Supporting Organizations

Arizona Association of Environmental Professionals

Arizona Department of Agriculture

Arizona Department of Environmental Quality

Arizona Department of Transportation

Arizona Department of Water Resources

Arizona Game and Fish Department

Arizona Native Plant Society

Arizona-Sonora Desert Museum

Arizona State Land Department

Arizona State Parks

Bureau of Indian Affairs

Bureau of Land Management

Bureau of Reclamation

National Park Service

Sonoran Institute

Southwest Vegetation Management Association

The Nature Conservancy in Arizona

University of Arizona Cooperative Extension

US Army Corps of Engineers

US Department of Defense

US Environmental Protection Agency

US Fish and Wildlife Service

USDA Forest Service

US Geological Survey

The appearance of an organization’s name above indicates that it: 

(1) recognizes that the list was developed in a transparent, unbiased, and 

scientific manner; (2) supports the conclusions reached during the Arizona

Wildlands Invasive Plant Working Group’s (AZ-WIPWG) listing and oversight process as reflected by the plants contained in the resultant list and

their categorization; and (3) intends to use and internally promote the list

to guide day-to-day management activities in accordance with its particular organizational mission and responsibilities.

2
Purpose 

Invasive non-native plants, especially those that alter ecological processes such as fire and hydrologic regimes, are a significant threat to Arizona’s wildlands. Federal and state noxious weed lists, however, are concerned primarily with agricultural pests that in some cases are not yet in the state. As a

result, these regulatory lists do not provide land managers and

other stakeholders with a complete picture of those non-native

plants that can impact native species, plant and animal communities, and ecosystems. What is needed is objective information that identifies and distinguishes those non-native

plants that can invade an area and cause adverse ecological

impacts from those non-native plants that, at least based on

current knowledge, are relatively innocuous.

Approach

To address the above need, over 20 federal and state agencies,

academic institutions, and private conservation, professional,

and commercial interests from across Arizona joined together

to form the Arizona Wildlands Invasive Plant Working Group

(AZ-WIPWG). The AZ-WIPWG was truly an example of

multiple organizations partnering and sharing resources to

accomplish a mutual goal. Over a two and a half-year period,

the AZ-WIPWG developed the enclosed categorized list of

invasive non-native plants that threaten Arizona’s wildlands.

This non-regulatory list was constructed through the application of a regionally developed assessment protocol, Criteria

for Categorizing Invasive Non-Native Plants that Threaten

Wildlands, released in February of 2003 (the Criteria). The

AZ-WIPWG implemented a comprehensive review and oversight process that ensured a consistent and objective evaluation of each non-native plant considered. To be evaluated a

non-native plant species had to be established in Arizona’s

wildlands (that is, outside of human cultivation and management).

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Categorized List

High: These species have severe ecological impacts on

ecosystems, plant and animal communities, and vegetational

structure; invasiveness attributes are conducive to moderate to

high rates of dispersal and establishment; and species are usually widely distributed, both among and within ecosystems

/communities. See page 9 for annotations.

Plants Ranked High (19)

• Acroptilon repens (Russian knapweed)

• Arundo donax (Giant reed)

• Bromus rubens (Red brome)

• Bromus tectorum (Cheatgrass)

• Centaurea solstitialis (Yellow starthistle)

• Eichhornia crassipes (Water hyacinth)

• Elaeagnus angustifolia (Russian olive)

• Eragrostis lehmanniana (Lehmann lovegrass)

• Euphorbia esula (Leafy spurge)

• Euryops multifidus (Sweet resinbush)

• Lepidum latifolium (Perennial pepperweed)

• Myriophyllum aquaticum (Parrot’s feather)

• Myriophyllum spicatum (Eurasian watermilfoil)

• Pennisetum ciliare (Buffelgrass)

• Pennisetum setaceum (Fountain grass)

• Salvina molesta (Giant salvinia)

• Tamarix chinensis (Fivestamen tamarisk)

• Tamarix parviflora (Smallflower tamarisk)

• Tamarix ramosissima (Saltcedar)

4

Elaeagnus angustifolia (Russian olive). Photo by 

John M. Randall, The Nature Conservancy
Medium: These species have substantial and apparent ecological impacts on ecosystems, plant and animal communities,

and vegetational structure; invasiveness attributes are conducive to moderate to high rates of dispersal, often enhanced

by disturbance; and ecological amplitude (diversity of ecosystems/communities) and distribution (within an

ecosystem/community) range from limited to widespread.

