Eastern Hemlock 

Forests: Guidelines to 

Minimize the Impacts of 

Hemlock Woolly Adelgid

United States

Department of

Agriculture

Forest Service

Northeastern Area 

State & Private 

Forestry

Morgantown, WV

NA-TP-03-04
Cover photographs (clockwise from upper left): hemlock woolly 

adelgid (Adelges tsugae) ovisacs on hemlock needles (Michael 

Montgomery, USDA Forest Service), hemlock-shaded stream 

(Jeff Ward, The Connecticut Agricultural Experiment Station), 

and black-throated green warbler (©Mike Hopiak, Cornell 

Laboratory of Ornithology).

Information about pesticides appears in this publication. 

Publication of this information does not constitute endorsement 

or recommendation by the U.S. Department of Agriculture, nor 

does it imply that all uses discussed have been registered. Use of 

most pesticides is regulated by State and Federal law. Applicable 

regulations must be obtained from appropriate regulatory 

agencies.

CAUTION: Pesticides can be injurious to humans, domestic 

animals, desirable plants, and fish or other wildlife if not handled 

or applied properly. Use all pesticides selectively and carefully. 

Follow recommended practices given on the label for use and 

disposal of pesticides and pesticide containers.

The United States Department of Agriculture (USDA) prohibits 

discrimination in all its programs and activities based on race, 

color, national origin, gender, religion, age, disability, political 

beliefs, sexual orientation, and marital or familial status. (Not all 

prohibited bases apply to all programs.) Persons with disabilities 

who require alternative means for communication of program 

information (Braille, large print, audiotape, etc.) should contact 

USDA’s TARGET Center at 202-720-2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, 

Office of Civil Rights, Room 326-W, Whitten Building, 14th and 

Independence Avenue SW, Washington, DC 20250-9410 or call 

202-720-5964 (voice or TDD). USDA is an equal employment 

opportunity provider and employer.
Eastern Hemlock Forests: 

Guidelines to Minimize the Impacts 

of Hemlock Woolly Adelgid

Jeffrey S. Ward1, Michael E. Montgomery2, Carole A.S.-J. Cheah1, 

Brad P. Onken3, and Richard S. Cowles1

1

TheConnecticut Agricultural Experiment Station, 

New Haven, Connecticut

2

USDA Forest Service, Northeastern Research Station, 

Hamden, Connecticut

3

USDA Forest Service, Northeastern Area 

State and Private Forestry, Morgantown, West Virginia

MAY 2004
ii Rehabilitation Guidelines for Hemlock Stands 

Acknowledgments

We are indebted to the participants of the Hemlock Forest Ecosystem Best Management Practices 

Workshop held in Haddam, Connecticut, in November 1994, for the conceptual framework of 

restoring damaged hemlock stands. Critical feedback on an early draft was provided by participants 

of the Hemlock Stand Management Workshop held at the Delaware Water Gap National Recreation 

Area in November 2003. Valuable insights were also gained from the contributors to the Symposium 

on Sustainable Management of Hemlock Ecosystems in Eastern North America held in Durham, New 

Hampshire, in June 1999, and the Hemlock Woolly Adelgid in the Eastern United States Symposium 

held in East Brunswick, New Jersey, in February 2002. We thank Kurt Gottschalk of the USDA 

Forest Service Northeastern Research Station and Larry Rousseau of the Connecticut Department 

of Environmental Protection-Division of Forestry for their constructive criticism and insights in 

improving this handbook. Lastly, we would like to thank Roberta Burzynski and Sandra Fosbroke of 

the USDA Forest Service Northeastern Area State and Private Forestry for editing this manuscript.

Funding for this research was provided, in part, by a grant from the USDA Forest Service, 

Northeastern Research Station No. 02-CA-11242343-085 to The Connecticut Agricultural 

Experiment Station.

