vehicles of visitors to the Forest

may contain seeds or other propagules. 

In addition to inadvertently spreading non-natives,

Forest visitors are also directly spreading non-native species

on the Forest by planting ornamentals or dumping yard

waste. Botanists have noted deliberate plantings of Hypericum calcyinum L., Prunus laurocerasus L., Narcissus sp.,

113

Figure 2—Plant families with the greatest number of noxious weeds and other non-native plants on the Mt. BakerSnoqualmie National Forest.
General Technical Report GTR-694

Hosta sp., ornamental Rhododendron spp., Rosmarinus

officinalis L.; they have also observed L. galeobdolon,

Hedera helix L., Symphytum officinale L., Rheum rhabarbarum L., Vinca minor L., and G. robertianum growing in or

adjacent to debris piles along Forest System roads. 

The use of rock, gravel, and fill that contain seeds and

other plant parts has also contributed to the spread and

establishment of noxious weeds on the Forest. For example,

P. × bohemicum occurs in and around riprap by the Johnson

Creek bridge in Snohomish County on the Skykomish

Ranger District. Because no other infestations of this species

are known from the reach above the bridge or along the road

to or from it, it is likely that the riprap contained pieces of 

P. × bohemicum. Forest engineering records state that the

riprap was used as part of the Johnson Creek bridge replacement in 1998.

Major floods may also contribute to the spread of invasive Polygonum spp. on the Forest. Results from repeat surveys of the Sauk River suggest that new infestations may

begin when entire plants, rhizomes, or other parts break off

and are carried downstream. Five of the nine infestations

known from the upper Sauk River were washed away by

floods in 2003 (Melisa Holman, The Nature Conservancy,

pers. comm). About 200 new patches were identified in previously uninfested areas downstream from the swept away

infestations.

Floodwaters are not the only way that invasive

Polygonum spp. are suspected to move within the floodplain. A contractor working on weed control on the

Darrington Ranger District reported that mountain beaver

(Aplodontia rufa Rafinesque) have cut down stems of P. ×

bohemicum and dragged them back to their burrows.

Within the past 10 years, the Forest has acquired several

new parcels of land. Unfortunately, many of these are infested with invasive plants. The SkiYou parcel, on the Mt.

Baker Ranger District, is in Skagit County along the Skagit

River. This parcel contains Rubus armeniacus Focke, P. ×

bohemicum, Tanacetum vulgare L., B. davidii, C. arvense,

and C. vulgare. The Snoqualmie Point parcel, on the

Snoqualmie Ranger District, is in King County, along the I90 corridor. Not only does this parcel contain all of the same

species as the SkiYou parcel, it also contains Rubus laciniatus Willd., S. jacobaea, H. helix, G. robertianum, and Ilex

aquifolium L.

Differences in detection

Because botany surveys often occur in areas associated 

with roads and trails and that survey areas are typically

larger than ecology plots (table 4), it is not surprising that

botany surveys detected a higher proportion of non-native

plants, including noxious weeds. Botany surveys target specific areas, rare species, and non-native plants, while ecology plot surveys represent a sample of the Forest vegetation. 

Given that controlling non-native plants is more likely

to be feasible and economical when infestations are small

(Rejmánek and Pitcairn 2002, Simberloff 2003), detecting

new infestations soon after they occur is crucial. Sampling

schemes for detecting rare species and events are applicable

to this effort. 

Cutler et al. (2001) reviewed different sampling

schemes for detecting rare events, grouped them into five

categories, and commented on their relevance for detecting

rare or uncommon Survey and Manage Species in Pacific

Northwest forests. We apply his same groupings and comment on their relevance for detecting new infestations of

non-native plants on the Forest. 

Collecting a very large sample. When little is known about 

the species of interest, then this approach can be a starting

point. However, it can be extremely expensive. The Forest

already has two large, high-quality, long-term data sets

regarding the distribution of vascular plants on the Forest:

the botany survey and ecology plot databases. Developing

new schemes to detect non-natives would be redundant and

expensive.

Stratified random sampling with disproportionate allocation.

If the Forest could predict where non-native plants are more

likely to be found, then botanists and ecologists could focus

their efforts in these areas. Because we know that most noxious weed sites on the Forest occur along roads, trails, and

other disturbed, accessible areas, and because we understand

many of the mechanisms that contribute to the spread and

establishment of noxious weeds, focusing our efforts in

these areas would likely result in higher detection rates.

114
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

However, the reproductive biology and habitat tolerances 

of all of the possible non-native species are not well understood, and it is possible that we will miss certain species if

we have too narrow a focus. If we only examined roads and

trails, for example, we would miss many of the riparian

weed infestations.

Snowball or network sampling. This method assumes that 

members of the population are linked. Finding one member

of the population increases the likelihood of finding another.

Indeed, many sites of non-native plants on the Forest contain two or more non-natives or may result from the same

source (infested materials used all along a road, for example). Expanding our search radius around known sites of

invasive plants of concern or following potential travel

routes from one infestation would likely result in more

detections. We may still, however, miss non-native plant

sites that are not “linked” in some way to the existing 

populations.

Sequential methods. These methods involve developing an 

initial sampling scheme that is then used to determine an

appropriate sampling scheme for the species of interest. This

approach would involve taking our existing data sets, focusing on the species of interest, and selecting new areas to 

survey based on surrounding detection rates. 

Model-based sampling. This scheme uses specific information about the species of interest to develop sample units

with different probabilities of presence or absence. We (ecologists) have used this method with considerable success to

detect rare and uncommon lichens (Lesher 2005),

bryophytes, and vascular plants on the Forest (unpublished

data). Developing these models has been time consuming

and expensive.

However, determining areas that are vulnerable to a

newly invading species is a high priority. Using information

from the few known sites to predict where else these new

invaders might occur would help focus our survey efforts for

these species.

CONCLUSIONS

Non-native plants are widely distributed on the Forest, and

complete eradication will never be feasible. The most effective ways to limit the harmful effects of invasive plants on

the Forest are preventing their spread and establishment and

identifying and controlling high priority infestations. 

Our work shows that multiple approaches to identifying

invasive plant infestations are necessary. Botany surveys

tend to be tied to specific project areas; survey results can be

used to help project managers prevent the spread and establishment of invasive plants. Ecology plots capture a representative sample of Forest vegetation changes, away from

the influence of roads and trails. Prompt reporting of infestations by knowledgeable persons increases the likelihood of

limiting invasive plants. 

One of the Forest’s highest priorities is controlling

species not widely distributed on the Forest. Managers need

to know which invasive plants are possible threats to Forest

resources, as well as how and where these species may

establish. Developing specific sampling schemes for specific, high priority invaders may be necessary. Network 

sampling or model-based sampling could be appropriate

approaches.

However, recognizing new invaders and high priority

infestations will be a continuing challenge. Professional

knowledge of the flora of the Forest must be combined with

a healthy suspicion of unfamiliar species. Skepticism should

increase if the species is located near roads or trails, is

unusually robust, or has ornamental plant characteristics,

such as unusual textures, showy flowers, or variegated

leaves. 

ACKNOWLEDGMENTS

Responding to the comments of Terry Lillybridge and Ellen 

Kuhlmann greatly improved the manuscript. Paula James

provided the Forest map.

LITERATURE CITED

Cutler, R.; Edwards Jr., T.C.; Alegria, J.; McKenzie, D. 

2001. A sample design framework for survey and 

manage species under the Northwest Forest Plan.

Proceedings of the Section on Statistics and

Environment, 2001 Joint Statistical Meeting, American

Statistical Association. Alexandria, Virginia.

115
General Technical Report GTR-694

Henderson, J.A.; Lesher, R.D.; Peter, D.H.; Shaw, D.C. 

1992. Field guide to the forested plant associations of

the Mt. Baker-Snoqualmie National Forest. R6-ECOLTP-028-91. USDA Forest Service, Pacific Northwest

Region.

Hitchcock, C.L.; Cronquist, A. 1973. Flora of the Pacific 

Northwest. Seattle, WA: University of Washington

Press.

Hitchcock, C.L.; Cronquist, A.; Ownbey, M.; 

Thompson, J.W. 1955–1969. Vascular plants of 

the Pacific Northwest. Seattle, WA: University of

Washington Press.

King County Noxious Weed Control Program. 2005.

King County Noxious Weed Control Program Weed

alert. Policeman’s helmet, Impatiens glandulifera.

[online] URL: http://www.nwcb.wa.gov/weed_info/

Impatiens_glandulifera.html.

Lesher, R.D. 2005. An environmental gradient model 

predicts the spatial distribution of potential habitat for

Hypogymnia duplicata in the Cascade Mountains of

northwestern Washington. PhD dissertation. Seattle,

WA: University of Washington

Nelson, J.R. 1985. Rare plant surveys: techniques for 

impact assessment. Natural Areas Journal. 5: 18–30.

Rejmánek, M.; Pitcairn, M.J. 2002. When is eradication 

of exotic plant pests a realistic goal? In: Vietch, C.R.;

Clout, M.N. eds. Turning the tide: the eradication of

invasive species. IUCN SSC Invasive Species

Specialist Group. IUCN, Gland, Switzerland, and

Cambridge, UK: 249–253.

Simberloff, D. 2003. How Much Information on 

Population Biology Is Needed to Manage Introduced

Species? Conservation Biology. 17: 83–92.

Statistics Canada. 2002. The 2001 census of Canada: 

population and housing estimates for British Columbia.

[online] URL: http://www.bcstats.gov.bc.ca/

census.asp#2001.

USDA APHIS. 2006. Federal noxious weed list, as of June 

30, 2006. [online] URL: http://www.aphis.usda.gov/

plant_health/plant_pest_info/weeds/downloads/

7cfr360-06.pdf.

U.S. Census Bureau. 2006. Washington State and County 

QuickFacts. Data derived from Population Estimates,

2000 Census of Population and Housing. [online]

URL: http://quickfacts.census.gov/qfd/states/

53000.html.

USDA Forest Service. 2005. Proposed treatment of 

invasive plants and new invaders strategy (Forest 

Plan Amendment #26) environmental assessment.

Mountlake Terrace, WA: Mt. Baker-Snoqualmie

National Forest, Pacific Northwest Region.

USDA Natural Resources Conservation Service. 2006.

The PLANTS Database. [online] URL: http://plants.

usda.gov. National Plant Data Center, Baton Rouge,

LA.

Washington State Noxious Weed Control Board. 2006.

2006 Noxious weed list. Olympia, WA. 

Wilson, L.M. 2006. Key to identification of invasive and 

native hawkweeds (Hieracium spp.) in the Pacific

Northwest. Kamploops, B.C.: British Columbia

Ministry of Forests and Range. Forest Practices

Branch.

116
ABSTRACT

Alaska has remained relatively unaffected by non-native plants; however, recently the state has started to experience an 

influx of invasive non-native plants that the rest of the U.S. underwent 60–100 years ago. With the increase in population,

gardening, development, and commerce there have been more frequent introductions to Alaska. Many of these species, 

such as meadow hawkweed (Hieracium caespitosum), Canada thistle (Cirsium arvense), and spotted knapweed (Centaurea

biebersteinii), have only localized populations in Alaska. Other species such as reed canary grass (Phalaris arundinacea)

and white sweetclover (Melilotus officinalis), both formerly used in roadside seed mixes, are now very widespread and are

moving into riparian areas and wetlands. We review the available literature and Alaska’s statewide invasive plant database

(AKEPIC, Alaska Exotic Plant Clearinghouse) to summarize changes in Alaska’s non-native flora over the last 65 years. We

suggest that Alaska is not immune to invasion, but rather that the exponential increase in non-native plants experienced elsewhere is delayed by a half century. This review highlights the need for more intensive detection and rapid response work if

Alaska is going to remain free of many of the invasive species problems that plague the contiguous U.S.

KEYWORDS: Alaska, invasion patterns, invasive plants, non-native plants, plant databases.

1 Matthew L. Carlson is an Assistant Professor and Program Botanist, Alaska Natural Heritage Program, University of Alaska Anchorage, 

707 A Street, Anchorage, AK 99501.

2 Michael Shephard is a Vegetation Ecologist, State and Private Forestry, USDA Forest Service, 3301 C Street, Suite 202, Anchorage, AK 99503.

117

INTRODUCTION

Most botanists and ecologists thought Alaska was immune 

to the invasion of non-native plants the rest of the United

States had experienced, and continue to experience, given

the great distances from source populations, relative lack of

agriculture, low levels of human disturbance, and cold climates. Non-native plants are well known to compose significant components of all the other states’ floras and biomass.

Their presence as naturalized members of most communities is generally accepted; however, numerous non-native

species are recognized for serious ecological and economic

damage and targeted for control. Habitats in Alaska are

extremely unique in this regard, being nearly free from the

presence of non-native plants. Recently, however, populations of many non-native species appear to be expanding

and most troubling, a number of species are spreading into

natural habitats. The same fate of degraded ecological communities, damaged ecosystem function, endangerment of

rare species, and lost economic revenue may be at the

doorstep of the 49th state.

The process of species introductions and establishment are quite varied and complex (Pimm 1991, D’Antonio

1993, Williamson 1996) and, despite our fragmented understanding, it informs our comprehension of patterns in

Alaska. In general, only a small proportion of total introductions results in the establishment of self-sustaining populations, a smaller proportion expands into natural areas,

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

IS THE SPREAD OF NON-NATIVE PLANTS 

IN ALASKA ACCELERATING?

Matthew L. Carlson1 and Michael Shephard2
General Technical Report GTR-694

and an even smaller proportion causes significant ecological damage (the familiar Ten’s Rule, Williamson 1996). The

reasons why an introduction fails may be due to the plant’s

inability to tolerate the new physical and ecological conditions or because of stochastic events. Short summers, cold

winters, and permafrost dominated habitats, among other

things, clearly preclude many temperate species from establishing in Alaska. However, successful introductions are

known to occur beyond species’ expected climatic zones.

Plants from temperate Europe have established as far north

as 78o N in Svalbard, for example (Elven and Elvebakk

1996). The frequency and size of introductions (i.e.,

propagule pressure) is well-accepted to be a primary determinant of the success of introductions (Colautti et al. 2006).

Low propagule pressure is likely one of the major reasons

why Alaska has remained relatively free from non-native

plants. 

Once populations have been established they generally

persist for some time without dramatic growth (i.e., lag

phase) and are very susceptible to local extirpation. As 

population sizes begin to grow they may enter a more dramatic phase of increase (i.e., exponential growth phase; cf. 

Kinlan and Hastings 2005). Numerous cases, such as purple

loosestrife (Lythrum salicaria L.) in the Midwest and starlings (Sturnus vulgaris L.) in New York, illustrate that a

non-native species may appear to be relatively benign and

restricted to a few small populations for many decades

before expanding dramatically.

Here we review changes in the non-native flora of

Alaska since the publication of the state’s first flora in 1941

(Hultén 1941). Specifically, we explore whether species 

of non-natives are entering exponential growth phases, if

species considered invasive differ in population expansion

from non-natives as a whole, and what the geographic 

patterns of non-native establishment have been.

Historic background

Between 1941 and 1950 Eric Hultén published the first 

comprehensive flora of Alaska, which in many cases

included non-native species and discussions of their 

origins. In 1968 the single volume, Flora of Alaska and

Neighboring Territories, was published (Hultén 1968), and

in it Hultén reported he used over three times the source

material than in the earlier volumes. We use these pioneering and comprehensive works as a baseline to gauge

changes in the region’s non-native flora.

More recently federal and state agencies in Alaska have

initiated non-native plant surveys to develop an understanding of the scope of the problem. In 1997 the USDA Forest

Service began conducting surveys (Duffy 2003) and in

2000 the National Park Service launched basic inventory

work (Densmore et al. 2001). Likewise the Alaska

Department of Transportation was concerned about the

spread of several species such as bird vetch (Vicia cracca

L.) along highways (Nolen 2002). The USDI Bureau of

Land Management and the USDI Fish and Wildlife Service

are now active in collecting information about non-native

plants on lands they manage (Cortés-Burns and Carlson

2006). The state’s public is also contributing to our understanding of non-native species occurrences as they are

becoming increasingly involved in invasive species issues

in general, such as impacts of rats on seabird colonies and

competition between Pacific salmon species and Atlantic

salmon. Non-native plant impacts have even been addressed

in recent legislation, whereby the selling of purple loosestrife (Lythrum salicaria L.) and orange hawkweed

(Hieracium aurantiacum L.) would be prohibited (Alaska

State House Bill 324). 

After reviewing inventory data across the country 

it became apparent that data management and sharing of

information lagged well behind actual infestations. In 2002,

an Alaskan statewide invasive plant database, AKEPIC, was

developed after many different land management agencies

came together in 2002 (see http://akweeds/uaa.alaska.edu).