Plants Ranked Medium (40)

• Alhagi maurorum (Camelthorn)

• Avena fatua (Wild oat)

• Brassica tournefortii (Sahara mustard)

• Bromus diandrus (Ripgut brome)

• Bromus inermis (Smooth brome)

• Cardaria chalapensis (Lenspod whitetop)

• Cardaria draba (Whitetop)

• Cardaria pubescens (Hairy whitetop)

• Carduus nutans (Musk thistle)

• Centaurea biebersteinii (Spotted knapweed)

• Centaurea diffusa (Diffuse knapweed)

• Centaurea melitensis (Malta starthistle)

• Chondrilla juncea (Rush skeletonweed)

• Cirsium arvense (Canada thistle)

• Conium maculatum (Poison hemlock)

• Convolvulus arvensis (Field bindweed)

• Cortaderia selloana (Pampas grass)

• Cynodon dactylon (Bermudagrass)

• Erodium cicutarium (Redstem filaree)

• Hordeum murinum (Mouse barley)

• Linaria dalmatica (Dalmatian toadflax)

• Linaria vulgaris (Yellow toadflax)

• Lolium perenne (Perennial ryegrass)

• Melilotus alba (White sweetclover)

• Melilotus officinalis (Yellow sweetclover)

• Mesembryanthemum nodiflorum (Slenderleaf iceplant)

• Rhus lancea (African sumac)

• Rubus armeniacus (Himalayan blackberry)

• Rubus discolor (Himalayan blackberry)

• Saccharum ravennae (Ravennagrass)

5
• Salsola collina (Slender Russian thistle)

• Salsola paulsenii (Barbwire Russian thistle)

• Salsola tragus (Prickly Russian thistle)

• Schismus arabicus (Arabian schismus)

• Schismus barbatus (Common Mediterranean grass)

• Sonchus asper (Spiny sowthistle)

• Sonchus oleraceus (Annual sowthistle)

• Sorghum halepense (Johnsongrass)

• Ulmus pumila (Siberian elm)

• Vinca major (Bigleaf periwinkle)

6

Cardaria draba (Whitetop). Photo by 

John M. Randall, The Nature Conservancy

Centaurea melitensis (Malta starthistle). Photo by 

John M. Randall, The Nature Conservancy

Brassica tournefortii (Sahara mustard).

Provided by U.S. Fish and Wildlife

Service 
Low: These species have minor yet detectable ecological

impacts; invasiveness attributes result in low to moderate rates

of invasion; ecological amplitude and distribution are generally limited, but the species can be problematic locally.

Plants Ranked Low (12)

• Aegilops cylindrica (Jointed goatgrass)

• Asphodelus fistulosus (Onionweed)

• Cirsium vulgare (Bull thistle)

• Cynoglossum officinale (Houndstongue)

• Echinochloa crus-galli (Barnyardgrass)

• Elymus repens (Quackgrass)

• Eragrostis curvula (Weeping lovegrass)

• Leucanthemum vulgare (Oxeye daisy)

• Mesembryanthemum crystallinum (Common iceplant)

• Onopordum acanthium (Scotch thistle)

• Panicum antidotale (Blue panicum)

• Tamarix aphylla (Athel tamarisk)

7

Asphodelus fistulosus (Onionweed). Photo by 

John M. Randall, The Nature Conservancy
Evaluated but not listed: Designation when current

information is inadequate to respond to the Criteria questions

or sum effects of ecological impacts, invasiveness, and ecological amplitude and distribution are below the threshold for 

listing.