Photo Credits: 

(t=top photo, m=middle photo, b=bottom photo, L=left photo, r=right photo)

USDA Forest Service: Vincent D’Amico (4b), Karen Felton (1L, 8b, 23), Lynn Jones (15t), Mike 

Montgomery (1m, 2t, 4t, 5m, 5b, 13, 19r), Brad Onken (1r, 8t), Ann Steketee (6) 

The Connecticut Agricultural Experiment Station: J.P. Barsky (3b, 17b, 18b), Carol Cheah (5t, 7t, 

12), Rich Cowles (14), Jeff Ward (2b, 7b, 10, 11, 15b, 16, 17m, 18t, 20, 21, 22) 

Connecticut Department of Environmental Protection, Division of Forestry: 9, 17t, 19L 

Cornell Laboratory of Ornithology: ©G.W. Lasley (3t)
Rehabilitation Guidelines for Hemlock Stands iii

Contents

Introduction.......................................................................................................................................... 1

Part 1. Biology and Ecology ............................................................................................................... 2

Hemlock in the Eastern United States ................................................................................................... 2

Habitat ............................................................................................................................................. 2

Ecological importance ..................................................................................................................... 2

Commercial value............................................................................................................................ 3

Hemlock Woolly Adelgid....................................................................................................................... 3

Life cycle ......................................................................................................................................... 4

Dispersal .......................................................................................................................................... 5

Damage to Hemlock by the Adelgid...................................................................................................... 7

Part 2. Guidelines for Managers........................................................................................................ 9

Planning and Monitoring ....................................................................................................................... 9

Prioritizing hemlock stands............................................................................................................. 9

Rehabilitation planning.................................................................................................................. 11

Monitoring adelgid populations..................................................................................................... 11

Monitoring infested stands ............................................................................................................ 12

Controlling Hemlock Woolly Adelgid ................................................................................................. 13

Cultural controls............................................................................................................................ 13

Chemical controls.......................................................................................................................... 13

Biological controls......................................................................................................................... 14

Managing Salvage Harvests ................................................................................................................ 15

Best management practices ........................................................................................................... 16

Before the harvest.......................................................................................................................... 17

During the harvest ......................................................................................................................... 18

After the harvest ............................................................................................................................ 18

Rehabilitation....................................................................................................................................... 19

Species selection............................................................................................................................ 20

Planting operations........................................................................................................................ 21

Reintroducing hemlock.................................................................................................................. 22

Summary............................................................................................................................................. 23

References........................................................................................................................................... 24
Rehabilitation Guidelines for Hemlock Stands 1

The primary purpose of this handbook 

is to help resource managers make informed 

decisions in preparing management plans and 

to provide an information bridge between 

managers with hemlock stands that are, or 

will be, infested with hemlock woolly adelgid, 

and the foresters who will implement salvage 

and rehabilitation activities. This handbook 

summarizes current scientifi c knowledge of the 

impact of hemlock woolly adelgid on eastern 

hemlock forests and the processes involved in 

rehabilitating hemlock stands that have been, or 

will be, damaged by hemlock woolly adelgid. 

Part 1 of this 

handbook presents the 

biology and ecology 

of eastern hemlock 

and hemlock woolly adelgid. Part 2 provides 

guidelines to help managers make decisions 

regarding the salvage and rehabilitation of 

damaged hemlock stands in order to recover, to 

the extent possible, the ecological and esthetic 

attributes associated with hemlock forests.

Introduction

Eastern hemlock (Tsuga canadensis) is the 

most shade-tolerant and long-lived tree species 

in Eastern North America. Its unsurpassed 

ability to tolerate low light enables it to form 

dense canopies and stands that provide a unique 

habitat for many plant and wildlife species. 

Carolina hemlock (Tsuga caroliniana) is a relic 

species limited to a small area in the southern 

range of eastern hemlock. Both species are 

being threatened by the accidentally introduced 

hemlock woolly 

adelgid (Adelges 

tsugae).

Control of 

the hemlock 

woolly adelgid 

is a challenge. 

An unusual life 

cycle, presence of 

susceptible hosts, 

and a lack of natural 

enemies contribute 

to explosive 

population increases 

and rapid spread of 

the adelgid. Until 

effective controls 

are discovered, 

it is anticipated 

that the adelgid 

will continue to spread in eastern forests 

and mortality of hemlock will increase. 