We hoped that this statewide database, modeled after the

Southwest exotic plant clearinghouse, would further

encourage information exchange concerning invasive plant

species. Indeed, the presence of a current database that

incorporates information from across the state offers an

opportunity to explore the patterns of non-native plant

establishment and contrast it with the baseline conditions 

of 1941.

Based on data present in floras and the statewide database, we show that Alaska is entering a phase of both

118
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

increased introductions and establishment of non-native

species. While only a handful of non-native species were

distributed widely in 1941 and 1968, many more have

become naturalized and are spreading rapidly across the

state, posing a serious threat to community integrity and

ecosystem function.

METHODS

We surveyed the literature, building on a list initiated by 

M. Duffy and A. Batten (unpublished), to compare the

number of non-native species known today to what was

reported by Hultén (1941, 1968), and assigned each taxon

as naturalized (or not) based on whether self-perpetuating

populations were known by the authors or other experts. We

also used a single taxonomic system (Integrated Taxonomic

Information System, ITIS) to resolve synonymy (table 1).

Taxa known from neighboring territories that have not been

recorded for Alaska were removed from the list.

To estimate how changes in the number of populations

have occurred over time, we compared the collection 

119

Table 1—Non-native plant species found in Alaska 

ITIS scientific name with 

authors

Achillea ptarmica L. - Yes Yes - 1, 2,

3

Achnatherum hymenoides -- - -3

(Roemer & J.A. Schultes) 

Barkworth 

Agropyron cristatum -- - -4

(Linnaeus) Gaertn. 

Agropyron desertorum -- - -5

(Fisch. ex Link) J.A. 

Schultes 

Agropyron fragile (Roth) Nat - - - 4

P. Candargy

Agrostemma githago L. - Yes Yes Ext 1, 2, 

3

Agrostis capillaris L. Nat Yes Yes - 1, 2,

3

Agrostis gigantea Roth Nat - Yes - 2, 4, 

6

Agrostis stolonifera L. - Yes Yes - 1, 2, 

4

Alchemilla monticola Opiz - - - - 3

Alliaria petiolata (Bieb.) Nat - - -4

Cavara & Grande 

Alopecurus geniculatus L. Nat Yes Yes - 1, 2, 

3, 6

Alopecurus pratensis L. Nat Yes Yes - 1, 2, 

3, 4

Alyssum alyssoides (L.) L. -- - -7

Amaranthus albus L. - Yes Yes Ext 1, 2, 

3

Amaranthus retroflexus L. - Yes Yes - 1, 2, 

3, 4

ITIS scientific name with 

authors

Amsinckia lycopsoides Nat Yes Yes - 1, 2, 

Lehm. 3

Amsinckia menziesii Nat Yes Yes - 1, 2, 

(Lehm.) A. Nels. & J.F. 6

Macbr. 

Anthemis cotula L. Nat Yes Yes - 1, 2, 

4

Anthemis tinctoria L. Nat Yes Yes - 1, 2

Anthoxanthum odoratum L. Nat Yes Yes - 1, 2

Anthriscus sylvestris Nat - - - 3

(L.) Hoffmann 

Arabis glabra (L.) Bernh. Nat Yes Yes - 1, 2, 

6

Arctium lappa L. - - - - 12

Arctium minus Bernh. - - - - 13

Arrhenatherum elatius (L.) Nat Yes Yes - 1, 2

Beauv. ex J.& K. Presl

Artemisia biennis Willd. Nat - Yes - 2, 10

Artemisia vulgaris L. Nat - - -8

Asparagus officinalis L. Nat - - - 7

Asperugo procumbens L. Nat - - - 3,10

Astragalus cicer L. Nat - - - 3, 10

Atriplex hortensis L. - Yes Yes Ext 1, 2

Atriplex patula L. Nat Yes Yes - 1, 2, 

6

Avena fatua L. - Yes Yes - 1, 2, 

4

Avena sativa L. - Yes Yes - 1, 2

Bellis perennis L. Nat Yes Yes - 1, 2

Berteroa incana (L.) DC. Nat - - - 3, 4, 

7

Bidens cernua L. Nat Yes Yes - 1, 2

Source

E

xtirpated

1968 H

ulten

1941 H

ulten

Naturalized

Source

E

xtirpated

1968 H

ulten

1941 H

ulten

Naturalized

Continued on page 125
General Technical Report GTR-694

ITIS scientific name with 

authors

Bidens frondosa L. Nat - Yes - 2, 6

Borago officinalis L. Nat - - - 3

Brassica juncea (L.) Czern. - Yes Yes Ext 1, 2

Brassica napus L. - Yes Yes - 1, 2, 

3, 4

Brassica rapa L. - Yes Yes - 4

Bromus hordeaceus L. Nat Yes Yes - 1, 2, 

4, 6

Bromus inermis ssp. Nat Yes Yes - 1, 2, 

inermis Leyss. 4, 6

Bromus secalinus L. Nat Yes Yes - 1, 2, 

Bromus tectorum L. Nat Yes Yes - 1, 2, 

4

Calystegia sepium (L.) Nat - - -4

R. Br.

Camelina sativa (L.) - Yes Yes Ext 1, 2

Crantz

Campanula glomerata L. - - - - 3

Campanula rapunculoides Nat - - - 3

L.

Capsella bursa-pastoris Nat Yes Yes - 1, 2, 

(L.) Medik. 3, 4

Caragana arborescens Lam. Nat - - - 3, 4

Cardamine oligosperma Nat - - - 6

Nutt. 

Carthamus tinctorius L. - - - - 12

Castilleja tenuis (Heller) - Yes Yes Ext 1, 2

Chuang & Heckard 

Centaurea biebersteinii DC. Nat - - - 4

Centaurea montana L. - - - - 3

Cerastium fontanum ssp. Nat Yes Yes - 1, 2,

vulgare (Hartman) 3, 4

Greuter & Burdet

Cerastium glomeratum Nat Yes Yes - 1, 2, 

Thuill. 3, 4

Cerastium tomentosum L. - - - - 6

Chenopodium album L. Nat Yes Yes - 1, 2, 

3, 4

Chenopodium berlandieri Nat Yes Yes - 1, 2, 

var. berlandieri Moq. 3, 4

Chenopodium rubrum L. Nat - Yes - 3, 7

Chrysanthemum segetum - - Yes Ext 2, 3

L. 

ITIS scientific name with 

authors

Cichorium intybus L. - - - - 4, 8

Cirsium arvense (L.) Scop. Nat Yes Yes - 1, 2, 

3, 4

Cirsium vulgare (Savi) Ten. Nat Yes Yes - 1, 2, 

3, 4

Clinopodium douglasii - - Yes Ext 2, 3

(Benth.) Kuntze 

Collomia linearis Nutt. Nat Yes Yes - 1, 2, 

3, 4

Convallaria majalis L. - - - - 6

Conyza canadensis (L.) Nat - - - 3, 4

Cronq. 

Coronilla varia L. Nat - - - 14

Cotula coronopifolia L. Nat Yes Yes - 1, 2, 

3

Crepis capillaris (L.) Wallr. - - Yes - 2, 3, 

7

Crepis tectorum L. Nat - Yes - 2, 3, 

4

Cryptantha torreyana - Yes Yes Ext 1, 2, 

(Gray) Greene 6

Cytisus scoparius (L.) Link Nat - - - 3, 4

Dactylis glomerata L. Nat Yes Yes - 1, 2, 

3, 4

Deschampsia danthonioides - Yes Yes Ext 6

(Trin.) Munro 

Deschampsia elongata Nat Yes Yes - 1, 2,

(Hook.) Munro 3, 4

Descurainia sophia (L.) Nat - Yes - 4

Webb ex Prantl 

Dianthus barbatus L. - - - - 3

Dianthus plumarius L. - - - - 4

Digitalis purpurea L. Nat Yes Yes - 1, 2, 

4

Digitaria ischaemum - - - -3

(Schreb.) Schreb. ex Muhl. 

Digitaria sanguinalis - - - -5

(L.) Scop. 

Echium vulgare L. - - - -3

Elodea canadensis Michx. - - - - 3

Elymus canadensis L. - - - - 5

Elymus repens (L.) Gould Nat Yes Yes - 1, 2, 

3, 4

Elymus sibiricus L. Nat Yes Yes - 1, 2, 

3, 4

120

Table 1—Non-native plant species found in Alaska (continued)

Source

E

xtirpated

1968 H

ulten

1941 H

ulten

Naturalized

Source

E

xtirpated

1968 H

ulten

1941 H

ulten

Naturalized
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

ITIS scientific name with 

authors

Eragrostis intermedia - - - -5

A.S. Hitchc. 

Erodium cicutarium - Yes Yes - 1, 2, 

(L.) L'Hér. ex Ait. 3, 4

Erucastrum gallicum - - - -4

(Willd.) O.E. Schulz 

Eschscholzia californica - - - -3

Cham. 

Euphorbia peplus L. - - - - 7

Euphrasia nemorosa Nat - - - 8

(Pers.) Wallr. 

Fagopyrum esculentum - - - -7

Moench 

Festuca trachyphylla - - - -7

(Hack.) Krajina 

Gaillardia pulchella Foug. - - - - 3

Galeopsis bifida Boenn. Nat Yes Yes - 1, 2, 

3, 4

Galeopsis tetrahit L. Nat Yes Yes - 1, 2, 

3, 4

Geranium bicknellii Britt. - - - - 6

Geranium carolinianum L. - - - -7

Geranium robertianum L. Nat Yes Yes - 1, 2, 

3, 7

Geranium sanguineum L. - - - -7

Gilia achilleifolia Benth. - Yes Yes Ext 1, 2, 

3

Gilia capitata Sims - Yes Yes Ext 1, 2, 

3

Glechoma hederacea L. - Yes Yes Ext 1, 2, 

3

Gnaphalium uliginosum L. Nat Yes Yes - 1, 2, 

3, 4

Gypsophila elegans Bieb. Nat - - - 7

Gypsophila paniculata L. Nat - - - 12

Hackelia micrantha - - Yes Ext 6

(Eastw.) J.L. Gentry

Helianthus annuus L. - Yes Yes - 1, 2, 

3, 4

Hesperis matronalis L. Nat Yes Yes - 1, 2, 

3

Hieracium aurantiacum L. Nat - Yes - 1, 2, 

3, 4

Hieracium caespitosum Nat - - -4

Dumort. 

ITIS scientific name with 

authors

Hieracium lachenalii Nat - - - 14

K.C. Gmel. 

Hieracium pilosella L. Nat - - - 4

Hieracium umbellatum L. Nat Yes Yes - 1, 2, 

6

Holcus lanatus L. Nat Yes Yes - 1, 2, 

3, 4

Hordeum comosum J. Presl - - - - 5

Hordeum jubatum L. Nat Yes Yes - 1, 2, 

3, 4 

Hordeum murinum L. ssp. - - - - 4, 12

leporinum (Link) Arcang.

Hordeum vulgare L. - Yes Yes - 1, 2, 

4, 8

Hypericum perforatum L. Nat - - - 4, 6

Hypochaeris radicata L. Nat Yes Yes - 1, 2, 

3, 4

Iberis amara L. - - - - 3

Impatiens glandulifera Nat - - - 4

Royle 

Lactuca serriola L. Nat - - - 4

Lactuca tatarica (L.) Nat Yes Yes - 8

C.A. Mey. 

Lamium album L. Nat Yes Yes - 3

Lamium maculatum L. - - - - 7

Lappula squarrosa (Retz.) Nat Yes Yes - 1, 2, 

Dumort. 3, 4

Lapsana communis L. Nat Yes Yes - 1, 2, 

3, 4

Lathyrus pratensis L. - - - -3

Leontodon autumnalis L. Nat - Yes - 2, 3, 

4

Lepidium densiflorum Nat Yes Yes - 1, 2,

Schrad. 6

Lepidium ramosissimum Nat - - - 4, 12

A. Nels. 

Lepidium virginicum L. - Yes Yes Ext 1, 2

Leucanthemum vulgare Nat Yes Yes - 1, 2, 

Lam. 3, 4

Levisticum officinale - - - - 12

W.D.J. Koch 

Linaria dalmatica (L.) - - - - 11 

P. Mill. 

Linaria pinifolia (Poir.) - - - - 4, 12

Thellung 

121

Table 1—Non-native plant species found in Alaska (continued)

Source

E

xtirpated

1968 H

ulten

1941 H

ulten

Naturalized

Source

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xtirpated

1968 H

ulten

1941 H

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Naturalized
General Technical Report GTR-694

ITIS scientific name with 

authors

Linaria vulgaris P. Mill. Nat Yes Yes - 1, 2, 

3, 4

Lolium arundinaceum Nat Yes Yes - 1, 2,

(Schreb.) S.J. 3, 4

Darbyshire 

Lolium perenne ssp. Nat Yes Yes - 1, 2,

multiflorum (Lam.) 3, 4

Husnot 

Lolium perenne ssp. Nat Yes Yes - 1, 2, 

perenne L. 3, 4

Lolium pratense (Huds.) - - - - 6

S.J. Darbyshire

Lonicera tatarica L. - - - - 8

Lotus corniculatus L. Nat - - - 4, 14 

Lupinus polyphyllus Lindl. Nat Yes Yes - 1, 2, 

4, 6

Lychnis chalcedonica L. Nat - - - 4

Lychnis coronaria (L.) - - - - 8

Desr.

Lythrum hyssopifolium L. - - - -7

Lythrum salicaria L. Nat - - - 4

Malva neglecta Wallr. - - - -4

Marrubium vulgare L. - Yes Yes Ext 1, 2, 

3

Matricaria discoidea DC. Nat Yes Yes - 1, 2, 

3, 4

Medicago lupulina L. Nat Yes Yes - 1, 2, 

3, 4

Medicago minima (L.) L. - - - - 4

Medicago polymorpha L. - Yes Yes Ext 1, 2, 

3

Medicago sativa L. ssp. Nat Yes Yes - 1, 2,

falcata (L.) Arcang. 4, 12

Medicago sativa L. Nat Yes Yes - 1, 2,

ssp. sativa 3, 4

Melilotus alba [officinalis Nat Yes Yes - 1, 2,

(L.) Lam.] 3, 4

Melilotus officinalis Nat Yes Yes - 1, 2,

(L.) Lam. 3, 4

Mentha × piperita L. Nat Yes Yes - 1, 2 

(pro sp.) [aquatica

× spicata]

Mentha spicata L. Nat Yes Yes - 1, 2, 

3, 4

Microsteris gracilis - Yes Yes Ext 1, 2,

(Hook.) Greene 3

ITIS scientific name with 

authors

Misopates orontium Nat - Yes - 1, 2,

(L.) Raf. 3

Mycelis muralis (L.) Nat - - - 4

Dumort. 

Myosotis scorpioides L. Nat Yes Yes - 4, 6

Nemophila menziesii - Yes Yes - 1, 2, 

Hook. & Arn. 3

Nepeta cataria L. - Yes Yes Ext 1, 2, 

3

Neslia paniculata (L.) - Yes Yes - 1, 2,

Desv. 3, 4

Nymphaea odorata - - - -6

Ait. ssp. odorata

Onobrychis viciifolia - - - - 3

Scop. 

Panicum miliaceum L. - - - - 5

Papaver nudicaule L. Nat Yes Yes - 1, 2, 

3, 4

Papaver rhoeas L. - Yes Yes Ext 1, 2, 

3

Pascopyrum smithii Nat Yes Yes - 1, 2,

(Rydb.) A. Löve 6

Pastinaca sativa L. - Yes Yes Ext 1, 2, 

3

Phalaris arundinacea L. Nat Yes Yes - 1, 2, 

3, 4

Phalaris canariensis L. Nat Yes Yes - 1, 2, 

3, 4

Phalaris minor Retz. - - Yes Ext 2, 3

Phleum pratense L. Nat Yes Yes - 1, 2, 

3, 4

Plagiobothrys figuratus - Yes Yes Ext 1, 2,

(Piper) I.M. Johnston ex 3

M.E. Peck ssp. figuratus

Plantago lanceolata L. - Yes Yes - 1, 2, 

3, 4

Plantago major L. Nat Yes Yes - 1, 2, 

3, 4

Poa annua L. Nat Yes Yes - 1, 2, 

3, 4

Poa compressa L. Nat Yes Yes - 1, 2, 

3, 4

Poa pratensis L. ssp. Nat Yes Yes - 1, 2, 

irrigata (Lindm.) Lindb. f. 3, 4

122

Table 1—Non-native plant species found in Alaska (continued)

Source

E

xtirpated

1968 H

ulten

1941 H

ulten

Naturalized

Source

E

xtirpated

1968 H

ulten

1941 H

ulten

Naturalized
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

ITIS scientific name with 

authors

Poa trivialis L. Nat Yes Yes - 1, 2, 

3, 4

Polygonum aviculare L. Nat - Yes - 2, 3, 

4

Polygonum convolvulus L. Nat Yes Yes - 1, 2, 

3, 4

Polygonum cuspidatum Nat Yes Yes - 1, 2, 

Sieb. & Zucc. 3, 4

Polygonum hydropiper L. - Yes Yes - 8

Polygonum hydropiperoides - Yes Yes Ext 1, 2, 

Michx. 3

Polygonum lapathifolium L. Nat Yes Yes - 1, 2, 

3, 4

Polygonum persicaria L. Nat Yes Yes - 1, 2, 

3

Polygonum ramosissimum - Yes Yes Ext 8

Michx. var. prolificum

Small 

Polygonum sachalinense F. Nat - - - 14

Schmidt ex Maxim.