Plants Evaluated but not listed (3)

• Hydrilla verticillata (Hydrilla)

• Tribulus terrestris (Puncturevine)

• Verbascum thapsus (Common mullein)

Alert: Additional designation for some species in either the

high or medium category, but whose current ecological amplitude and distribution are limited. This designation alerts site

managers to species capable of invading unexploited natural

communities, based on initial, localized observations or

behavior in similar ecosystems/communities elsewhere.

Plants with an Alert Designation (19)

• Bromus diandrus (Ripgut brome)

• Cardaria chalapensis (Lenspod whitetop)

• Cardaria draba (Whitetop)

• Cardaria pubescens (Hairy whitetop)

• Chondrilla juncea (Rush skeletonweed)

• Conium maculatum (Poison hemlock)

• Eichhornia crassipes (Water hyacinth)

• Euphorbia esula (Leafy spurge)

• Lepidum latifolium (Perennial pepperweed)

• Linaria vulgaris (Yellow toadflax)

• Mesembryanthemum nodiflorum (Slenderleaf iceplant)

• Myriophyllum aquaticum (Parrot’s feather)

• Myriophyllum spicatum (Eurasian watermilfoil)

• Rhus lancea (African sumac)

• Rubus armeniacus (Himalayan blackberry)

• Rubus discolor (Himalayan blackberry)

• Saccharum ravennae (Ravennagrass)

• Salvina molesta (Giant salvinia)

• Vinca major (Bigleaf periwinkle)

8
Red Flag : Additional designation assigned by the

AZWIPWG to identify and document a critical piece of information not evident in the overall ranking. 

Red Flag Annotations (21)

Aegilops cylindrica—Above 1,220 meters (4,000 feet) elevation, Aegilops cylindrica can replace native herbaceous and

shrub vegetation subsequent to its removal on highly disturbed soil surfaces. Aegilops cylindrica infestations alter natural fire regimes during the summer months when wildfires

are most likely to occur by increasing fine-fuel loads relative

to native vegetation. Roadside populations of A. cylindrica

connect rights-of way that serve as fire corridors to wildlands

and, as a result, increase the risk of wildfires in the wildlandurban interface. Because A. cylindrica can occur as a contaminant in revegetation seed lots, seed mixes should be checked

for the presence of this species.

Brassica tournefortii—Abundant rainfall during the latter

part of 2004 and early 2005 resulted in an undocumented

response by Brassica tournefortii in terms of number of individuals and total biomass. These increases potentially contributed to the altered fire regimes (that is, increased number

and areal extent of fires) that occurred in Arizona at lower elevations during 2005. Should these trends persist in future

years, then the scores and rank reported here for B. tournefortii should be revisited.

Bromus inermis—Bromus inermis should not be used for

reclamation purposes in wildlands because of its persistence

and invasive potential.

Centaurea biebersteinii—Centaurea biebersteinii likely has

not yet reached its full invasive potential in Arizona. Its ecological impacts and reproductive capacity are well documented in other states, especially in Montana. Centaurea biebersteinii has great potential to increase its abundance and areal

extent in Arizona on sites that are subjected to fire suppression, mechanical fuel treatment (that is, thinning), or timber

harvest activities on public lands.

9
Cirsium arvense—Cirsium arvense has been observed in a

variety of ecosystems/plant communities across Arizona and

in even more ecological types in other states, but it currently

has few occurrences within any specific ecological type in

Arizona. Above elevations of 1,525 meters (5,000 feet), C.

arvense has a high potential to invade many ecological types.

It may not have had, however, enough time or opportunity to

exploit these types. Because this plant is extremely difficult

to control, land managers currently without infestations may

want to consider this plant as a priority for early detection and

monitor accordingly.