Decline of eastern hemlock will lead to a 

loss of habitat diversity and a decrease in 

the esthetic beauty in many of our forests 

as hemlock is replaced by hardwood 

species such as black birch (Betula lenta) and 

red maple (Acer rubrum). Because hemlocks 

can survive for several years after initial 

infestation by hemlock woolly adelgid, forest 

managers have time to take steps to minimize 

its impact. 

Healthy hemlock. Dead hemlock.

Hemlock woolly adelgid 

ovisacs.
2 Rehabilitation Guidelines for Hemlock Stands 

Part 1. Biology and Ecology

Hemlock in the Eastern United States

Habitat

Eastern hemlock is an important component 

of northeastern and Appalachian forests, often 

forming nearly pure stands. It grows on almost 

19 million acres of forest in the Eastern United 

States (Schmidt and McWilliams 1996) and 

is the predominant species on 2.3 million 

acres (McWilliams and Schmidt 2000). The 

range of eastern hemlock extends from Nova 

Scotia to Georgia and westward to Minnesota. 

The other hemlock growing in the East, 

Carolina hemlock, is limited to the Blue Ridge 

Mountains of the Southern Appalachians. 

Eastern hemlock is a large, majestic tree 

that can live for 800 years or more (Godman 

and Lancaster 1990). A mature tree reaching 

over 175 feet tall with a diameter of more than 

6 feet is among the largest specimens recorded. 

Although similar 

in appearance, 

mature Carolina 

hemlock is 

typically smaller, 

reaching heights 

of 55 feet and 

2 to 3 feet in 

diameter on 

larger specimens.

Eastern 

hemlock is the 

most shadetolerant tree 

species in the 

Eastern United States and is able to survive in 

the understory with as little as 5 percent of full 

sunlight (Godman and Lancaster 1990). This 

ability to survive in low light helps explain 

the deep canopy of hemlock that often extends 

almost to the forest fl oor. 

Maximum rates of photosynthate storage 

in hemlocks occur on mild days during the 

dormant period 

(winter) when 

hardwood trees 

have shed their 

leaves (Hadley 

and Schedlbauer 

2002). This 

adaptation 

enhances 

the ability of 

hemlock to 

develop under 

deciduous 

hardwoods. 

Eastern 

hemlock can be 

found on a wide 

range of sites 

from ridgetops 

to swamps. Like all hemlock species, it is 

intolerant of drought and typically occurs where 

soil is constantly moist 

(Farjon 1990). Although 

its soil requirements are 

not exacting, it occurs 

characteristically on moist 

to very moist, acidic 

soils with good drainage. 

Eastern hemlock is found 

on most topographic 

positions in the northern 

part of its range. It is 

restricted largely to coves 

and north- and eastfacing lower slopes in the 

Southern Appalachians.

Ecological importance

The dense, evergreen canopy associated 

with mature hemlock forests creates a unique 

environment that is a critical habitat for many 

animal and plant species. The deep, dense 

Hemlocks add texture to the landscape.

Hemlock foliage often 

extends to the ground.
Rehabilitation Guidelines for Hemlock Stands 3

canopy of eastern hemlock increases vertical 

structure heterogeneity in forests, and more 

than 120 vertebrate species utilize mature 

stands (DeGraaf and others 1992). Hemlock 

forests also 

provide thermal 

cover and forage 

for a variety 

of mammals, 

including 

porcupines 

(Erethizon 

dorsatum) and 

white-tailed deer 

(Odocoileus 

virginianus) 

(Reay and others 

1990, Wydeven 

and Hay 1996). 

Nearly 90 species 

of birds can be 

found in hemlock forests (Benzinger 

1994). Several species are signifi cantly 

associated with hemlock forests 

(Tingley and others 2002), including 

the black-throated green warbler 

(Dendroica virens), Blackburnian 

warbler (Dendroica fusca), and Acadian 

fl ycatcher (Empidonax virescens).