Polygonum × bohemicum Nat - - - 14

(J. Chrtek & Chrtková) 

Zika & Jacobson 

[cuspidatum ×

sachalinense]

Polypogon monspeliensis Nat - Yes - 2, 3

(L.) Desf. 

Prunus padus L. Nat - - - 3, 4

Prunus virginiana L. Nat - - - 14

Ranunculus acris L. Nat - Yes - 2, 3, 

4

Ranunculus repens L. Nat Yes Yes - 1, 2, 

4, 6

Raphanus sativus L. - Yes Yes - 1, 2, 

3, 7

Rorippa nasturtium- - Yes Yes - 1, 2,

aquaticum (L.) Hayek 7

Rosa rugosa Thunb. Nat - - - 3

Rubus discolor Weihe & - - - - 4

Nees 

Rubus idaeus ssp. idaeus L. Nat - - - 7

Rudbeckia hirta L. - - - - 3

Rumex acetosa ssp. acetosa - Yes Yes - 1, 2, 

L. 3

ITIS scientific name with 

authors

Rumex acetosella L. Nat Yes Yes - 1, 2, 

3, 4

Rumex crispus L. Nat Yes Yes - 1, 2, 

3, 4

Rumex longifolius DC. Nat Yes Yes - 1, 2, 

3, 4

Rumex maritimus L. Nat Yes Yes - 1, 2, 

3

Rumex obtusifolius L. Nat Yes Yes - 1, 2, 

3, 4

Sagina procumbens L. Nat - - - 6

Salix × pendulina - - - - 3

Wenderoth 

Saponaria officinalis L. - - - - 4

Schedonorus pratensis Nat - - - 3

(Huds.) Beauv.

Secale cereale L. - Yes Yes - 1, 2, 

3

Senecio eremophilus - - Yes Ext 2, 7

Richards. 

Senecio jacobaea L. Nat - - - 4, 6

Senecio sylvaticus L. - - - -4

Senecio viscosus L. - - - - 4

Senecio vulgaris L. Nat Yes Yes - 1, 2, 

3, 4

Setaria viridis (L.) Beauv. - - - - 3, 4

Silene armeria L. - - - - 3

Silene dioica (L.) Clairville Nat - - - 4, 12

Silene latifolia Poir. ssp. - - - - 3, 4

alba (P. Mill.) Greuter & 

Burdet 

Silene noctiflora L. Nat Yes Yes - 1, 2, 

3

Silene vulgaris (Moench) Nat - - - 3, 7

Garcke 

Sinapis arvensis L. - Yes Yes Ext 1, 2, 

3, 4

Sisymbrium altissimum L. Nat Yes Yes - 1, 2, 

3, 4

Sisymbrium officinale (L.) - Yes Yes - 1, 2,

Scop. 3

Solanum nigrum L. - Yes Yes Ext 1, 2, 

3

Solanum physalifolium - - - - 7

Rusby

123

Table 1—Non-native plant species found in Alaska (continued)

Source

E

xtirpated

1968 H

ulten

1941 H

ulten

Naturalized

Source

E

xtirpated

1968 H

ulten

1941 H

ulten

Naturalized
General Technical Report GTR-694

ITIS scientific name with 

authors

Sonchus arvensis L. Nat Yes Yes - 1, 2,

3, 4

Sonchus arvensis ssp. Nat - - - 3, 4

uliginosus (Bieb.) Nyman 

Sonchus asper (L.) Hill Nat Yes Yes - 1, 2, 

3, 4

Sonchus oleraceus L. Nat Yes Yes - 1, 2, 

3, 4

Sorbaria sorbifolia (L.) Nat - - - 3, 4

A. Braun 

Sorbus aucuparia L. Nat Yes Yes - 1, 2, 

3, 4

Spergula arvensis L. Nat Yes Yes - 1, 2, 

3, 4

Spergularia rubra (L.) Nat Yes Yes - 1, 2, 

J.& K. Presl 3, 4

Spinacia oleracea L. - Yes - Ext 1

Stellaria media (L.) Vill. Nat Yes Yes - 1, 2, 

3, 4

Symphytum asperum - - - -3

Lepechin 

Symphytum officinale L. - - - - 4

Tanacetum vulgare L. Nat Yes Yes - 1, 2, 

3, 4

Taraxacum laevigatum Nat Yes Yes - 1, 2,

(Willd.) DC. 3, 7

Taraxacum officinale ssp. Nat Yes Yes - 1, 2,

officinale G.H. Weber ex 3, 4

Wiggers 

Thlaspi arvense L. - Yes Yes - 1, 2, 

3, 4

Tragopogon dubius Scop. Nat - - - 3, 4

Trifolium aureum Pollich Nat - Yes - 2, 3, 

4, 7

Trifolium campestre Schreb. - - Yes Ext 2, 3, 

7

Trifolium dubium Sibthorp - Yes Yes Ext 1, 2, 

3

Trifolium hybridum L. Nat Yes Yes - 1, 2, 

3, 4

Trifolium lupinaster L. - Yes Yes - 1, 2

Trifolium microcephalum - Yes Yes Ext 1, 2, 

Pursh 3, 7

ITIS scientific name with 

authors

Trifolium pratense L. Nat Yes Yes - 1, 2, 

3, 4

Trifolium repens L. Nat Yes Yes - 1, 2, 

3, 4

Trifolium variegatum Nutt. - Yes Yes Ext 1, 2, 

3

Tripleurospermum Nat Yes Yes - 1, 2, 

perforata (Merat) M. 3, 4

Lainz 

Triticum aestivum L. - Yes Yes - 1, 2, 

3, 4

Urtica urens L. - Yes Yes Ext 1, 2, 

3

Vaccaria hispanica - Yes Yes Ext 1, 2,

(P. Mill.) Rauschert 3

Veronica anagallis- - Yes Yes Ext 1, 2, 

aquatica L. 3

Veronica arvensis L. - Yes Yes - 1, 2, 

3

Veronica chamaedrys L. - Yes Yes - 1, 2, 

3

Veronica longifolia L. Nat - - - 3

Veronica persica Poir. - Yes Yes Ext 1, 2, 

3

Veronica serpyllifolia ssp. Nat Yes Yes - 1, 2, 

serpyllifolia L. 3, 4

Viburnum opulus L. - - - -4

Vicia cracca L. Nat Yes Yes - 1, 2, 

3, 4

Vicia sativa ssp. nigra (L.) Nat Yes yes - 1, 2,

Ehrh. 3

Vicia villosa Roth Nat Yes Yes - 1, 2, 

3

Viola tricolor L. - - - - 1, 2, 

3, 4

Zea mays L. - - - - 12

Sources cited are coded as follows: 1- Hultén 1941-50, 2- Hultén 1968, 

3- UAF Herbarium database, 4- AK Exotic Plant Clearinghouse -Sept 

2006, 5- Jeff Conn unpublished- search of Palmer Herbarium, 6- Mary

Stensvold unpublished, 7- Welsh, S. Anderson's Flora of Alaska, 8- Al 

Batten unpublished work, 9- Bruce Bennett unpublished -Yukon weeds 

Oct 2004, 10- Mike Duffy unpublished work, 11- Jeff Heys personal 

communication, 12-Carlson, M., I. Lapina 2004, 13- Jeanne Standley 

personal communication, 14-Michael Shephard personal communication

124

Table 1—Non-native plant species found in Alaska (continued)

Source

E

xtirpated

1968 H

ulten

1941 H

ulten

Naturalized

Source

E

xtirpated

1968 H

ulten

1941 H

ulten

Naturalized
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

history of three groups of Alaskan plants: native, nonnative, and non-native species considered to be invasive.

The collection history of native species serves as a null

expectation of overall collection intensity through time.

Rather than comparing the actual number of records, we

standardized the records to a proportion of total records for

each group for a given year, thus allowing for comparisons

among rare and common species or groups of species. For

example, 36 percent (i.e., 488 collections) of the total 1,344

collections of the native plants were recorded by 1941,

while 30 percent (i.e., 90) out of 305 total collections of 

the invasive plants were recorded by the same year.

From the updated list of non-native species, we selected 15 that are considered to be invasive or very widespread

non-native species in the state, and contrasted their history

of collections with their closest native relatives (phylogenetically and ecogeographically). Additionally, we compared the 15 species considered invasive with 15 randomly

chosen non-native species. The number and location of 

collections from the earliest record to 2006 were examined.

We tallied the number of herbarium samples collected within the following: Hultén (1941), Hultén (1968), and the

University of Alaska, Herbarium (ALA – online database

current up through 2003; see http://arctos.database.museum/SpecimenSearch.cfm). To explore how Alaska might

differ from the Pacific Northwest, we compared the collection history of the chosen 15 invasive species with records

in the Oregon State University Herbarium (see http://ocid.

nacse.org/cgi-bin/qml/herbarium/ plants/vherb.qml). Three

of these species were removed because of too few records.

We conducted a second analysis combining these data with

those of AKEPIC to contrast the relative proportion of all

records for the species at three landmark years: 1941, 1968,

and 1985 (table 2). Differences in proportion of total

records in the three years were tested using a non-parametric test (Kruskal-Wallis) since the data did not meet normality assumptions. An experiment-wide Bonferroni correction

was made to maintain significance at p ≤ 0.05. 

We attempted to reduce potential bias among the

datasets regarding what constitutes a population record by

filtering the AKEPIC inventory points through a 25 x 25

mile grid. We chose 25 mile grid cells because this is

roughly the size of the ‘dots’ used by Hultén (1968), where

a single dot may represent one or more individual collections. Additionally, this makes AKEPIC data comparable to

ALA, which has not entered all collections into their database from a single location (A. Batten, pers. comm.).

The grid cells produce a maximum of a single collection point per grid cell. For example, although there are 209

current points of Cirsium arvense (L.) Scop. in the AKEPIC

database, most occur within Anchorage and Haines, so

when filtered through the grid there are 16 ‘sites,’ which 

are then comparable to ALA.

RESULTS

Reviewing the literature indicates there have been 283 nonnative plant taxa recorded in Alaska to date, relative to a

total flora of approximately 2,100 taxa. In 1941 Alaska had

154 non-native plant taxa, and 174 in 1968 (Hultén 1941,

1968); of the latter, 110 have since naturalized; i.e., formed

self-perpetuating populations. By 2006 an additional 109

new species were added to Alaska’s flora, 47 of which have

naturalized (table 1). Thus, from 1941 to 1968 roughly one

non-native plant taxon was added per year, while from 1968

to 2006 nearly three taxa were annually added to the flora.

We find that 36 species listed in 1941 and 1968 have not

been recorded since and are presumed extirpated (table 1).

The pattern of plant collections in Alaska through time

suggests that the number of herbarium collections has

increased steadily for both native and non-native plants (fig.

1). There is a trend for a greater proportion of collections

known by 1941 in native plants than in non-native or invasive plants. Relative to native plants, the proportion of 

collections for the non-native and invasive plants increases

more quickly from 1985 to the present. In 1985 roughly 50

percent of the total collections had been made for invasives

and non-natives, contrasting with 68 percent for the native

species. Interestingly, the data of the same invasive species

in Oregon show a very different relationship, with a significantly higher proportion of total collections having been

made by 1968 and 1985 (p < 0.001, Kruskal-Wallis). This

suggests that, in Oregon, collection frequency is declining

over time, while in Alaska it is increasing, especially for

invasive and non-native species.

125
General Technical Report GTR-694

Table 2—List of 15 native and non-native Alaskan plant taxa used in the analysis. The number of records is

presented prior to 1941, 1941–1967, 1968–1984, 1985–2006, and the 2002–2006 AKEPIC records filtered

through a 25 x 25 mile grid.

1941– 1968– since AKEPIC 

Native Species <1941 1967 1984 1985 Grid TOTAL

Cerastium arvense L. 0 15 9 19 0 43

Cirsium kamtschaticum Ledeb. ex DC. 0 3 3 6 0 12

Crepis elegans Hook. 4 30 15 10 0 59

Descurainia sophioides (Fisch. ex Hook.) O.E. Schulz 10 33 20 28 0 91

Scutellaria galericulata L. 1 32 6 10 0 49

Hieracium triste Willd. ex Spreng. 1 43 23 59 0 126

Impatiens noli-tangere L. 4 29 7 7 0 47

Penstemon gormanii Greene 1 20 11 12 0 44

Lupinus arcticus S. Wats. 6 95 48 42 0 191

Phleum alpinum L. 0 170 10 45 0 225

Polygonum caurianum B.L. Robins. 2 28 5 5 0 40

Ranunculus occidentalis Nutt. 5 31 32 67 0 135

Tephroseris palustris (L.) Fourr. 7 95 33 31 0 166

Taraxacum phymatocarpum J. Vahl 0 13 10 68 0 91

Lathyrus palustris L. 1 60 26 21 0 108

Invasive or widely distributed species

Cerastium fontanum Baumg. 8 4 7 21 30 70

Cirsium arvense (L.) Scop. 2 2 1 5 16 26

Crepis tectorum L. 0 1 4 14 59 78

Descurainia sophia (L.) Webb ex Prantl 5 0 0 3 8 16

Galeopsis bifida Boenn. (combined with G. tetrahit) 5 0 0 0 31 36

Hieracium aurantiacum L. 0 5 4 1 19 29

Impatiens glandulifera Royle 0 0 0 0 3 3

Linaria vulgaris P. Mill. 1 2 7 10 54 74

Melilotus alba Medik. 2 2 2 4 55 65

Phleum pratense L. 25 1 3 12 65 106

Polygonum aviculare L. 8 17 10 22 55 112

Ranunculus acris L. 4 3 2 10 4 23

Senecio jacobaea L. 0 0 0 0 6 6

Taraxacum officinale ssp. officinale 29 0 4 18 139 190

Vicia cracca L. 1 4 5 5 27 42

Randomly selected non-native species

Agrostis stolonifera L. 12 2 4 1 6 25

Caragana arborescens Lam. 0 0 1 3 3 7

Conyza canadensis (L.) Cronq. 0 0 132 6

Erodium cicutarium (L.) L’Hér.Ait. 1 0 131 6

Geranium robertianum L. 1 0 1 0 0 2

Hesperis matronalis L. 1 0 1 1 0 3

Lamium album L. 1 0 1 01 3

Leontodon autumnalis L. 0 0 0 1 10 11

Lolium perenne L. ssp. multiflorum 1 0 0 4 18 23

Nemophila menziesii Hook. & Arn. 2 0 010 3

Neslia paniculata (L.) Desv. 3 0 0 0 1 4

Polygonum cuspidatum Sieb. & Zucc. 0 0 2 2 16 20

Rumex longifolius DC. 0 0 6 1 5 12

Secale cereale L. 1 1 0 0 0 2

Silene vulgaris (Moench) Garcke 0 0 1 0 1 2

126
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

127

Figure 1—Mean proportion of collections over

time relative to the cumulative total in 2006 

for 15 invasive, non-native, and native plants in

Alaska. Proportion of collections in Oregon is

displayed for 12 of 15 of the invasive species.

The means of the 15 species in each group are

displayed as symbols and standard errors are

also included.

If records from the AKEPIC database are added to the

herbarium records, a recent and exponential growth pattern

is observed for both non-native and invasive species (fig.

2). The two groups began with roughly the same proportion

of records in 1941. While the non-natives increased more

steadily, displaying a significantly greater proportion by

1985 (p = 0.024, Mann-Whitney), the invasive group rose

dramatically after 1985, with 81 percent of all species collections having been made in the last 20 years. 

The overall number of records is on average an order

of magnitude greater for native species than for non-native

and invasive species. When AKEPIC data are included, the

number of records of invasive and non-native species is still

less than half that of native species for any given year (table

2). 

The pattern of records over time differed for individual

species among the three Alaskan species groups (native,

non-native, and non-native and invasive). Individual native

species differed substantially among one another in 1941,

with more than half of the total collections recorded by this

time for four of the 15 species (fig. 3). Four native species,

including Taraxacum alaskum Rydb., the rare Cirsium

kamtschaticum Ledeb. ex DC., and geographically restricted

Penstemon gormani Greene had very few of the total collections known until after 1985. Individual invasive species

all showed a consistent pattern of exponential growth (fig.