Cortaderia selloana—Cortaderia selloana is widely sold as

both a live plant or seed in Arizona and on the internet. It

also is promoted as a low water-use plant in Arizona. As a

relatively new plant to Arizona, C. selloana has only started to

appear in wildlands. Based on the species’ broad ecological

amplitude, it potentially can become as problematic in

Arizona as it now is in California and other places. At present

C. selloana exists only in small patches in the state; however,

plenty of unoccupied niches, such as riparian corridors, are

available to this species to invade.

Eichhornia crassipes—At present no wildland aquatic

ecosystems within Arizona are known to be infested with

Eichhornia crassipes. Records at the Arizona Department of

Agriculture, however, indicate several small (< 0.4 hectares [1

acre]) populations have been discovered and eradicated from

Arizona wildland streams, park ponds, and irrigation tailwater pits during the past 20 years. Eichhornia crassipes is

listed as a regulated and restricted noxious weed in Arizona.

Eragrostis curvula—This assessment does not pertain to

Eragrostis curvula var. conferta (Boer lovegrass). This taxon

has different moisture and temperature limits relative to the

species as whole and likely behaves differently in regard to its

ecological impacts, invasiveness, and ecological amplitude.

Eragrostis curvula var. conferta as a valid taxon is ambiguous

as the U.S. Department of Agriculture Plants Database regards

it as a synonym of E. curvula. Because of the differences in

environmental tolerances and ploidy between E. c. var. con10
ferta and the species as a whole, for the purposes of this list

E. c. var. conferta is considered a separate taxon and is not

evaluated as part of E. curvula.

Euryops multifidus—Only about 10 known populations of

Euryops multifidus occur in Arizona. Those populations have

been mapped and most locations have active control efforts.

Vegetation survey projects should be aware that undocumented populations may exist on historic Civilian Conservation

Corps project sites.

Hordeum murinum—Some authorities recognize Hordeum

glaucum, H. leporinum, and H. murinum as separate species;

however, based on the use of the U.S. Department of

Agriculture Plants Database as the authority for reconciling

taxonomic questions, H. murinum is recognized herein as the

valid species taxon and H. m. glaucum, H. m. leporinum, and

H. m. murinum are recogized as subspecies.

Hydrilla verticillata—Although based on its question and section scores related to Impact and Invasiveness Hydrilla verticillata potentially could have been ranked as a High, Alert

taxon, it was assigned an Evaluated but not listed designation

to reflect its current distribution status: present in the state

but only in human-constructed water bodies. If inadvertently

introduced into natural, low-elevation water bodies in

Arizona, H. verticillata easily could establish and flourish in

Arizona’s wildlands.

Lepidium latifolium—Lepidium latifolium is not widely distributed in Arizona. Established populations occur mostly

near the northern borders of the state. Land managers should

be on the alert for isolated plants or small nascent populations

that can be eradicated before they can spread. Lepidium latifolium is a difficult species to eradicate so addressing infestations while they are small is critical.

Linaria dalmatica—Linaria dalmatica occurs within a variety

of ecosystems/plant communities that experience different

natural fire regimes. Linaria dalmatica, however, established

in these various ecological types after the onset of habitat

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alteration and fire exclusion that characterizes these types

today. Because L. dalmatica was not present when historic

(natural) fire regimes were functioning, it is unclear how the

presence of L. dalmatica might affect the ability to restore a

natural fire regime. Little empirical evidence exists to enable

anticipating these potential effects. The expanding wildlandurban interface and projected increases in the intensity of forest restoration/fuel treatments may provide new opportunities

for L. dalmatica to spread into forested areas. Only a portion

of L. dalmatica seeds may germinate in any given year. As a

result, dormant seeds may germinate at sites following herbicide applications or other site disturbances that reduce native

plant competition.

Melilotus spp. (M. alba, M. officinalis)—Melilotus spp. is

invasive in a number of ecosystems/plant communities in

Arizona. Melilotus spp. also may be used, however, in semiarid habitats in northern Arizona for reclamation purposes

where it has been difficult to reestablish native species after

disturbances such as fire. Once suitable native alternatives

can be identified and successfully restored in these areas, use

of Melilotus spp. for reclamation purposes should be discontinued.