A wide variety of aquatic species 

is more likely to be found in streams 

sheltered by hemlock than streams 

sheltered by hardwoods. For example, 

both brook trout (Salvelinus fontinalis) 

populations and macroinvertebrate 

diversity were greater in hemlock 

streams in the Delaware Water Gap National 

Recreation Area by 300 and 37 percent, 

respectively (Evans 2002). Hemlock maintains 

aquatic habitat integrity by regulating 

streamfl ow and moderating water temperature 

(Evans 2002). Hemlock-shaded streams were 

found to have lower summer temperatures and 

were less likely to dry up. 

Commercial value

Although overshadowed by its importance 

in augmenting terrestrial habitat diversity, 

maintaining aquatic habitat integrity, 

and providing esthetic appeal, the 

commercial value of hemlock is 

signifi cant to rural economies. 

Eastern hemlock comprises 22 

percent of the total volume of 

softwood growing stock in the 

Northeast (Powell and others 1993, 

table 18). Harvested trees are used 

for products as diverse as pulp and 

paper, lumber, and mulch. Hemlock 

was estimated to be 7 percent of the 

sawlog and 12 percent of the annual 

pulpwood harvest in New England 

in 1999 (McWilliams and Schmidt 

2000, page 9).

Hemlock Woolly Adelgid

The hemlock woolly adelgid is tiny, less 

than 1.5 millimeters (1/16 inch) long, and varies 

from dark reddish-brown to purplish-black in 

color. Except for the newly hatched nymphs, it 

has a covering of wool-like wax fi laments that 

it produces to protect itself and its eggs from 

desiccation and natural enemies. This “woolly” 

wax is a characteristic of all adelgids and is 

Acadian fl ycatcher.

Hemlocks are a valuable commodity in some areas.
4 Rehabilitation Guidelines for Hemlock Stands 

most conspicuous 

when the adelgid is 

mature and laying 

eggs. The best time 

to see these woolly 

“ovisacs” (see cover) 

is from late fall to 

early summer. 

The adelgid is 

an unusual insect 

that feeds during 

the cooler part of 

the year and is 

inactive during the 

hot summer months. 

By feeding during 

above-freezing periods in late fall, winter, and 

early spring, the hemlock woolly adelgid has 

access to photosynthate and other nutrients that 

are abundant in the hemlock twigs then, and 

also avoids generalist natural enemies that are 

inactive at this time. Evidence of the nutritional 

benefi t of feeding during “leaf off’ is that this 

overwintering generation lays about twice as 

many eggs as the following spring generation.

Life Cycle

The life cycle 

of hemlock woolly 

adelgid in Eastern North 

America consists of two 

complete parthenogenetic 

(all-female) wingless 

generations on hemlock: 

the sistens (winter 

generation), whose 

development spans 9 to 

10 months from early 

summer to midspring of 

the following year, and 

the progrediens (spring 

generation), which 

occurs from spring to 

early summer. These 

generations overlap in 

middle to late spring 

(McClure 1989) and the 

exact time of occurrence 

of each phase of the 

life cycle is infl uenced 

by local climate. 

Generally, egg laying is 

well underway before 

the fi rst wildfl owers 

appear in early spring. 

The egg masses, or 

ovisacs, produced by 

the unmated females 

in late winter/early 

spring contain up to 

300 eggs (McClure and others 2001). These 

eggs usually start to hatch in April and May into 

mobile, reddish-brown nymphs that crawl on 

the branches and can be dispersed between trees 

by wind and animals. Most of these “crawlers” 

settle on the twigs produced the previous 

year, the same twigs that their mothers settled 

on 9 months earlier. The nymphs have long 

mouthparts, or stylets, that are inserted at the 

Hemlock woolly adelgid life cycle.

Adult hemlock woolly adelgid and eggs 

in the ovisac.
Rehabilitation Guidelines for Hemlock Stands 5

base of the hemlock 

needles and travel 

to the xylem ray 

parenchyma cells in 

the twig to obtain 

nutrients (Young 

and others 1995). 

They remain at the 

same feeding site on 

the hemlock twig, 

developing through 

four nymphal stages 

before they become 

adults in early June.