4), while the non-native species differed dramatically from

one another in the proportion of records over time (fig. 5).

No more than 35 percent of the total collections had been

made for any of the species in the invasive group by 1941,

and in two cases no records were known until after 1985.

By comparison, five of 15 non-native species had more

than 50 percent of the records made by 1941, while seven

species were not known until after 1968.

DISCUSSION

The flora of Alaska, like all other states, is clearly in flux 

due to introductions of non-native species. A relatively
General Technical Report GTR-694

128

Figure 2—Mean proportion (and standard error)

of all records, including the AKEPIC database,

over time of 15 invasive, non-native, and native

plants in Alaska.

severe climate and low levels of anthropogenic landscape

changes have not offered an effective barrier to non-native

plants, and a more proactive approach is necessary if the

state wishes to maintain its natural ecosystems. While the

proportion of non-native to native species is still relatively

small (14 percent vs. ca. 30 percent for Oregon; T. Kaye

pers. comm.), a growing number of non-native plants is

being collected every year. Further, there is an increase in

observations of species moving off of the anthropogenic

footprint and into more intact ecosystems, often in habitats

with natural disturbance (Carlson and Lapina 2004).

The increase in non-native plants mirrors a similar

increase in human population for the state, which has

tripled since 1968 (U.S. Census 2000). With it, the amount

of ground disturbing activities related to oil development,

agriculture, housing, and roads has also dramatically

increased. When human disturbance was low, such as the

decade after Alaska became a state, the probability that a

non-native plant species would find its way to Alaska and

become established was small. In this context, it is interesting to note that the majority of non-native plants recorded

in Alaska by the mid 20th century were restricted to southeastern Alaska, which was the population center, and is also

the region that has seen the greatest proportion of species

actually establish. Currently, we are witnessing a geographic shift in the center of introductions, with an increasing number of non-native species establishing in south central and central Alaska, where the human population and

development is now greatest.

Overall, our study indicates that the number of nonnative plant population records (including those considered

invasive) follows an exponential growth pattern, in contrast

to that of native species, which is linear. The greater

increase in non-native plant records is likely due to both an

escalation in establishment and a stepped-up survey effort,

two factors that are difficult to disentangle with these data

alone. However, comparisons of only herbarium records

indicate that the number of collections of non-native and
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

in the establishment and rapid population growth phases.

Taken together, these trends suggest that Alaska is not 

necessarily less susceptible to invasion, but that the process

of invasion has been delayed by a number of decades. We

further propose what we are now seeing in Alaska is the

establishment of individual foci in various locations of

where human disturbance and the propagule pressure are

large enough to promote establishment. For example, 

in Valdez, there is now a large infestation of Hieracium

caespitosum Dumort. that must have been established 

some time in the last decade. Otherwise, this species is 

only known from a few disparate and small populations 

(M. Shephard pers. obs.).

invasive taxa is increasing more quickly than those of

natives. Additionally, when comparing species considered

invasive relative to a random sample of non-native species

in the combined data set, we see that the invasives are

showing a greater increase in number of populations

recorded in recent years. Given these trends, we propose

that this increase is not just due to increased survey effort.

Species invasions can be characterized as going

through the phases of 1) establishment (when population

growth is often highly irregular), 2) rapid population

growth and expansion, and 3) reduced growth and slowed

spread (Kinlan and Hastings 2005). In Oregon, invasive

species appear to be in the reduced growth phase. The same

species in Alaska, however, are all behaving as populations

129

Figure 3—Proportion of all records over time of

15 native plants in Alaska.
General Technical Report GTR-694

In many instances, the spatial and temporal patterns of

expansion and establishment of non-natives across Alaska

mirror those known from the rest of the continent. Crepis

tectorum L., Hieracium aurantiacum L., and Hypochaeris

radicata L. all are expanding rapidly in the western United

States and they appear to be in an exponential growth phase

within Alaska. In 1968 there were only one or two collections of these plants in Alaska, whereas now AKEPIC 

and additional unpublished data suggest these taxa are all

spreading rapidly. The Kodiak Wildlife Refuge and the

Koniag Native Corporation have been jointly trying to control a large infestation of H. aurantiacum within native

plant communities across a remote 40 acre island.

Hypochaeris radicata was one of the most common roadside weeds, often exceeding 30 percent cover, on Prince of

Wales Island in southeastern Alaska (unpublished report,

for the USDA Forest Service). Crepis tectorum is now

extremely abundant along roads throughout Alaska, and 

is beginning to show up on glacial river floodplains.

Other taxa, such as Phalaris arundinacea L., Trifolium

repens L., T. hybridum L., and Melilotus alba Medikus

were previously used for roadside seeding. Today these

species visually dominate many road systems across the

state, and are known to be spreading into wetlands and

riparian areas.

There are still other species such as Impatiens 

glandulifera Royle, Senecio jacobaea L., and Centaurea 

130

Figure 4—Proportion of all records over time of

15 invasive plants in Alaska.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

bierbersteinii D.C. that were completely unknown to be

naturalized in the state before 1985. Impatiens glandulifera

was probably planted in gardens, then escaped and is now

well entrenched in a beach meadow in southern Alaska.

Senecio jacobaea likely arrived via road and logging equipment, and is now very widespread in Ketchikan and occurs

in small, scattered populations further north. Likewise, C.

bierbersteinii has probably hitchhiked to Alaska on equipment, and is currently known from at least ten different

locations along roads, from Anchorage to Ketchikan (all

populations ranged from 1- 50 plants). 

Some taxa such as Descurainia sophia (L.) Webb ex

Prantl, Ranunculus acris L., and Lonicera tatarica L. are

considered problematic invasive species outside of Alaska.

In Alaska D. sophia, and R. acris are naturalized but populations tend to be small and isolated and are not particularly

problematic. Lonicera tatarica, which is widely planted in

southern and central Alaska, never has naturalized to our

knowledge. Perhaps these taxa are in a ‘lag phase’ or will

never become established in Alaska. 

Alaska does have some invaders that have not been

particularly problematic in the contiguous U.S. states. For

example, Siberian rye (Elymus sibiricus L.) was introduced

at the University of Alaska experimental station in Palmer

and is now showing up on sandy soils in south-central

Alaska and even relatively remote river bars. Siberian peashrub (Caragana arborescens) has been planted as an ornamental shrub and hedge in interior and southern Alaska for

131

Figure 5—Proportion of all records over time of

15 non-native and invasive plants in Alaska.
General Technical Report GTR-694

many decades and it is now readily recruiting in undisturbed boreal forests in Alaska. It has also recently been

found to be quite invasive in Elk Island National Park in

Alberta, Canada (Henderson and Chapman 2006). 

A number of introductions also appear to have failed,

as expected. Species such as Spinacia oleracea, Nepata

cataria, and Plagiobothrys figuratus were known only from

a few collections 65 years ago and no additional records

have been noted since. The majority of the failed introductions are agricultural species or agricultural weeds, which

often are not effective competitors outside of cultivation.

Nonetheless, it should be noted that even apparently poorly

adapted agricultural species have responded quickly to natural selection and are now invading native habitats (e.g.

Melilotus officinalis and M. alba, cf. Klebesadel 1992). 

Undoubtedly, many of the current and future introductions in Alaska will go extinct locally, but others will result

in establishment and potentially affect habitats and ecosystem functioning. Efforts to identify which of those species

will cause greatest ecological harm are currently being

undertaken (Carlson et al. manuscript in prep.).

Many land management agencies, as well as the public,

are becoming mobilized to reduce potentially negative

impacts due to non-native plants. Primarily, the response

has been to identify which species are here, where they are

located, and how fast they are spreading. This information

is critical in designing effective and efficient control measures. Additionally, we need to develop a better understanding of the pathways of dispersal and establishment, and of

how ground disturbing activities contribute to invasion.

CONCLUSION

Alaska occupies a unique and advantageous position relative to the rest of the states: the majority of land has not

been impacted by human development, and non-native

plants are still largely concentrated in high-use areas.

However, invasive non-native plants are quickly colonizing

undeveloped areas (cf. Conn et al. in press). Once they

become established in undeveloped areas, eradication and

control efforts will be extremely expensive and logistically

challenging, if not impossible. Consequently, the only

effective way of maintaining the uniquely native flora of

Alaska is by reducing the influx of non-native species into

developed areas and by controlling the invasive species

before they reach natural systems.

ACKNOWLEDGMENTS

Melinda Lamb graciously collected, analyzed, and reviewed 

much of the data. Alan Batten and Michael Duffy supplied

an extensive list of non-native plants recorded for Alaska.

Keith Boggs and Helen Cortes-Burns offered useful comments and insights in this manuscript.

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Alaska. Alaska Agriculture and Forestry Experiment

Station Bulletin 89. 27 p.

Lapina, I.; Carlson, M.L. 2004. Non-native plant inventories of southcentral Alaska. Report on file with USDA

Forest Service, State and Private Forestry and Alaska

Natural Heritage Program. 30 p.

Nolen, A. 2002. Vetch infestations in Alaska. Prepared 

for: Alaska Department of Transportation and Public

Facilities. Alaska Plant Materials Center, Division of

Agriculture. Palmer, AK: Department of Natural

Resources. 38 p. 

Oregon State University Herbarium Database. 2006. 

[online] URL: http://ocid.nacse.org/cgi-bin/qml/

herbarium/plants/vherb.qml.

Pimm, S. 1991. The balance of nature? Ecological issues 

in the conservation of species and communities.

University of Chicago Press. 434 p.

University of Alaska, Fairbanks, Herbarium Database. 

2006. [online] URL: http://arctos.database.museum/

SpecimenSearch.cfm.

U.S. Census Bureau. 2000. Census 2000 data for the state 

of Alaska. [online] URL: http://www.census.gov/ 

census2000/states/ak.html.

Williamson, M. 1996. Biological invasions. London: 

Chapman Hall.

133
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ABSTRACT

Habitat distribution models (HDMs) are increasingly used in conservation biology and have the potential to inform efforts 

to monitor invasive species. However, several challenges complicate the use of HDMs for invasive species. First, these models assume that the species are in equilibrium with the environment, which is generally not the case with invasive species.

Second, data on current distributions of invasive species usually come from a variety of sources, often leading to a clumped

distribution of sample points. Third, many modeling techniques are sensitive to the recorded frequency of the species.

Fourth, modeling techniques vary in their assumptions and data requirements and it is unclear which techniques work best

for invasive species. We illustrate these challenges by modeling the potential distribution of three invasive species on the

Olympic Peninsula, Washington. We tested three modeling techniques and assessed their sensitivities to these challenges.

We found that models were sensitive to the level of clumping in the current species and are affected by the fact that species

are still spreading. We provide suggestions for improving modeling efforts in the future.

KEYWORDS: Habitat distribution models, HDM, monitoring.

1 Chad C. Jones is a Research Ecologist, College of Forest Resources, Box 352100, University of Washington, Seattle, WA 98195-2100.

2 Steven A. Acker is the Supervisory Botanist, Olympic National Park, 600 East Park Avenue, Port Angeles, WA 98362.

3 Charles B. Halpern is a Research Professor, College of Forest Resources, Box 352100, University of Washington, Seattle, WA 98195-2100.

135

INTRODUCTION

Monitoring for invasive species is both time consuming and

costly. Therefore, if land managers can target monitoring

efforts to areas where the invasive species are likely to

occur, they can maximize the effectiveness of limited

resources. Habitat distribution models (HDMs) are currently widespread in conservation biology ecology (Guisan and

Zimmermann 2000) and are beginning to be used to predict

the potential distributions of invasive species. 

However, several challenges complicate the use of

HDMs for invasive species. First, these models assume that

species are in equilibrium with their environments (Guisan

and Zimmermann 2000), but this is generally not the case

with invasive species, which are still spreading. Dynamic

species distributions may bias model results by creating

spurious correlations with habitat variables. Most importantly, we cannot test how well the models predict the

potential distributions of spreading species, only how well

they predict current distributions. Furthermore, data on current distributions of invasive species usually come from a

variety of sources and are usually not from a coherent or

balanced sampling design (although this is changing). This

can lead to clumped distributions of sample points and may

affect how common a species is in the dataset. Some modeling techniques only require information on species presence but most require information on both presence and

absence. Those that require both presence and absence

information are sensitive to how frequent the invasive

species is in the dataset (i.e., the percentage of plots where

the species is present). Therefore, if the distribution of the

sample points affects frequency, it can greatly affect results. 

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

CHALLENGES IN PREDICTING THE POTENTIAL

DISTRIBUTION OF INVASIVE SPECIES USING 

HABITAT DISTRIBUTION MODELS

Chad C. Jones1, Steven A. Acker2, and Charles B. Halpern3
General Technical Report GTR-694

We illustrated these challenges by modeling the potential distributions of three invasive species on the Olympic

Peninsula, Washington: Canada thistle (Cirsium arvense

(L.) Scop.), herb robert (Geranium robertianum L.) and

evergreen blackberry (Rubus laciniatus Willd.). We used

three different modeling techniques and assessed their 

sensitivity to these factors. We asked four questions: 1)

How does clumping of sample distributions affect model

results? 2) How might spreading species affect model

results? 3) How does frequency affect the predictions of

presence/absence methods? 4) How do the results of the

different modeling techniques compare? We use specific

examples to illustrate the answers to these questions.

METHODS AND RESULTS

HDMs require data on the species locations (presence or 

absence) and maps of habitat variables thought to affect the

potential species distributions. We combined 13 datasets

from a variety of sources with a total of 4142 data points

across the Olympic Peninsula. For each model we divided

the data into five equal parts and used four-fifths to build

the model and one-fifth to test it. We repeated this with the

five partitions of the data to yield five replicate models. We

developed models using one presence/absence technique,

logistic regression (Nicholls 1989), and two presence-only

techniques, Ecological Niche Factor Analysis (ENFA, Hirzel

et al. 2002) and Genetic Algorithm for Rule-set Prediction

(GARP, Stockwell and Peters 1999). Models for GARP

have not yet been completed and at this point include only

one replicate. Because of the low frequency of the species

in the dataset, we artificially set the frequency to 50 percent

by keeping all presence records and randomly removing

absence points until there were equal numbers of presence

and absence points. We used 12 habitat variables in the

models including measures of climate (e.g., annual precipitation, number of frost days), topography (e.g., slope, heat

load) and vegetation (e.g., conifer and total vegetation

cover).

To test the effect of clumped distribution on modeling

results we set a minimum distance between plots and randomly removed plots that were closer than this distance.

Increasing the minimum distance between plots decreased

clumping in the dataset but also reduced sample size. We

ran models at minimum distances of 100, 500, and 5000 m.

Sample size decreased from 219-559 presences at 100 m to

35-67 presences at 5000 m. We found that model accuracy,

136

Figure 1—Model accuracy with different minimum distances for three species and three modeling techniques. Values of Kappa >0.6 are good; 0.2-0.6 are moderate, and <0.2 are poor (Landis and Koch 1977).

Values of AUC >0.9 are good, 0.7-0.9 are moderate, and <0.7 are poor (Pearce and Ferrier 2000). 
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

as measured by Kappa and AUC (see Fielding and Bell

1997), generally decreased with increasing minimum 

distance (fig. 1), likely a result of smaller sample size.

However, habitat suitability maps suggest a different result.

For example, logistic regression models for Rubus laciniatus predict that the most suitable habitat occurs on the west

side of the peninsula when the minimum distance is set at

100 m (fig. 2a). This is because most occurrences of Rubus

are in the west. However, Rubus can and does occur on the

eastern side of the peninsula. When the minimum distance

is increased to 5000 m, the model more accurately reflects

the full distribution of the species (fig. 2b).

The effect associated with spread of species is illustrated by model results for Geranium robertianum, which

first arrived on the northeastern part of the peninsula near

Port Angeles. In the past 5-10 years it has spread to the

western side of the peninsula, but is still much more common in the northeast. This dispersal gradient is closely

matched by a gradient in precipitation frequency, which

appears as an important habitat variable in the models.

However, this relationship may be spurious. It is likely that

the current distribution of Geranium robertianum reflects

patterns of introduction and spread rather than precipitation.

We tested the effects of frequency on logistic regression models by artificially varying the numbers of absence

data points to create frequencies of 50, 33 and 20 percent

for each species. As frequency increased, model accuracy

increased and habitat suitability increased. At a frequency

of 20 percent only a small area in the immediate vicinity 

of current locations was considered suitable.

Some recent studies have suggested that presence-only

techniques would perform better than logistic regression for

spreading species (e.g., Hirzel et al. 2001). However, we

found that logistic regression was generally more accurate

in predicting the current distribution of the species (fig. 1). 