Pennisetum ciliare—At present Pennisetum ciliare is only

occasionally observed in semi-desert grasslands and

Chihuahuan desertscrub and has not been observed in southwestern interior chaparral scrub, and Madrean evergreen

woodland. Invasion into these “cooler” ecological types

could increase or begin if the new cold-tolerant cultivar “Frio”

is released into Arizona. Continued development of cold tolerance or drought tolerance in P. ciliare cultivars poses a significant ecological threat if such cultivars are released into

Arizona wildlands.

Pennisetum setaceum—Although Pennisetum setaceum is

established in a number of ecosystems/plant communities, it is

not yet present in many individual occurrences of these types.

Large areas of suitable wildland habitat still remain for this

species to colonize. The misnamed Pennisetum setaceum

“Rubrum” (with dark purplish foliage and purplish crimson

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spikes) or P. setaceum purple-type is actually a distinct

species, P. advena. Pennisetum advena is sold commercially

in Arizona as an ornamental but reportedly does not reproduce

reliably from seed and, as a result, was not evaluated.

Tamarix aphylla—Tamarix aphylla currently has a limited

distribution within Arizona wildlands even though many thousands of populations are present in agricultural and urban

areas of southwestern Arizona. The species was introduced to

provide windbreaks for homesteads. Until recently seeds

were thought to be sterile and the only means of spread into

wildlands was via vegetative reproduction. It is now known

that T. aphylla can hybridize with other Tamarix spp. One

documented occurrence of this is along the Gila River in

western Maricopa County. It is unclear at this point what the

morphology, physiology, reproduction by seed, and invasiveness of the hybrids will be, as well as the attributes of any

subsequent backcross progeny. 

Tamarix spp. (T. chinensis, T. parviflora, T. ramosissima)—

The ecological impacts associated with invasion by Tamarix

spp. should be considered within the context of the specific

riparian community invaded. In addition, such impacts may

be mediated by previous changes to a variety of ecological

processes associated with the particular riparian community.

Land managers planning riparian restoration projects involving the control of Tamarix spp. should consider and address,

as appropriate, other factors, such as existing hydrologic

regimes, fluvial processes, and whether Tamarix spp. stands

are providing habitat for southwestern willow flycatchers

(Empidonax traillii extimus) before proceeding with such

projects.

13
Applications

Some of the intended uses of the list are to: (1) be a tool for

land managers to assist in setting priorities, allocating

resources, and developing and justifying management strategies; (2) educate a variety of stakeholders about the ecological

impacts and distribution of specific non-native plants; and (3)

modify public and industry behavior regarding particular

plants that adversely impact wildlands and may be sold commercially. The AZ-WIPWG recognizes that the list should be

updated periodically as new information is acquired about a

currently listed species’ behavior in wildlands or as additional

non-native plants become established in Arizona’s wildlands.

Usage

Based on the use of the Criteria as currently conceived, the

ranking categories are designed to reflect the degree of ecological impact a particular plant species is having on a statewide basis. Some species, however, potentially threaten wildlands in only a portion of Arizona. In other geographic

locales, they may not be able to establish and survive outside

human cultivation. The answers to the Criteria question

regarding ecological amplitude indicate those ecosystems/

plant communities occurring within Arizona within which a

species likely will have an ecological impact based on current

information. These ecosystems/plant communities often have

a close correspondence with geographic location. Although a

lack of information on impacts does not rule out that a particular plant may be problematic in other locales in the state, it

might suggest geographic areas where such a plant may not be

of ecological concern. 