These spring generation adults generally 

lay eggs between mid-June and early July 

(McClure 1989), although timing is again 

infl uenced by 

temperature, 

latitude, and 

elevation. The 

number of eggs 

produced (around 

20-75 eggs) is 

smaller than that 

of the winter 

generation. The 

eggs hatch in 

early July and the 

emerging crawlers 

disperse and settle 

preferentially on 

the new shoots of 

hemlock produced 

in June. However, these nymphs do not feed 

and instead enter a summer dormancy period 

called estivation that lasts from July to October. 

The nymphs are inconspicuous at this time, 

producing only a tiny halo of woolly wax 

(above). 

A portion of the spring generation nymphs 

develops into winged adults (sexuparae) that fl y 

from the hemlock in search of a spruce tree to 

begin sexual reproduction. In North America, 

however, a suitable species 

of spruce does not occur 

and their offspring do not 

survive (McClure 1989). 

The relative abundance of 

the winged form is density 

dependent, and a greater 

proportion is produced when 

the health of the host tree is 

declining (McClure 1991a).

When cool temperatures 

begin to prevail in October, 

the nymphs of the winter 

generation break dormancy and swell as they 

begin feeding. The nymphs develop during 

the winter; thus, the life cycle on hemlock is 

completed. This life cycle of two all-female 

generations each year, timed to avoid 

periods when predation would be 

high, makes the hemlock woolly 

adelgid capable of explosive rates 

of population increase, especially 

during the initial stages of infestation 

on healthy hemlocks.

Dispersal

First discovered and described 

in Oregon (Annand 1924), hemlock 

woolly adelgid was initially 

detected in the Eastern United 

States in a private plant collection 

Winged adult hemlock woolly adelgid.

Hemlock woolly adelgid crawler nymphs.

Hemlock woolly adelgid nymphs in 

summer estivation.
6 Rehabilitation Guidelines for Hemlock Stands 

in Richmond, Virginia, in 1951. For several 

decades, the hemlock woolly adelgid was not 

considered a significant pest because it had 

not damaged hemlocks in Japan, Taiwan, and 

Western North America. 

At first, hemlock woolly adelgid spread 

slowly in the Eastern United States. Its 

discovery in southeastern Pennsylvania did not 

occur until 1969. Infestations now range from 

northeastern Georgia to southeastern Maine 

and westward to eastern Tennessee. In the last 

decade, hemlock woolly adelgid populations 

have been expanding rapidly at the southern, 

western, and northern fringes of its range. 

Rates of hemlock woolly adelgid dispersal 

have been estimated at 20 to 30 km per year 

(McClure and others 2001, Morin and others in 

press).

Adelgid eggs and crawlers are easily 

transported by wind, wildlife (especially birds), 

humans (McClure 1990), and infested nursery 

stock (Gibbs 2002, Ouellette 2002). Eggs and 

crawlers of both generations of hemlock woolly 

adelgid generally are present from late March 

through early July in the Northeast; therefore, 

care should be taken when transporting 

hemlock products from infested areas through 

healthy hemlock regions during this period.

The spread of hemlock woolly adelgid 

northward has been more rapid along larger 

watercourses and coastal areas. This may 

reflect the moderating effect of large bodies 

of water on climate. Another factor favoring 

the spread along watercourses may be the 

movement of migratory birds along riparian 

corridors.

Distribution of hemlock (green) with area infested by hemlock woolly adelgid (red).

Uninfested

Infested

(2003)

First Observed

(1951)
Rehabilitation Guidelines for Hemlock Stands 7

Climate may limit the spread of hemlock 

woolly adelgid, or at least slow its spread 

northward. Populations can be dramatically 

reduced by extreme winter temperatures or 

sharp downward temperature fl uctuations 

during abnormally mild winters. Laboratory 

studies of northern fi eld-collected adelgids 

showed that none survived at -30 to -40 oF 

(Parker and others 

1999, Skinner 

and others 2003). 

In the forest, 

hemlock woolly 

adelgid mortality 

rates greater than 

90 percent were 

observed when 

temperatures 

fell below -5 oF 

in Connecticut 

(Cheah and 

McClure 2002). 