CONCLUSION

In conclusion, HDMs can be used to model the distributions

of invasive species, but must be used with care. Clumped

sample distributions and the fact that species are still

spreading can greatly affect modeling results. Logistic

regression performed somewhat better than ENFA and

GARP, but is very sensitive to frequency. However, measures of accuracy, such as Kappa and AUC, can only be used

to assess predictions of current species distributions. For

invasive species, we are more interested in predicting

potential distributions, which cannot be tested directly. To

better achieve this goal and overcome the challenges of

using HDMs for invasive species, we recommend the following: first, when using logistic regression, absences

137

Figure 2—Habitat suitability maps for Rubus laciniatus based on logistic regression with a minimum distance of

100 m (A) and 5000 m (B). Darker shading indicates greater habitat suitability and open circles indicate points

where the species is present at the given minimum distance between plots.
General Technical Report GTR-694

should be removed to obtain a high frequency (e.g., 50 

percent). Second, sampling designs should capture the

entire current distributions of the species rather than focusing on areas with the greatest invasion. Third, decisions

about model performance should not be based entirely on

mea-sures of accuracy. Examining the distribution maps

produced by models can provide insights into which models

are best. Additionally, combining model results with information on the biology of species from their native ranges 

or other invaded areas can allow better model assessment.

These approaches will enable the use of HDMs to more

accurately inform invasive species monitoring.

ACKNOWLEDGMENTS

We thank Collette deFerrari, Connie Harrington, Robin 

Lesher, Ann Lezberg, Ed Schreiner, Andrea Woodward,

Joan Ziegltrum, and the Olympic National Park staff for

providing data. Olympic National Park provided funding

for this project.

LITERATURE CITED

Fielding, A.H.; Bell, J.F. 1997. A review of methods for 

the assessment of prediction errors in conservation

presence/absence models. Environmental Conservation.

24: 38–49.

Guisan, A.; Zimmermann, N.E. 2000. Predictive habitat 

distribution models in ecology. Ecological Modelling.

135: 147–186.

Hirzel, A.H.; Hausser, J.; Chessel, D.; Perrin, N. 2002. 

Ecological-niche factor analysis: How to compute 

habitat-suitability maps without absence data? Ecology.

83: 2027–2036.

Hirzel, A.H.; Helfer, V.; Metral, F. 2001. Assessing 

habitat-suitability models with a virtual species.

Ecological Modelling. 145: 111–121.

Landis, J.R.; Koch, G.G. 1977. The measurement of 

observer agreement for categorical data. Biometrics.

33: 159–174.

Nicholls, A.O. 1989. How to make biological surveys 

go further with generalized linear models. Biological

Conservation. 50: 51–75.

Pearce, J.; Ferrier, S. 2000. Evaluating the predictive 

performance of habitat models developed using logistic

regression. Ecological Modelling. 133: 225–245.

Stockwell, D.; Peters, D. 1999. The GARP modelling 

system: problems and solutions to automated spatial

prediction. International Journal of Geographical

Information Science. 13: 143–158.

138
ABSTRACT

Washington State is facing an invasion of non-native, highly competitive noxious weeds. For example, in the mid-1980’s, 

approximately 60,000 acres of Ferry County were mapped by Washington State University (WSU) as noxious weed infested, with more than 400,000 acres of rangeland threatened in the county. The survey also mapped 420,000 acres infested in

Washington State with diffuse knapweed (Centaurea diffusa) with spread calculations of over 12 million acres by 2007!

These non-native noxious weeds reduce biological diversity, decrease forage, increase erosion potential, and decrease land

values across the state and throughout the western U.S. The health of these grasslands is vital for wildlife, livestock, and

people of Washington. Washington State land managers and owners often do not have the time, funds, or expertise to implement fully integrated weed control as part of their weed management strategies. The Integrated Noxious Weed Invasive

Species Project (INWISP) addresses this technical transfer need by enhancing education and engaging land managers for

better understanding of weed control. It focuses on enhancing early detection, rapid response, and integrated control to help

address weed problems. 

KEYWORDS: Integrated weed control, biological control, early detection-rapid response, INWISP.

1 Daniel Fagerlie is the Director of INWISP and the Colville Reservation Extension EIRP Project, WSU Ferry County Extension, 350 E Delaware 

Ave, #9, Republic, WA 99166-9747, phone: 509-775-5235, fax: 509-775-5218, fagerlie@wsu.edu.

2 Jennifer Andreas is the Program Coordinator, Western Washington Integrated Weed Control Project, WSU King County Extension, 919 SW

Grady Way, Ste 120, Renton WA 98057, phone: 206-205-3135, fax: 206-296-0952, jennifer.andreas@metrokc.gov.

3 Washington State University, King County Extension.

4 Washington State University, Chelan County Extension.

5 Washington State University, Ferry County Extension.

139

PROJECT DESCRIPTION

Since 1999, Dan Fagerlie, INWISP Director, has worked 

with the USDA Forest Service, WSU, USDA APHIS,

Washington Department of Fish and Wildlife, Colville

Tribes, and the Extension and Weed-Board Offices of several counties, to assemble a team to work on these efforts.

Initially, the growing team worked to expand the use of biocontrol agents in northeastern Washington to fight invasive

weeds. With additional Forest Service funding, the program

expanded across Central Washington in 2002 and part of

western Washington in 2003, and expanding to serve

throughout western Washington in 2006. The statewide

project added enhanced weed control education in 2004,

and an early detection and rapid response component in

2006. The Confederated Tribes of the Colville Reservation

have been major partners from the beginning of the project,

with the Yakama Tribe participating in recent years.

Significant reductions of targeted invasive species are

occurring as a result of this collaborative project. We are

observing not only a great reduction in the rate of spread,

but landscape changes with diffuse knapweed, Centaurea

diffusa Lam., monocultures returning to grasses and forbs

across eastern Washington. There has also been a dramatic

reduction in the rate of spread of Dalmatian toadflax,

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

THE INTEGRATED NOXIOUS WEED INVASIVE SPECIES 

PROJECT (INWISP) OF WASHINGTON STATE

Daniel L. Fagerlie1, Jennifer E. Andreas2, Tara J. Zimmerman3,

Brad W. Gaolach3, Dale K. Whaley4, and Tyler W. Wilson5
General Technical Report GTR-694

Linaria dalmatica (L.) P. Mill., across the state of

Washington and the western United States. The project 

collection areas are the major source for insects that are

having a remarkable impact on the toadflax in these areas.

Areas of initial releases are already showing dramatic

reductions in density of this invasive weed and the insects

are seeking out scattered, isolated infestations. The project

also provides the base funding, matched by collaborators,

for the western Washington Coordinator.

CHALLENGES OF INVASIVE PLANT 

CONTROL IN WESTERN WASHINGTON

Invasive plant control in western Washington presents some 

interesting challenges to land managers. First, environmental conditions can vary widely between regions. For

instance, dramatically different precipitation levels can produce a range of conditions from desert-like grassland (e.g.,

Sequim) to temperate rainforest (e.g., Forks). The result of

such differences in water availability can alter plant size

and possibly growth patterns, particularly in high precipitation areas. Environmental conditions can also impact the

biological control agents used to manage weed species.

Most biocontrol agents are native to environments similar

to eastern Washington habitats. Using these same agents 

in western Washington conditions and predicting similar

results as other areas in North America may not be realistic

and requires further exploration. 

Second, invasive species can have different and often

stronger impacts on island plant communites compared to

mainland communities. Special considerations are required

with all aspects of invasive species management in such

landscapes to minimize these impacts. 

Third, diverse land use practices, including farm (largescale to “hobby farms”), range, timber and intensely urban

areas are widespread in western Washington. Challenges lie

in effectively controlling invasive species while considering

many landowners each with different land-use goals, weed

problems and control strategies. 

Fourth, although land managers have been implementing biological control as part of their integrated weed 

management programs, they rarely have the time, funds 

or expertise to use this tool to its full potential. WSU

Extension’s INWISP is assisting land managers in meeting

these challenges by providing information, education,

resources and the first organized biocontrol effort in western Washington. 

PROJECTS CRITICAL TO WESTERN 

WASHINGTON

Two projects that are particularly important in western 

Washington are the control of purple loosestrife (Lythrum

salicaria L.) in tidal-influenced water and the potential new

biocontrol agent for Scotch broom (Cytisus scoparius (L.)

Link). Galerucella calmariensis L. and G. pusilla (Duft.)

(Coleoptera: Chrysomelidae), (purple-loosestrife foliage

feeding beetles) have been greatly successful in many areas

of the U.S., including much of Washington. However, the

beetles rarely tolerate the regular inundation of water at

purple loosestrife infestations that occur in areas influenced

by tides. Examples of such areas include the Columbia

River, rivers in Pacific and Grays Harbor Counties and 

the Snohomish Estuary. Two other biocontrol agents are

available and appear to be tolerant of tidal-influenced environments: the flower-bud feeding weevil, Nanophyes marmoratus Goeze (Coleoptera: Brentidae), and the root-feeding weevil, Hylobius transversovittatus Goeze (Coleoptera:

Curculionidae). Nanophyes marmoratus is available for 

limited redistribution. Hylobius transversovittatus, however,

is difficult to collect and large numbers are rarely obtained.

In 2007, rearing of H. transversovittatus will begin in

Washington in order to build large numbers of weevils 

for redistribution. 

Scotch broom is a dominant invasive species in much

of western Washington. In 2005, the accidentally introduced

mite, Aceria genistae Nalepa (Acarina: Eriophyidae) 

(confirmation still required) was discovered in the Tacoma/

Seattle area. The mite appears to reduce seed production

and biomass and when abundant may cause stem-dieback.

WSU, King County Extension, USFS, Rocky Mountain

Research Station and Oregon Department of Agriculture are

collaborating to determine if the mite is host-specific to

Scotch broom. Greenhouse and open-field tests were conducted in 2006 to test if the mite would attack native plants

(e.g., species in the genera Lupinus, Thermopsis, and

140
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

Lathyrs) and crop species (e.g., soybeans). Tests will likely

continue in 2007. If Scotch broom is the only species

attacked by the mite a request will be submitted to USDA

APHIS for its approval as a biocontrol agent. Once

approved, the mite will be available to distribute throughout Washington, Oregon and California.

CONCLUSION

INWISP’s implementation of integrated weed management 

tools assists in the effort to impact invasive plant species

throughout Washington. Collaborations and funding through

federal, state and local entities have been critical in allowing the successes we have had to date. These successes are

evidenced through changes in landscapes towards more

desirable conditions and reductions in the rate of landscape

degradation from new invaders.

141
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ABSTRACT

The ongoing arrival and invasion of Pacific Northwest ecosystems by non-native plant species is well-known amongst most 

botanists and land managers in the region. However, there are few comprehensive sources of information to describe the

extent and impact of these invasions to policy-makers and the public. The objective of this study was to assess the ability of

different types of information systematically collected across the region to describe the distribution and abundance of invasive species in the forests of the Pacific Northwest. Cover of the most abundant species was measured on 4,169 “standard”

Forest Inventory and Analysis (FIA) sample points across Oregon and Washington from 2001–2005. Intensive measurements of all vascular plant species were collected on a systematic “intensive” subset of the standard plots, with 201 assessed

to date. Plant cover was estimated on four 0.017 ha subplots at each plot location. Sixty-three percent of all intensive sample

points had at least one non-native species recorded; proportions ranged from 100 percent in the dry Columbia Plateau and

Basin and Range eco-regions to 33 percent in the North Cascades eco-region. The majority of the non-native species found

were composites or graminoids, tended to be shade-intolerant, and were found in dry or recently-disturbed forest types. The

most frequent invasive species found were Bromus tectorum L., Hypericum perforatum L., and Rubus laciniatus Willd., but 

several non-native species that are not considered invasive were equally common, including Mycelis muralis (L.) Dumort.,

Tragopogon dubius Scop., and Digitalis purpurea L. Although not common, some shade-tolerant species are locally important, particularly Ilex aquifolium L. in lowland west-side forests. The average cover of non-native plants on plots in Oregon

and Washington was 5.4 percent, suggesting that non-native plants cover 1,153,000 ha of forest land in the two states.

Results indicate that non-native plants are already an important component of forests in the Pacific Northwest, even though

many of the recently-arrived invasive species of concern have yet to make a substantial impact.

KEYWORDS: Forest Inventory and Analysis program, Oregon, Washington, non-native plants, forest.

1 Andrew Gray is a Research Ecologist, Forest Inventory and Analysis Program, USDA Forest Service, PNW Research Station, 3200 SW

Jefferson Way, Corvallis, OR 97331, 541-750-7252, agray01@fs.fed.us.

143

INTRODUCTION

The invasion of non-native plants into new areas has had a 

large impact on natural and managed ecosystems. Invasive

plants can directly affect the composition and function of

ecosystems. They may also have a large economic impact,

by changing or degrading land use, or through the costs of

eradication efforts. These impacts are estimated to cost the

U.S. economy at least $35 billion per year (Pimentel et al.

2005). Non-native plant invasions affect ecosystems and

land use by competitively excluding desired species and

altering disturbance regimes, and they are a primary cause

of extinction of native species (D’Antonio and Vitousek

1992, Vitousek et al. 1996, Mooney and Hobbs 2000).

As a result of their impact on managed and natural

ecosystems, the prevalence of non-native invasive plant

species is a key element of many efforts that assess 

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

DISTRIBUTION AND ABUNDANCE OF INVASIVE 

PLANTS IN PACIFIC NORTHWEST FORESTS

Andrew Gray1
General Technical Report GTR-694

ecosystem health and sustainable management (Anonymous

1995, National Research Council 2000, Heinz Center

2002). Nevertheless, there is little data on the abundance,

distribution, and impact of invasive plants available

(Blossey 1999). Information is often incomplete or available for only a few species in a few areas for selected time

periods. As a result, it is cur-rently not possible to provide a

comprehensive understanding of the abundance and impacts

of non-native invasive plants in the United States (National

Research Council 2000, Heinz Center 2002).

The objective of this study was to assess the ability of

different types of information systematically collected

across the region to provide insights on the distribution and

abundance of invasive species in the forests of the Pacific

Northwest.

METHODS

The Forest Inventory and Analysis program (FIA) is a 

nationwide program of the USDA Forest Service with a

mandate to report on the status and trends of the nation’s

forest resources. Classified remotely-sensed images (historically aerial photo points, currently satellite images) are used

to post-stratify field plot data and improve the accuracy of

inventory estimates. A systematic grid of field locations has

been established on a 4.9 km spacing (one point per 2,400

ha), hereafter referred to as the “standard” plots. Ground

data are collected on any field location with “forest land,”

defined as land areas ≥0.4 ha in size that support, or have

supported, ten percent canopy cover of trees and are not

primarily managed for a non-forest land use. Additional

measurements are taken on one out of every 16 standard

locations (one point per 38,800 ha), hereafter referred to as

“intensive” plots. The plot grids extend across all land types

and all ownerships, and a nationally standardized sampling

approach has been in place since 2001. Earlier inventory

designs, which did not sample all ownerships and/or used

different plot designs in different areas, are not discussed

here, although data have been useful for assessing some

invasive species (Gray 2005).

This study describes measurements of non-native

plants on 4,169 “standard” and 201 “intensive” FIA plots

across Oregon and Washington from 2001–2005. Each plot

consisted of four 0.017 ha (7.32 m radius) subplots systematically arranged at each field grid location. On standard

plots, the cover of the most abundant species was estimated

on each subplot. The criterion for “most abundant” was the

three species from each growth form (tree seedling, shrub,

forb, or graminoid) with the greatest cover and any additional species with cover ≥3 percent. On intensive plots,

cover of each vascular plant species present was estimated

on each subplot. In addition, the presence or absence of

species with canopy cover within 0-1.8 m above each of

three 1 m2 quadrats was recorded on each subplot (Gray

and Azuma 2005). Standard plots were measured during a

long sample window (April to October) by crews with general forest resource measurement skills; many graminoid

species and some forbs were only identified at the generic

or family level due to phenology or the lack of botanical

skills. On intensive plots, understory vegetation was measured by skilled botanists during the summer growing season

(June to August), with provisions for collecting voucher

specimens for later identification by herbarium experts.

RESULTS

One or more non-native species were recorded on 62 percent of all sampled plots in Oregon and Washington (table

1). This percentage varied among eco-regions, from 100

percent for the Northern Basin and Range to 33 percent 

for the North Cascades; the latter was the only eco-region

where more than half the plots had no non-native species

recorded. The number of plots sampled to date was low for

some eco-regions, so results are less reliable than for ecoregions with greater sample sizes. The mean percentage of

species on a plot that were non-native differed by ecoregion, but was less than 10 percent except for the

Willamette and Blue Mountains eco-regions. The nonnative proportions of summed plot-level cover generally

followed the same patterns found with species richness, but

suggest that in some eco-regions, cover of individual nonnative species tends to be lower than cover of natives. 