Plants sold commercially and on the list may behave in the

manner described above. Two approaches are possible relative to the use of the list in these situations. First, plant taxa

presumed to be non-invasive in particular geographic areas of

Arizona might still be used. Because, however, the Criteria

and its application for Arizona were not designed to provide

positive recommendations on where (or within which ecosystems/plant communities) particular plants might be non-invasive, each commercially sold plant should be assessed on a

14
case-by-case basis and used with caution. Future modifications to the Criteria that enable plant assessments on a bioregional basis may improve the above determination. Second,

ecosystems/plant communities within which invasion does not

occur by a particular plant may occur in close proximity to

ecosystems/plant communities that are invaded and impacted

by the same plant. Moreover, plant materials sold in one

location may be used in another inappropriate location. It

would be difficult to manage all the potential pathways for

moving a plant within the state to inappropriate locations. As

a result, a conservative approach to this situation would be to

err on the side of caution and not sell the plant within

Arizona. Because the categorized list of Invasive Non-Native

Plants that Threaten Wildlands in Arizona is a non-regulatory

list, the AZ-WIPWG cannot dictate which of the two preceding approaches should be followed. To the extent that noninvasive plant alternatives can be identified and promoted for

any of the commercially sold plants on the list that may fit

under this situation, the ideal situation is that industry itself

will eventually have sufficient incentive and motive not to sell

listed plants anywhere within the state.

15

Linaria vulgaris (Yellow toadflax). Photo by 

John M. Randall, The Nature Conservancy
16
17
18
19
20
21
Definitions

Section Scores (Ecological Impacts, Invasiveness,

Distribution)

Section scores can range from A to D. In some cases U or

Unknown is used when insufficient information is available to

assign a score. Section scores are based on scores (A to D, or

U) assigned to individual questions within each section. For

Ecological Impacts, the scores represent a range of severity

with A assigned for the most severe impacts and D assigned

for a negligible impact. For Invasiveness, A represents the

greatest potential to invade an ecosystem/community, whereas

D would indicate a low potential. For Distribution, A indicates that the species has a wide ecological amplitude, is

widespread within particular ecosystems/communities, or

both. A D score would indicate the converse.

Other Useful Definitions

Invasive non-native plants that threaten wildlands: Plants

that: (1) are not native to, yet can spread into, the wildland

ecosystems under consideration; and that also (2) do any of

the following within wildland ecosystems—displace native

species, hybridize with native species, alter biological communities, or alter ecosystem processes.

Non-native plants: Species introduced to the ecosystems

under consideration [here in reference to Arizona] after

European contact and as a direct or indirect result of human

activity.

Wildlands: Public and private lands [and waters] that support native ecosystems, including national, state, and local

parks and forests, ecological reserves, wildlife areas, Bureau

of Land Management lands, and so on. Working landscapes—such as grazed rangeland and active timberlands—

that support native ecosystems are included in the definition.

22
Acknowledgments

Over 20 stakeholder organizations contributed staff time and

other logistical support to make this project possible. The

Southwest Vegetation Management Association sponsored the

project, and The Nature Conservancy in Arizona secured

funding to support and coordinate the efforts of the Arizona

Wildlands Invasive Plant Working Group through both private—including Packard Foundation, Fear Not Foundation,

and the Conservancy’s Invasive Species Initiative—and public—Department of Defense Legacy Resource Management

Program—sources. The U.S. Geological Survey provided

website services and hosts all project materials. The Bureau

of Reclamation provided graphics support, and U.S. Fish and

Wildlife Service covered initial printing costs for this

brochure.

For More Information

Additional pdf copies of this booklet are available at the

Southwest Vegetation Management Association website

(http://www.swvma.org/) and U.S. Geological Survey’s

Southwest Exotic Plant Information Clearinghouse

(SWEPIC) website (http://www.usgs.nau.edu/swepic). Go

to the “AZ-WIP” link. Individual plant assessment information, as well as additional information about the AZ-WIPWG

and the Criteria, also are posted at the SWEPIC website.

Cover Photos: Sorghum halepense (Johnsongrass) by John M.

Randall, The Nature Conservancy. 

Pennisetum ciliare (Buffelgrass) by John M. Randall, The

Nature Conservancy.

Salvinia molesta + Tamarix spp. by Theresa Olson, Bureau of

Reclamation. 

Mesembryanthemum nodiflorum (Slenderleaf ice plant) by

Barry A. Rice, The Nature Conservancy.

23