Research 

and management 

efforts to slow 

the rate of spread and limit the impacts caused 

by hemlock woolly adelgid are underway, 

but it is unknown to what extent these efforts 

will be successful, particularly in the forest 

environment. Therefore, it is possible that most 

eastern hemlock stands in the Eastern United 

States could become infested within several 

decades, except perhaps for the very coldest 

regions. 

Damage to Hemlock by the Adelgid

Adelgids cause damage by depleting 

the hemlock’s starch reserves, which in turn 

reduces the tree’s ability to grow and produce 

new shoots. While hemlocks in Western 

North America and Eastern Asia are believed 

to be resistant, eastern hemlock and Carolina 

hemlock are very susceptible and can die within 

a few years from unchecked, heavy infestations 

of hemlock woolly adelgid. All age and size 

classes of hemlock are susceptible to adelgid 

infestations. 

Mortality of eastern hemlock is usually a 

slow process, occurring over a 2- to 12-year 

period, or longer, after the initial infestation 

(McClure 2001, Mayer and others 2002, Orwig 

2002). Hemlock decline is characterized by 

a reduction in new shoot 

production in infested parts 

of the crown followed 

by needle drop, branch 

tip dieback, thinning of 

the foliage, lower branch 

dieback, and fi nally tree 

mortality (McClure and 

others 2001). 

Other major factors 

that infl uence the rate 

of hemlock decline are 

stressors such as drought, 

poor site conditions, and Cold may limit the spread of hemlock 

woolly adelgid.

Dying hemlock.
8 Rehabilitation Guidelines for Hemlock Stands 

insect and disease pests—elongate hemlock 

scale (Fiorinia externa), hemlock looper 

(Lambdina fi scellaria fi scellaria), spruce 

spider mite (Oligonychus ununguis), hemlock 

borer (Melanophila fulvogutta), root rot 

disease (Armillaria mellea), and the needlerust 

Melampsora parlowii (Godman and Lancaster 

1990, Souto and Shields 2000). Hemlock 

mortality has been 

most severe in parts 

of Connecticut, New 

Jersey, Virginia, and 

parts of Pennsylvania 

(Orwig and Foster 

1998, Bair 2002, 

Evans 2002, Mayer 

and others 2002, 

Orwig and others 

2002).

As mortality 

occurs, many hemlock 

stands will be replaced 

with deciduous 

species, such as black 

birch and red maple 

(Orwig and Foster 

1998, Orwig 2002). 

The replacement 

of the hemlock 

component of the 

eastern forest by deciduous hardwoods 

will have long-term ecological and 

esthetic impacts at both local and 

regional scales. One important 

consequence will be the loss of habitat 

diversity and quality for terrestrial 

ecosystems. 

Loss of eastern hemlock will also 

impact adjacent riparian habitats. 

Increased nitrate leaching is possible 

following extensive hemlock mortality 

due to increased nitrifi cation rates and 

inorganic nitrogen availability (Jenkins 

and others 1999). These changes 

have potential ramifi cations for both stream 

productivity and deterioration of water quality 

in watersheds that provide public drinking 

water.

Another insidious threat, especially in 

forest preserves, is the steadily increasing deer 

herd. Eastern hemlock is a preferred browse 

species of whitetailed deer (Alverson 

and others 1988). 

Survival and growth 

of seedlings are 

greatly reduced by 

deer browsing (Ward 

2002). In areas with 

large deer herds, 

hemlock seedlings 

may be sparse 

or almost absent 

(Frelich and Lorimer 

1985). Thus, dying 

hemlocks are not 

replaced by hemlock 

seedlings, but by 

other species such as 

black birch and red 

maple.

Hemlock regeneration is limited by deer browsing.

Hemlock borer infestations become evident 

when woodpeckers remove bark in search of 

beetles.
Rehabilitation Guidelines for Hemlock Stands 9

Part 2. Guidelines for 

Managers

Planning and Monitoring

One of the concerns of forest managers is 

predicting when stands infested with hemlock 

woolly adelgid will experience damage and 

mortality. Until effective biological controls, 

tree resistance, or practical chemical pesticides 

can be developed (see the section on controlling 

hemlock woolly adelgid), managers should 

anticipate damage to hemlock stands within 

several years of the initial infestation. 