Assuming that each species is randomly dispersed

across each subplot, it is possible to combine the cover of

non-native species, remove the overlap among them, and

estimate the area covered. A simpler and perhaps more

144
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

robust method (which tends to result in lower, more conservative estimates) is to sum the individual cover of nonnative species and divide by the summed cover of all

species. This estimated mean cover across all intensive

plots in Oregon and Washington was 5.4 percent, with a

standard error of 0.87 percent (table 1). Since the area of

forest land in the two states is estimated to be 21,284,000

ha, we can calculate the area covered by non-native plants

as 1,153,000 ± 186,000 ha. This is the basic metric needed

for national reporting of non-native plants (Anonymous

1995, Heinz Center 2002).

The percentage of species that were non-native

declined with increasing stand size class (table 2). In general, stand size class is highly correlated with time since

severe disturbance in this region, and the results indicate

that most non-native plant species on forest land in the

region are associated with recently-disturbed areas.

The most important non-native species on forest land

in Oregon and Washington was Bromus tectorum (table 3),

a species known in the region for its dramatic impacts on

rangelands, and the most common non-native species east

of the Cascade Mountains. However, the next two most

commonly-encountered species (Mycelis muralis and

Tragopogaon dubius) have not received much attention on

the various state and agency lists of invasive species in the

region. Mycelis was the most common non-native species

on the west slopes of the Cascades, while Tragopogon was

important in the Blue Mountains and mountains in northeastern Washington. Some of the common non-native

species are found on several invasive species lists (e.g.,

Hypericum perforatum and Cirsium vulgare (Savi) Ten.),

but other prevalent species are on few or no lists (e.g.,

Rubus laciniatus and Holcus lanatus L.). 

145

Table 1—Abundance of non-native plant species on forest land in Oregon and 

Washington, by eco-region (Omernik 1987)

N plots Plots with Non-native percentages of total (%)

Ecoregion sampled non-natives (%) Species SE Cover SE

Coast Range 35 51.4 7.5 1.9 4.4 2.1

Puget Lowland 5 60.0 6.4 3.3 6.5 4.8

Willamette Valley 5 80.0 25.3 7.2 25.4 11.6

Western Cascades 41 61.0 6.1 1.3 3.8 1.7

Eastern Cascades 24 62.5 7.2 1.8 6.7 3.1

Blue Mountains 34 85.3 10.7 1.7 7.3 1.9

Northern Rockies 15 73.3 7.6 1.9 7.0 3.0

North Cascades 27 33.3 2.7 1.1 2.9 2.2

Klamath Mountains 9 55.6 5.2 1.9 0.7 0.3

N. Basin and Range 6 100.0 6.7 1.2 3.5 2.4

Total 201 62.2 7.4 0.7 5.4 0.9

Table 2—Percentage of all species that were non-native on forest land in 

Oregon and Washington by stand size class

Non-native percentage

Size class (DBH) Plots (n) Species (n) Mean SE

<12.5 cm 26 33.2 13.8 0.98

12.5-22.9 cm 49 30.7 8.7 0.57

22.9-50.8 cm 97 33.7 6.6 0.42

>50.8 cm 29 36.3 2.0 0.37
General Technical Report GTR-694

Since the 4,169 standard plots with forest land in

Oregon and Washington were sampled over long field seasons by crews with a variety of botanical skills, we examined the data and interviewed crew members to determine

which species could be reliably detected. The selected

species were examined to determine their distribution and

their association with vegetation and climate variables

using logistic regression. The distribution of two reliablyidentified invasive species is illustrated in figure 1. Bromus

tectorum (cheatgrass) was most frequently recorded in the

Eastern Cascades and Blue Mountains eco-regions, but was

well-distributed across the eastern areas of the two states.

The frequency of B. tectorum was primarily associated with

low annual precipitation and low tree basal area. In contrast, Rubus discolor Weihe & Nees (Himalaya blackberry)

was well-distributed in the western parts of the two states,

but was primarily associated with low elevations. Although

some of these relationships were similar to those found in 

a portion of western Oregon (Gray 2005), this analysis

described a broader range of conditions, and climate tended

to be more important.

CONCLUSIONS

The results from the strategic FIA inventory on non-native 

invasive species in Oregon and Washington illustrate the

power of having a comprehensive assessment with consistent protocols and sampling effort. The high percentages of

plots with non-native species are novel data and may be

somewhat surprising to policy-makers and the general public, many of whom regard the regions’ forestlands as rather

pristine and consider invasive species to still be an emerging or future threat. Compared to many monitoring protocols with subjective sampling approaches, the lack of bias

in sample location and sampling effort provides high confidence in the applicability of the data. The data collected

with the “intensive” all-species protocols can be readily

summarized to address national reporting needs with appropriate estimates of confidence in the mean.

The “standard” FIA plot grid provides a much higher

density of points with which to assess the distribution 

and abundance of selected non-native invasive species.

However, it is currently not feasible to sample a large 

number of (or all) plant species on the standard plot grid.

Several FIA regions have developed lists of invasive

species that crews are trained to detect on standard plots

146

Table 3—Non-native species found on 5 or more forest health plots, showing the number 

of plots the species was recorded on (out of 201), mean characteristic cover at the plot 

level, and the number of invasive lists the species was found on (out of 8)

Number of 

Scientific name Common name N plots Mean Cover Inv. lists

Bromus tectorum cheatgrass 40 7.11 4

Mycelis muralis wall-lettuce 27 1.17 0

Tragopogon dubius yellow salsify 24 0.43 1

Hypericum perforatum common St. Johnswort 21 1.73 6

Digitalis purpurea purple foxglove 20 1.89 3

Cirsium vulgare bull thistle 19 2.31 6

Dactylis glomerata orchardgrass 18 1.55 2

Rumex acetosella common sheep sorrel 18 0.43 1

Hypochaeris radicata hairy catsear 17 3.18 3

Rubus laciniatus cutleaf blackberry 17 2.90 0

Senecio jacobaea stinking willie 16 1.09 7

Holcus lanatus common velvetgrass 15 17.02 2

Rubus discolor Himalayan blackberry 15 7.21 6

Leucanthemum vulgare oxeye daisy 14 0.88 4

Lactuca serriola prickly lettuce 14 0.25 2
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

(Rudis et al. 2005). If adopted in the Pacific states, an invasive list would need to be limited to less than 40 species

and be biased towards those that are readily identifiable for

much of the year. The open question to experts concerning

the choice of FIA protocols is: Which is more useful,

greater accuracy about the distribution of selected invasive

species, or a comprehensive, lower-resolution assessment 

of all species (native and non-native)? 

LITERATURE CITED

Anonymous. 1995. Statement on criteria and indicators for 

the conservation and sustainable management of temperate and boreal forests. Journal of Forestry. 93: 18.

Blossey, B. 1999. Before, during and after: the need for 

long-term monitoring in invasive plant species management. Biological Invasions. 1(3): 301.

147

Figure 1—Distribution of the two most abundant non-native species found on standard FIA plots in

Oregon and Washington, showing county boundaries.
General Technical Report GTR-694

D’Antonio, C.M.; Vitousek, P.M. 1992. Biological invasions by exotic grasses, the grass/fire cycle, and global

change. Annual Review of Ecology and Systematics.

23: 63.

Gray, A. 2005. Eight nonnative plants in western Oregon 

forests: associations with environment and management. Environmental Monitoring and Assessment.

100(1): 109.

Gray, A.N.; Azuma, D.L. 2005. Repeatability and implementation of a forest vegetation indicator. Ecological

Indicators. 5(1): 57.

Heinz Center (H. John Heinz III Center for Science, 

Economics, and the Environment). 2002. The state of

the nation’s ecosystems: measuring the lands, waters,

and living resources of the United States. Cambridge,

UK: Cambridge University Press.

Mooney, H.A.; Hobbs, R.J.H. 2000. Invasive species in 

a changing world. Washington, D.C.: Island Press.

National Research Council Committee to Evaluate 

Indicators for Monitoring Aquatic and Terrestrial

Environments. 2000. Ecological indicators for the

nation. Washington, D.C.: National Academy Press.

Omernik, J.M. 1987. Ecoregions of the conterminous 

United States, Map (scale 1:7,500,000). Annals of the

Association of American Geographer. 77(1): 118.

Pimentel, D., Zuniga, R. and Morrison, D. 2005. Update 

on the environmental and economic costs associated

with alien-invasive species in the United States.

Ecological Economics. 52(3): 273. 

Rudis, V.A.; Gray, A.; McWilliams, W.; O’Brien, R.; 

Olson, C.; Oswalt, S.; Schulz, B. 2005. Regional

monitoring of non-native plant invasions with the

Forest Inventory and Analysis program. In: McRoberts,

R.E. et al., eds. Proceedings of the 6th Annual Forest

Inventory Analysis Symposium. USDA Forest Service

General Technical Report. WO-70: 49.

Vitousek, P.M.; D’Antonio, C.M.; Loope, L.L.; 

Westbrooks, R. 1996. Biological invasions as global

environmental change. American Scientist. 84(5): 468. 

148
ABSTRACT

It is widely accepted that herbarium specimens have been and continue to be fundamental to understanding the biology and 

taxonomy of invasive plant species. Over the last 60 years, specimens from Pacific Northwest (Oregon, Washington, Idaho,

Montana, British Columbia, and Alaska) herbaria have supported seminal research by several regional invasive species biologists (Ownbey 1950c, Reichard and Hamilton 1997, Novack and Mack 2001, Zika 2003). However, there remains a considerable gap between the recognized and the documented distribution of many plants considered invasive in this region. 

KEYWORDS: Herbaria, invasive species, Washington, WTU.

1 David Giblin is the Collections Manager, University of Washington Herbarium, Burke Museum, Box 355325, University of Washington, 

Seattle, WA 98195-5325, dgiblin@u.washington.edu, (206) 543-1682.

2 Ben Legler is the Database Manager, University of Washington Herbarium, Burke Museum.

3 Richard G. Olmstead is the Curator, University of Washington Herbarium, and Professor, Department of Biology at the University 

of Washington, olmstead@u.washington.edu.

149

INTRODUCTION

The University of Washington Herbarium (also known as 

WTU) is the largest herbarium in the Pacific Northwest 

and contains Washington State’s most comprehensive collection of invasive plant species. There are currently over

11,800 specimens of introduced species in WTU’s Pacific

Northwest collections dating back to the 1880s, with 3,300

of these specimens having been added in the last ten years

through a combination of efforts by individual researchers

and WTU-sponsored collecting trips (fig. 1).

In the last two years, WTU botanists have made firsttime collections in Washington natural areas of potentially

invasive species (e.g., Sorbus hybrida L., Paulownia tomentosa (Thunb.) Sieb. & Zucc. ex Steud.), an aggressive nonnative species (Impatiens glandulifera Royle) not previously recorded on Mt. Baker-Snoqualmie National Forest, and

a species known to be invasive in Oregon and California

(Geranium lucidum L.). Contrary to the situation at WTU,

there is an overall decline in the growth of herbaria 

collections throughout the U.S. (Prather et al. 2004a). This

decline is compromising efforts to detect when potentially

invasive species become established in the wild and the

ability to assess how quickly such species may spread

(Prather et al. 2004b).

Examining WTU’s holdings for several well-established invasive species in the Pacific Northwest, we found

substantial information gaps regarding documented occurrences and recognized distributions. For example, there are

a total of four specimens of Spartina alterniflora Loisel. at

WTU, with only one of these specimens deposited before

1994. The rapidly spreading Buddleja davidii Franch. is

represented at WTU with a total of ten Washington State

specimens from six counties, and only five of these specimens were collected prior to 2000. 

DEVELOPMENT OF AN ONLINE 

DATABASE

The emergence of the digital age has transformed the rate 

and means by which invasive species data can be shared.

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

HERBARIUM COLLECTIONS AND INVASIVE 

SPECIES BIOLOGY: UNDERSTANDING 

THE PAST, PRESENT, AND FUTURE

David E. Giblin1, Ben Legler2, and Richard G. Olmstead3
General Technical Report GTR-694

Through a grant from the National Science Foundation,

WTU staff have documented within a database and created

online access to label information from over 150,000 specimens, as well as the ability to download these data. Ready

access to herbarium specimens that reflect contemporary

distributions for invasive plant species, specimen collections that are comprehensive in capturing the morphological

variation of such species, and specimen label data that are

readily available online are invaluable tools for academic

researchers, conservation biologists, and natural resource

managers interested in theoretical and applied issues associated with the biology and taxonomy of invasive plant

species. We recommend that state and federal agencies provide financial support for field collecting activities that document the distribution of non-native species, that duplicate

specimens from such activities be deposited at regionally

significant herbaria, and that small herbaria maximize

emerging databasing and data sharing opportunities so that

their collections data are also available to the broadest 

possible audience.

LITERATURE CITED

Novack, S.J.; Mack, R.N. 2001. Tracing plant introduction 

and spread: genetic evidence from Bromus tectorum

(Cheatgrass). BioScience. 51(2): 114–122.

Ownbey, M. 1950c. Natural hybridization and amphiploidy 

in the genus Tragopogon. American Journal of Botany.

37: 487–499.

Prather, L.A.; Alvarez-Fuentes, O.; Mayfield, M.H.; 

Ferguson, C.J. 2004a. The decline of plant collecting

in the United States: a threat to the infrastructure of

biodiversity studies. Systematic Botany. 29(1): 15–28.

Prather, L.A.; Alvarez-Fuentes, O.; Mayfield, M.H.; 

Ferguson, C.J. 2004b. Implications of the decline in

plant collecting for systematic and floristic research.

Systematic Botany. 29(1): 216–220.

Reichard, S.; Hamilton, C.W. 1997. Predicting invasions 

of woody plants introduced into North America.

Conservation Biology. 11(1): 193–203.

Zika, P.F.; Jacobson, A.L. 2003. An overlooked hybrid 

Japanese knotweed (Polygonum cuspidatum × sachalinense; Polygonaceae) in North America. Rhodora. 

105: 143–152.

150

Figure 1—The number of specimens of non-native species added to the WTU vascular plant collections between

1937-2004. We chose 1937 as the beginning date because this is the year that C.L. Hitchcock, author of Vascular

Plants of the Pacific Northwest, arrived at the University of Washington and began expanding the collection. The

last year for which we have all specimens documented in the database is 2004.
PARTNERSHIPS,

EDUCATION, AND

OUTREACH

Hound’s-tongue (Cynoglossum officinale) (Lisa K. Scott)
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ABSTRACT

The “Weeds Cross Borders Project” is a partnership of land managers, agencies and regional invasive plant committees in 

the greater Okanagan region of Canada and the United States. Partners include the South Okanagan-Similkameen Invasive

Plant Society, Okanogan County Noxious Weed Control Board, Boundary Weed Management Committee, Ferry County

Noxious Weed Control Board, WSU Extension Ferry County, B.C. Ministry of Transportation and Washington State

Department of Transportation. The project facilitated cooperation and improved coordination of the laws, regulations and

policies that have differed in the past. A Cooperative Weed Management Area was established to help prevent and control

non-native invasive plant species that have an adverse effect on native plant communities, wildlife habitat and agricultural

lands. The project provides an integrated and coordinated approach to invasive plant management, sharing resources for

education, training, inventory and control. Invasive plants do not recognize our political boundaries, but freely travel our

waterways, railways and highways, dispersing their seed along the way. Coordinating treatment and education across our

borders is necessary to effectively control invasive plants in the long term over the entire geographic area. In 2004 and

2005, priority weeds were inventoried and mapped along both sides of the border. Treatments of selected weeds included

seeding, hand-pulling, cutting, biological control and herbicide applications. With public education deemed an essential

component of the project, the partners developed an education/outreach program to effectively communicate invasive plant

management issues, including the production of posters, press releases and a Cross Borders flyer that was distributed to

thousands of landowners on both sides of the border.

KEYWORDS: Invasive plant management, inventory, monitoring, treatment, public education, international.

1 Lisa K. Scott is the Invasive Plant Program Coordinator, South Okanagan-Similkameen Invasive Plant Society, C/O 8703 Palmer Pl.,

Summerland, B.C. V0H 1Z2 Canada.

153

INTRODUCTION

Invasive plants are a serious threat to the economy and 

biodiversity of the greater Okanagan Region of British

Columbia and Washington State. Invasive plants threaten

both species and habitats at risk. Non-native weeds are

invading new habitats at a rate of 12–14 percent per year,

in the absence of efforts to contain their spread. 

Invasive plants do not recognize political boundaries.

They freely travel our waterways, railways and highways,

dispersing their seed along the way. Coordinating treatment

and education across borders is necessary to effectively

control invasive plants in the long term over an entire geographic area. As a result of this identified need, the “Weeds

Cross Borders Project” was established in 2004. The project

is a partnership of land managers, agencies and regional

invasive plant committees in the greater Okanagan region

of Canada and the United States. Partners include the South

Okanagan-Similkameen Invasive Plant Society, Okanogan

County Noxious Weed Control Board, Boundary Weed

Management Committee, Ferry County Noxious Weed

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

WEEDS CROSS BORDERS PROJECT:

A CANADA – UNITED STATES COLLABORATION

Lisa K. Scott1
General Technical Report GTR-694

Control Board, WSU Extension Ferry County, B.C. Ministry

of Transportation and Washington State Department of

Transportation. Within Canada the project covers the jurisdictions of the Regional District Okanagan-Similkameen

and Regional District Kootenay Boundary. Within the U.S.,

the project area includes Ferry County and Okanogan

County. The entire project area is referred to as the Weeds

Cross Borders Coordinated Weed Management Area.

PROJECT ACTIVITIES

Target invasive plant species were identified within each 

jurisdiction. Table 1 outlines the target species and lists the

different respective legal designations for Washington State

and British Columbia. As part of the cooperative agreement,

each jurisdiction agreed to match designations for target

weed species. For example, wild four o’clock Mirabilis

nyctaginea (Michx.) MacM. is currently not categorized 

in British Columbia, yet it is a Class A designate in

Washington State. This means that in WA, it is a high priority species and eradication is required by the Okanogan

County Weed Board. As a consequence of collaboration, the

Canadian partners agreed to control all identified infestations of wild four o’clock occurring within the Okanagan

and Boundary Regions adjacent to the U.S. Border.

Similarly, the status of puncturevine Tribulus terrestris L.

was elevated in the U.S. to match the higher Canadian designation. These examples of changes to existing weed management approaches ensured that the same level of control

was being achieved on both sides of the border for each of

the respective target weed species. 

Infestations of target invasive plants were inventoried

and mapped along all State/Provincial Highways, as well 

as identified secondary or county roads. Mapping was also

conducted along connecting waterways, including Lake

Osoyoos and Kettle River, and along the Canadian – United

States Border. The inventory was necessary to identify

infestations for control and to provide the baseline data 

necessary to monitor and evaluate treatment effectiveness. 

Control activities were conducted using an integrated

approach of prevention, in combination with physical, 

chemical and biological control options. For both the

Canadian and U.S. infestations, plants were treated with a

combination of mowing, site-specific herbicide applications, hand pulling, re-vegetation and biological control

agent releases. 

Education and outreach were also identified as a 

priority action by each of the project partners. General 

invasive plant information as well as species-specific

resources were produced and distributed throughout the

Weed Management Area. These resources included posters,

154

Table 1—Target weed species with legal corresponding designations or control requirements in Washington

State and British Columbia

Common Name Latin Name WA Designation B.C. Designation

Leafy spurge Euphorbia esula Class B Designate Provincially Noxious

Meadow hawkweed Hieracium pratens New Invader Not categorized

Musk (nodding) thistle Carduus nutans Class B Designate Not categorized

Orange hawkweed Hieracium aurantiacum New Invader Regionally Noxious – Bulkley-Nechako, 

Cariboo, Central Kootenay, ColumbiaShuswap, East Kootenay, ThompsonNicola

Puncturevine Tribulus terrestris Class B Designate Regionally Noxious – OkanaganSimilkameen

Purple loosestrife Lythrum salicaria Class B Designate Not categorized

Wild four o’clock Mirabilis nyctaginea Class A Designate Not categorized

Class A Designate – top priority, their removal is completed free of charge by the Okanogan County Weed Board. 

Class B Designate - mandatory control is required to stop seed production.

The B.C. Weed Control Act imposes a duty on all land occupiers to prevent the spread of designated noxious plants. Provincially Noxious weeds are 

classed as noxious within all regions of B.C. Regionally Noxious weeds are classed as noxious within the boundaries of the corresponding regional 

districts.
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

press releases, bulletins, calendars and flyers. These

resources were distributed by each of the project partners at

community events and were posted at libraries, municipal

offices, government agencies, bike shops and community

bulletin boards. The flyers were sent to over 20,000

landowners living within the Weed Management Area.

The issue of cross border invasive plant management

was also raised with Canadian and U.S. politicians.

Legislative tours were conducted in 2004 and 2005, highlighting the target species, the concerns surrounding their

management and the importance for continued and

increased financial support.

FUTURE OUTLOOK

The resources provided through the Cross Borders Project 

allowed partners to more effectively manage weeds in a

cooperative fashion. The project provided the partners with

an opportunity to improve individual weed management

programs through information exchange, education and

training, coordination of inventory and control efforts, and

sharing of resources when appropriate. This cooperative

project is a unique British Columbia-Washington endeavor

that sets an example for other agencies and other jurisdictions to follow. With the partnerships established, we look

forward to continuing this program into the future. With

limited funding, the partnerships allow each agency to

make the most of existing budgets and control projects,

because weeds know no boundaries.

ACKNOWLEDGMENTS

Many people contributed to the successful completion of 

the past three years of cross border weed management.

These individuals represent a wide variety of affiliations

and backgrounds.

Significant financial contributions were provided by

the U.S. Federal Highway Administration, B.C. Ministry of

Transportation and the (B.C.) Inter-Ministry Invasive Plant

Committee.

155
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ABSTRACT

Invasive plants are a serious problem affecting our ability to preserve native organisms, ecosystems, and functions. Many 

invasive plants are introduced through horticultural channels. In 2001 a group of people involved in professional horticulture

developed the St. Louis Codes of Conduct, a voluntary framework to assess how professional activities affect the introduction and spread of invasive plants. At the University of Washington Botanic Gardens we have implemented the codes of

conduct. This paper reviews the codes for botanic gardens and how the staff addressed them. This included revising the collections policy, removing plants, developing partnerships, and engaging the administration of the University of Washington.

KEYWORDS: Invasive plants, horticulture, codes of conduct.

1 Sarah H. Reichard is an Associate Professor, University of Washington Botanic Gardens, Box 354115, Seattle, WA 98195, reichard@u.washington.edu.

157

INTRODUCTION

Invasive plants have emerged as a leading threat to the 

conservation of species, threatening endangered species

(Wilcove et al. 1998). They are known to alter ecosystem

processes such as nutrient cycling (Vitousek et al. 1987)

and those alterations may persist even after removal of the

species (Dougherty and Reichard 2004). Invasive species

have substantial economic impact as well. Estimates of the

economic costs associated with invasive species (plants and

animals) were conservatively estimated to be $137 billion

annually in 2000 (Pimental et al. 2000) and the federal 

government reported spending more than $631 million on

invasives species activities in fiscal year 2000 (GAO 2000).

Seven states were surveyed and reported spending a total 

of more than $232 million for the same fiscal year (GAO

2000).

Populations of invasive species already present are

often controlled using various methods, but increasing

attention is being given to preventing their introduction and

spread. Prevention may be achieved through risk assessment, either at time of introduction or at first detection, as

well as by determining possible pathways and implementing ways to mitigate those pathways. In some cases, educating those involved may help to alleviate the movement of

species through the pathway by alerting them to the problem and providing suggestions for solutions. 

Invasive plants are often introduced and widely sold 

as landscape plants. Estimates are that more than 60 percent

of all invasive species and over 80 percent of woody invasive species were introduced through horticulture (Reichard

1997). Those introducing the plants are usually unaware of

their potential to spread and have a negative impact to

native plant and ecosystems and they do not bear the costs

asso-ciated with controlling them.

The National Invasive Species Council produced a

National Management Plan (NMP) that was signed by 

then-President William Clinton in 2001. The NMP includes

a chapter on prevention; action item 15 states the Federal

agencies will develop screening systems and other comparable management measures, possibly including codes of

conduct (National Invasive Species Council 2001). 

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

THE ST. LOUIS CODES OF CONDUCT: 

PROVIDING A FRAMEWORK TO PREVENT

INVASIONS FROM HORTICULTURE

Sarah H. Reichard1
General Technical Report GTR-694

Codes of conduct, also called codes of practice or

codes of ethics, provides a formal statement of values

and/or best management practices. Many businesses and

industries establish codes of conduct to communicate their

standards to the world. Codes of conduct also make an

excellent framework for education about issues and communication between the industry and others. 

In 2001 the Missouri Botanical Garden in St. Louis, 

in collaboration with the Royal Botanic Garden at Kew,

England convened a workshop to develop codes of conduct for horticulture professionals, as well as amateurs.

Participants were invited from five categories: retail and

wholesale nurseries, botanic gardens and arboreta, landscape architects, government agencies, and the gardening

public (Reichard 2004). 

Over three days the participants developed mutually

agreed upon Findings and Guiding Principles (table 1).

These provided the foundation for the disciplinary codes 

of conduct (Reichard 2004). The codes of conduct had substantial overlap among the disciplines, e.g., assess risk of

new introductions, know current invasives in the region,

use and promote alternative species, and provide leadership

and educational opportunities for members. The codes differed in some of the areas more specific to the disciplines,

such as botanic gardens examining their collections policy,

a policy that would not be found in the other disciplines.

The government also addressed their role in early detection

of new species and seeking international cooperation, and

acknowledged their eventual need to evaluate the effectiveness of the codes of conduct. The codes for each discipline

may be found at http://www.centerforplantconservation.org/

invasives/codesN.html.

In 2003 I gathered a group of graduate students 

interested in implementing the codes of conduct at the

University of Washington Botanic Garden (UWBG), made

up of the Washington Park Arboretum (WPA) and the

Center for Urban Horticulture, using the codes for botanic

gardens. When considering the first code, to perform an

assessment of all of the policies of the institution, however,

we decided that our institution was not the UWBG, but 

the University of Washington. This somewhat altered our

process and we proceeded to address all of the public 

garden codes with the University in mind. Below I list the

codes of conduct for botanic gardens and arboreta and

describe how we addressed them for the UWBG and, where

appropriate, the UW. 

Codes of Conduct for Botanic Gardens and 

Arboreta

1. Conduct an institution-wide review examining all 

departments and activities that provide opportunities to stem the proliferation of invasive species 

and inform visitors. For example, review or write 

a collections policy that addresses this issue; 

examine such activities as seed sales, plant sales, 

book store offerings, wreath-making workshops, 

etc. 

As discussed, we chose to review the University 

of Washington rather than just the botanic gardens. The 

gra-duate students met with the UW Campus Landscape

Advisory Committee to present the codes to them and discuss concerns. There was concern that implementation

would increase costs in a very tight budget situation, but

most apprehension was easily addressed. It was decided an 

invasive species policy would be developed for campus,

including the various properties that the UW owns beyond

the main campus, such as the Friday Harbor Marine

Laboratories (FHML) in the San Juan Islands. Thus, another class worked collaboratively with FHML to develop a

vegetation management plan to help them manage existing

invasive species and to prevent the introduction of new

ones. While it was outside our expertise, we also discussed

with them their policies to prevent release of marine organisms into Puget Sound via the laboratories. It is intended

that future classes will develop plans for other UW

properties. 

Public gardens typically have a collections policy,

which guides how collections will be acquired, used, and

cared for. At the UWBG we revised our collections policy

to ensure that each aspect was compliant with the Codes of

Conduct. We continue to discover new issues that should be

included in the collections policy, such as introduction of

soil borne seeds and organisms through plants donated by

individuals, so this will be an on-going activity.

158
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

2. Avoid introducing invasive plants by establishing 

an invasive plant assessment procedure. Predictive 

risk assessments are desirable, and should also 

include responsible monitoring on the garden site 

or through partnerships with other institutions.

Institutions should be aware of both direct and 

indirect effects of plant introduction, such as 

biological interference in gene flow, disruption 

of pollinator relationships, etc. 

I have studied the biology of invasive plants and 

worked on predictive methods, including methods that are

used by other public gardens and nurseries (Reichard 1997,

Reichard and Hamilton 1997, Reichard 2001). These methods combine aspects of species biology with invasive history elsewhere to suggest if a species is likely to become

invasive, not likely, or if further evaluation is needed. I

assess all species proposed for addition to the collection at

UWBG, but main campus has its own selection criteria and

oversight committee, the Campus Landscape Advisory

Committee. The campus landscape architect, sometimes

working with an outside firm, suggests plants, which are

then approved by the committee. 

3. Consider removing invasive species from plant 

collections. If a decision is made to retain an 

159

Table 1—The St. Louis Declaration

Findings • People are major dispersers of plants. The magnitude of this dispersal is unprecedented 

and has allowed dispersal of species that manifest aggressive traits in new areas. 

• Plant introduction and improvement are the foundation of modern agriculture and

horticulture, yielding diversity to our supply of plants used for food, forestry, landscapes 

and gardens, medicinal and other purposes. 

• A small proportion of introduced plant species become invasive and cause unwanted 

impacts to natural systems and biological diversity as well as economies, recreation, and

health. 

• Plant species can be invasive in some regions, but not in others. The impacts of invasive 

plant species can occur at times and places far removed from the site of introduction. 

Guiding Principles 1. Plant introduction should be pursued in a manner that both acknowledges and minimizes 

(a.k.a. “The St. Louis unintended harm. 

Six”) 2. Efforts to address invasive plant species prevention and management should be

implemented consistent with national goals or standards, while considering regional 

differences to the fullest extent possible. 

3. Prevention and early detection are the most cost-effective techniques that can be used 

against invasive plants. 

4. Research, public education and professional training are essential to more fully understanding the invasive plant issue and positively affecting consumer demand, proper plant

use, development of non-invasive alternatives, and other solutions. 

5. Individuals from many fields must come together to undertake a broad-based and 

collaborative effort to address the challenge, including leaders in horticulture, retail and

wholesale nurseries, weed science, ecology, conservation groups, botanical gardens, garden

clubs, garden writers, educational institutions, landscape architects, foundations and 

government. 

6. A successful invasive plant species strategy will make use of all available tools including

voluntary codes of conduct, best management practices, and appropriate regulation. Codes

of conduct for specific communities of interest are an essential first step in that they

encourage voluntary initiative, foster information exchange, and minimize the expense 

of regulation.
General Technical Report GTR-694

invasive plant, ensure its control and provide 

strong interpretation to the public explaining 

the risk and its function in the garden. 

As part of a regional project I have been working with 

the Washington State Nursery and Landscape Association to

remove five species invasive in maritime Washington from

commercial trade. These include Buddleja davidii (butterfly

bush), Ilex aquifolium (English holly), Iris pseudacorus

(yellow flag iris), Foeniculum vulgare (common garden

fennel), and selected cultivated varieties of Hedera hibernica/H. helix (English and Atlantic ivies). The UWBG therefore decided to remove these species from the collection as

well. There are no Iris or Foeniculum in the collection, nor

are the invasive cultivars of Hedera in the WPA, except as

invasive populations. All B. davidii plants were removed.

The English holly, however, required a more nuanced

approach. The WPA has an internationally recognized collection of hollies and English holly is an important species

in the genus. The curatorial decision was made to remove

all but one plant and to provide interpretation about its

invasiveness. The single plant will be enclosed in a structure designed to exclude birds, the dispersers of the fruits.

This mirrors a much earlier decision to leave the less invasive Sorbus aucuparia in the important Sorbus collection,

but to interpret its attractiveness to birds and subsequent

invasion. 

4. Seek to control harmful invasive species in 

natural areas managed by the UWBG and assist 

others in controlling them on their property, 

when possible. 

The UWBG manages the Union Bay Natural Area 

which is 90 acres including grassland, extensive lake shoreline, and some deciduous forest. It is one of the largest open

areas in the city of Seattle and contains a large percentage

of invasive species. One part-time employee works mostly

on invasive species control, with the occasional assistance

of other employees. University of Washington restoration

classes have used it as laboratory space for restoration for

about 15 years. In addition, the UWBG is the home of the

Restoration Ecology Network, which partners university

students with restoration projects throughout the region

(Gold et al. 2006). 

5. Promote non-invasive alternative plants or, when 

possible, help develop non-invasive alternatives

through plant selection or breeding. 

Working with members of the Washington State 

Nursery and Landscape Association (WSNLA) and the

Washington State Noxious Weed Board (WSNWB), the

UWBG and other environmental groups developed alternatives to 16 species that are invasive in western Washington

and sold in the landscape trade. We designed an attractive

32 page booklet that pictured the invader and described

why it is a problem, along with photographs and descriptions of safe alternatives (http://plonenw.onenw.org/

InvasiveSpeciesCoalition/GardenPlants/). This required

understanding why consumers wanted these species and

then finding alternatives that met all the various needs,

although not necessarily in one species. The initial printing

of 26,000 booklets was given out through flower shows,

noxious weed boards, master gardeners, nurseries, libraries,

and other methods. Its success has lead to a second printing

and to development of a similar booklet for eastern

Washington species. 

6. If your institution participates in seed or plant 

distribution, including through Index Seminum,

do not distribute known invasive plants except 

for bona-fide research purposes, and consider the 

con-sequences of distribution outside your biogeographic region. Consider a statement of caution

attached to species that appear to be potentially 

invasive but have not been fully evaluated. 

Index seminum, which translates as “seed list,” is an 

informal seed exchange between many public gardens. Not

all gardens participate and a few of those that do have policies dictating that they trade seed only for research or within their bioregion. The UWBG participates, but mostly lists

wild-collected seeds of species native to the Pacific

Northwest. Recognizing that some species from this region,

such as Pseudotsuga menziesii Mirb.el (Franco), Douglasfir, are invasive when introduced to others parts of the

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Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

world, such species are identified and marked with an asterisk. The asterisk leads to the following statement: “The

Washington Park Arboretum is concerned about the impact

of alien plant introductions on local native plant populations. Those species followed by an asterisk have been

reported to naturalize in some regions of the world. It is

assumed that institutions or individuals receiving seed will

take appropriate steps to evaluate the invasive potential of

all plant introductions.”

7. Increase public awareness about invasive plants. 

Inform why they are a problem, including the 

origin, mechanisms of harm, and need for prevention and control. Work with the local nursery 

and seed industries to assist the public in environmentally safe gardening and sales. Horticulture 

education programs, such as those at universities,

should also be included in education and outreach

efforts. Encourage the public to evaluate what they 

do in their own practices and gardens. 

As previously discussed, I have worked with the 

WSNLA to teach their members about invasive species and

remove some from sale. In addition, we offer adult education classes in weed identification to amateur and professional horticulturists. I teach a landscape plant identification

class, required for landscape architecture undergraduates, in

which I discuss the invasiveness of the species covered and

I teach another class entitled the “Biology, Ecology, and

Management of Invasive Plants.” I also give presentations

to garden clubs and other community groups. 

8. Participate in developing, implementing, or 

supporting national, regional, or local early warning systems for immediate reporting and control.

Participate also in the creation of regional lists 

of concern. 

The UWBG includes the Otis Douglas Hyde 

Herbarium, a collection of pressed and dried plant specimens. For several years it has been developing a collection

of area invasive species and houses the collection of the

Washington State Noxious Weed Board. Many duplicates of

the invasive species in the collection have been traded with

other institutions to increase the number of specimens and

to include species that may be found in other parts of North

America, but are not currently found here. This will facilitate confirmation of identity should the species be found

here. In addition, we encourage members of the public to

bring in unknown plants for identification. We notify the

WSNWB of anything that appears to be potentially invasive. In addition, I chaired a committee which developed a

prioritized list of invasive plants known in Washington and

Oregon as of 1997. This list needs updating. 

9. Botanical gardens should try to become informed 

about invasiveness of their species in other biogeographic regions, and this information should be

compiled and shared in a manner accessible to all. 

As discussed in code number 6, the UWBG does 

include information about the invasiveness of our native

species when we participate in the Index Seminum program.

We do not publicize it otherwise. 

10. Become partners with other organizations in the 

management of harmful invasive species. 

The Restoration Ecology Network (see code number 4)

has been active with a wide array of agencies and organizations, managing invasive species and educating students

and the public. In the past six years they have worked on

over 25 unique projects, partnering with city parks, colleges

and universities, private interests, and citizen's groups. 

11. Follow all laws on importation, exportation, quarantine, and distribution of plant materials across 

political boundaries, including foreign countries. 

Be sensitive to conventions and treaties that deal 

with this issue, and encourage affiliated organizations (plant societies, garden clubs, etc.) to do the

same. 

This is, of course, something that is always done. 

When seed is imported for research, through Index

Seminum, or through other sources, all permits are obtained.

When appropriate, permission from the exporting country is

sought. 

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General Technical Report GTR-694

CONCLUSION

Ultimately the best solution for dealing with invasive 

plants with horticultural origins is likely increased regulation regarding importation and sale. Other countries have

successfully implemented such restrictive regulations

(Pheloung 2001). However, voluntary Codes of Conduct

serve an important role in education about invasive species

and provide a framework for individuals, businesses, organizations, and agencies to assess their own actions. We also

found it was a useful tool to engage our university administrators in a dialogue about activities on a wide scale at the

institution. The St. Louis Declaration provides Codes of

Conduct for the five groups invited in that endeavor but

efforts are also underway to develop such codes for the pet

industry, specifically for water gardening (J. Reaser, Pet

Industry Joint Advisory Committee). Learning from the

experiences of developing the St. Louis Declarations

(Reichard 2004), other industries and interest groups may

also want to develop codes of conduct.

ACKNOWLEDGMENTS

I would like to thank the students that helped me assess the 

University of Washington and make recommendations:

Mitchell Almaguer-Bay, Wendy DesCamp, Wendy Gibble,

Bronwyn Scott and Lauren Urgenson. Their efforts greatly

impressed the University administration and lead to adoption of an invasive species policy. Tanya DeMarsh-Dodson

worked with the nursery industry to remove some species

from sale, suggest alternatives and develop the booklet and

discuss it with nurseries. All of the participants at the 2001

workshop at the Missouri Botanic Garden deserve a huge

amount of credit for stepping forward, as do the members

of the WSNLA task force and participating nurseries. 

LITERATURE CITED

Dougherty, D.; Reichard, S.H. 2004. Factors affecting 

the control of Cytisus scoparius and restoration of

invaded sites. Plant Protection Quarterly. 19: 137–142.

GAO (General Accounting Office). 2000. Invasive 

species: federal and selected state funding to address

harmful, nonnative species. GAO/RCED-00-219. 

Gold, W.; Ewing, K.; Banks, J.; Groom, M.; Hinckley, 

T.; Secord, D.; Shebitz, D. 2006. Collaborative 

ecological restoration. Science. 312: 1880–1881.

National Invasive Species Council. 2001. Meeting the 

invasive species challenge: National Invasive Species

Management Plan. [online] URL: http://www. 

invasivespecies.gov/council/nmp.shtml.

Pheloung, P. 2001. Weed risk assessment for plant introductions into Australia. In: Groves, R.H.; Panetta F.D.;

Virtue J.G., eds. Weed Risk Assessment. Melbourne,

Australia: CSIRO Publishing: 83–92.

Pimental, D.; Lach, L.; Zuniga, R.; Morrison D. 2000. 

Environmental and economic costs of non-indigenous

species in the United States. BioScience. 50: 53–65.

Reichard, S. 1997. Preventing the introduction of invasive 

plants. In: Luken, J.; Thieret, J., eds. Assessment and

management of plant invasions. New York: SpringerVerlag: 215–227.

Reichard, S.H.; Hamilton. C.W. 1997. Predicting invasions of woody plants introduced into North America.

Conservation Biology. 11(1): 193–203.

Reichard, S.H. 2001. The search for patterns that enable 

invasion prediction. In: Groves, R.H.; Panetta F.D.;

Virtue J.G., eds. Weed Risk Assessment. Melbourne,

Australia: CSIRO Publishing: 10–19.

Reichard, S. 2004. Conflicting values and common goals: 

codes of conduct to reduce the threat of invasive

species. Weed Technology. 18: 1503–1507. 

Vitousek, P.M.; Walker, L.R.; Whiteaker, L.D.; MuellerDombois, D.; Matson, P.A. 1987. Biological invasion

by Myrica faya alters ecosystem development in

Hawaii. Science. 238: 802–804.

Wilcove D.S.; Rothstein D.; Dubrow J.; Phillips A, 

Losos, E. 1998. Quantifying threats to imperiled

species in the United States. BioScience. 48: 607–615.

162
ABSTRACT

Invasive alien plant species cause both economic losses and ecological damages. However, many potentially invasive plant 

species are deliberately introduced for economic benefit, such as for horticultural trade, despite the risks associated with

them. Nonetheless, prevailing policies aimed at addressing the invasive alien plant problem fail to explicitly consider both

the role of the horticulture industry as a cause of the problem and the substantial revenues generated through the introduction and sale of ornamental plants. Here, I present a range of credible policy options for reducing the risk of invasive plant

introductions via horticultural trade. In addition, I outline how my graduate research will discern stakeholder perceptions of

the invasive plant problem while determining and explaining stakeholder preferences with respect to the alternative policy

interventions.

KEYWORDS: Horticulture, invasive plants, policy alternatives, risk, North America.

1 Arianne Ransom-Hodges is an MS candidate, School of Resource and Environmental Management, Simon Fraser University, 8888 University 

Drive, Burnaby, BC, V5A 1S6, Canada.

163

INTRODUCTION

Invasive alien plant species are known to cause both economic losses and ecological damages (Pimentel et al. 2005,

Wilcove et al. 1998). However, the introduction of a nonnative species can be simultaneously classified as destructive by one segment of society and desirable by another

(Reichard 2005). Indeed, many potentially invasive plant

species are deliberately introduced for economic benefit

despite the risks associated with them (Haber 2002).

Notably, ornamental species comprise the vast majority of

invasive plants in many countries (Baskin 2002). Therefore,

the horticulture industry is the most significant pathway for

the intentional introduction of invasive alien plants.

Nonetheless, prevailing policies aimed at tackling the

invasive alien plant problem in North America fail to

explicitly address its horticultural dimension. Specifically,

conventional policies fail to explicitly consider both the

role of the horticulture industry as a cause of the problem

and the substantial revenues generated via the introduction

and sale of ornamental plants (Simberloff et al. 2005,

Knowler and Barbier 2005, Perrings et al. 2002). Although

new government strategies are being devised and implemented in Canada and the United States, economic benefits

derived from the horticultural use of alien plant species

continue to be overlooked.

RESEARCH OBJECTIVES

The purpose of my research is to identify policy options 

that address the problem of invasive alien plant species 

and assess their acceptability to stakeholder groups. The

specific objectives of my research are thus to:

• identify plausible policy options to address the 

introduction of invasive alien plant species via 

horticultural trade

• discern stakeholder perceptions of the invasive 

plant problem

Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

POLICIES TO REDUCE THE RISK OF INVASIVE PLANT

INTRODUCTIONS VIA HORTICULTURAL TRADE: 

STAKEHOLDER PERCEPTIONS AND PREFERENCES

Arianne Ransom-Hodges1
General Technical Report GTR-694

• determine stakeholder preferences with respect 

to various policy options

• determine stakeholder perceptions of key concerns/

obstacles with respect to policy development (e.g.,

potential costs, institutional problems, unfair 

distribution of cost/benefits, etc.)

• identify determinants of stakeholder preferences 

(e.g., individual background, geographic location, 

etc.).

METHODOLOGY

I will collect data through the use of a web-based survey of 

stakeholders belonging to the following groups: professional horticulturists, amateur gardeners, agriculturalists, park

managers and staff, invasive plant experts, and naturalists.

After the completion of a pilot test, the survey will be

launched in October 2006. The study sample will consist of

stakeholders residing in four Canadian provinces (British

Columbia, Saskatchewan, Ontario, and New Brunswick)

and five American states (California, Montana, Ohio,

Connecticut, and Florida) (fig. 1). The selection of the nine

key jurisdictions listed above ensures that stakeholders

from across North America will be surveyed.

Through the survey, study participants will be asked to

select the stakeholder category (e.g., professional horticulturist, amateur gardener, agriculturalist, etc.) that corresponds to the capacity in which they are confronted most by

the alien invasive plant problem. Furthermore, respondents

will disclose their perceptions of the problem and preferences among alternative strategies aimed at addressing the

problem. Participants will also provide general socio-demographic information. In order to assess stakeholder preferences for the different policy options, survey respondents

will rank five alternative policies from most to least preferred. Furthermore, a list of considerations will be provided and participants will be asked to identify the consideration that most influenced the order in which they ranked the

policy options. In order to prevent arbitrary selections from

the list of considerations, a category labeled ‘other’ is

included.

Once the data are collected, a contingent ranking

approach will be used to analyze stakeholder preferences

for various policy options. A multinomial LOGIT model

will estimate the probabilities of observing potential rankings. The model will be used to explain how changes in

sample attributes, including demographic characteristics

and stakeholder group membership, might affect predicted

probabilities.

Policy Options

Five plausible policy options have been identified from a 

review of the literature. By focusing on three key considerations for designing policies aimed at tackling the horticultural dimension of the invasive plant problem, these policies were subsequently simplified such that they can be 

easily described and presented to respondents (table 1). 

The three main considerations are: (1) the method used to

determine which plant species will be targeted by the policy, (2) the approach used to limit the introduction and 

dispersal of the specified plant species, and (3) the way 

the policy will be implemented and enforced.

The stakeholder survey outlines two methods for determining which plant species will be considered invasive.

The first is black listing - the listing of all non-native plant

species known to be invasive in a given region. As such,

only species that have already invaded will be listed. The

second approach, screening, consists of assessing the likelihood that newly imported non-native plant species will

become invasive. The screening approach results in policies

that only target species that have a high likelihood of invasion.

Three approaches for limiting the introduction and dispersal of plant species considered invasive are described in

the survey. Ban/quarantine, where the import and sale of all

plant species that are considered invasive is completely prohibited, is the first approach. The second, a variable tax,

involves imposing a variable monetary charge on the sale of

all newly imported non-native plant species. The tax rate is

dependent on the likelihood of invasion of a species. Lastly,

a fixed environmental fee can be applied such that a fixed

monetary charge is imposed on the sale of all non-native

plant species.

Finally, respondents will be presented two ways to

implement and enforce policies. The first is a mandatory

164
Meeting the Challenge: Invasive Plants in Pacific Northwest Ecosystems

approach by which polices are implemented and enforced

by the government. The second approach consists of voluntary self-regulation, in which the horticulture industry is

responsible for implementation and enforcement of the 

policy (e.g., by establishing voluntary codes of conduct).

The first two considerations can be combined to produce policies governing the import and sale of non-native

species. The last consideration specifies how these policies

are implemented and enforced. For the purposes of the survey, five credible policy options are devised by combining

all three considerations (table 1).

165

Figure 1—Map of jurisdictions where study will be conducted.
General Technical Report GTR-694

POTENTIAL IMPLICATIONS

The proposed research will support the development of 

policies to address the intentional introduction of invasive

plant species by the horticulture industry.

While some of the policy options presented in the survey represent existing policies (Option 1 - black listing) or

new strategies currently being considered by governments

in Canada and the United States (Option 2 - risk assessment), others represent less familiar approaches. Option 3

corresponds to industry self-regulation, Option 4 mirrors an

optimal pollution tax (Knowler and Barbier 2005), and

Option 5 characterizes an environmental charge. As such,

my research will determine the acceptability of novel policy

approaches as compared to conventional policy options.

Importantly, the study will explore preferences for incentive-based policies that consider both the benefits and risks

associated with the import and sale of non-native plants.

Furthermore, my research will provide information

about how different stakeholder groups perceive the invasive alien plant problem. Thus, the study may potentially

expose points of convergence between different stakeholder

groups with respect to preferences for the various policy

options. Thus, this study may help decision makers to isolate policies with high acceptability across many segments

of society.

LITERATURE CITED

Baskin, Y. 2002. The greening of horticulture: New codes 

of conduct aim to curb plant invasions. Bioscience.

52(6): 464–471. 

Haber, E. 2002. Spread and impact of alien plants across 

Canadian landscapes. In: Claudi, R.; Nantel, P.;

Muckle-Jeffs, E., eds. Alien invaders in Canada's

waters, wetlands, and forests. Ottawa: Canadian Forest

Service, Natural Resources Canada. 320 p.

Knowler, D.; Barbier, E. 2005. Importing exotic plants 

and the risk of invasion: Are market-based instruments

adequate? Ecological Economics. 52(3): 341–354.

Perrings, C.; Williamson, M.; Barbier, E.B.; Delfino, 

D.; Dalmazzone, S.; Shogren, J.; Simmons, P.;

Watkinson, A. 2002. Biological invasion risks and the

public good: an economic perspective. Conservation

Ecology. 6(1): 1. [online] URL: http://www.consecol.

org/vol6/iss1/art1/.

Pimentel, D.; Zuniga, R.; Morrison, D. 2005. Update on 

the environmental and economic costs associated with

alien-invasive species in the United States. Ecological

Economics. 52(3): 273–288.

Reichard, S. 2005. The tragedy of the commons revisited: 

Invasive species. Frontiers in Ecology and the

Environment. 3(2): 103–109.

Simberloff, D.; Parker, I.M.; Windle, P.N. 2005.

Introduced species policy, management, and future

research needs. Frontiers in Ecology and the

Environment. 3(1): 12–20.

Wilcove, D.S.; Rothstein, D.; Dubow, J.; Phillips, A.; 

Losos, E. 1998. Quantifying threats to imperiled

species in the United States. Bioscience. 48(8): 607.

166

Table 1––Policy options presented to survey respondents

Policy options

Option 1 Option 2 Option 3 Option 4 Option 5

Import & sale Black list and Screen and ban Screen and ban Screen and Fixed

ban all species species with species with a variable tax environmental

listed a high likelihood high likelihood fee

of invasion of invasion

Implement & enforce Mandatory Mandatory Voluntary Mandatory Mandatory
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