Fortunately, steps can be taken to 

speed recovery and minimize the 

impact that the loss of hemlock 

will have on ecosystem functions 

and esthetic values. To avoid crisis 

management, these steps should be 

initiated before the onset of widespread 

hemlock decline and mortality. As 

noted above, there are several years 

between initial infestation and marked 

stand mortality. Thus, there is usually 

ample time for thorough planning.

Management decisions are 

generally driven by 1) resource 

management objectives, 2) site and 

stand conditions, and 3) economic 

resources available to implement a course of 

action. The decision of which option to follow 

will take careful consideration and planning. 

Management options include harvesting stands 

before notable decline, salvage harvesting 

(removal of dead and dying trees), felling trees 

without removing them, protecting individual 

trees using biological controls or insecticides, 

and doing nothing. 

Development of a management plan 

that will establish priorities, standards, and 

practices will facilitate objective decisions 

and allocation of limited resources. Engaging 

local input during the planning stages will 

provide an opportunity to educate concerned 

citizens about the potential negative 

consequences of “letting nature take its 

course” (such as hazard trees, lower esthetic 

values, and spread of alien plants) and about 

the fi nancial considerations of alternative 

methods of dealing with stands damaged 

by hemlock woolly adelgid. Local public 

offi cials should be included in the planning 

process to identify regulatory issues and 

remedies, if needed, before implementing any 

management activities.

This section provides information 

not only for prioritizing the location of 

management operations (e.g., salvage cutting) 

and reestablishment of an evergreen conifer 

component, but also for monitoring the 

progression of an adelgid infestation.

Prioritizing hemlock stands

Criteria for determining management 

priorities should include economic, 

ecological, and esthetic factors:

• Economic criteria for determining priorities 

should include public safety (hazard tree 

Obtain public input during fi eld tours.
10 Rehabilitation Guidelines for Hemlock Stands 

management), restoration costs, hemlock 

woolly adelgid control costs, public 

water supply quality, wildfi re potential, 

and potential for viable salvage harvest 

(accessibility and merchantability). 

• Ecological criteria should include habitat 

protection of endangered and threatened 

species, regeneration potential, current 

condition of hemlock and rate of potential 

decline, streambank and water quality 

protection, vulnerability to invasive species, 

and availability of alternative evergreen 

cover for wildlife.

• Esthetic criteria (visual impact) should 

include declines in tourism and visitors 

due to decreased quality of the recreational 

experience, trail 

or facility closure, 

and the presence 

of standing dead 

trees or signifi cant 

logging slash. 

Slash is the residue 

(twigs, branches, 

and unmerchantable 

logs) left after 

logging (Helms 

1998).

Locating all stands with a signifi cant 

hemlock component, in most cases, is easily 

done using existing aerial photography. Other 

useful sources for locating hemlock stands 

include infrared digital orthophotographs and 

Landsat (satellite) images. Some States and 

Federal units have GIS maps of hemlock stands. 

For each stand with a signifi cant hemlock 

component, consider stand characteristics, site 

quality, tree characteristics, and the potential for 

rehabilitation.

Stand characteristics

Each stand should be examined for relevant 

stand characteristics, such as age, diameter, 

volume, and accessibility. In addition to the 

usual forest stand inventory measurements, 

stands should be ranked 

(or rated) by the esthetic, 

cultural, and ecological 

values provided by 

their existing hemlock 

component. Ecological 

values should include 

water quality (riparian 

buffer integrity) and 

wildlife habitat, especially 

for species of special 

concern, such as the 

Blackburnian warbler and 

brook trout.

Site quality

An important component for prioritizing 

stands should be site quality. Hemlocks 

growing in better moisture regimes generally 

survive a hemlock woolly adelgid infestation 

better than those on drought-prone or 

waterlogged sites (Mayer and others 2002). 

Generally, hemlocks on southwest-facing 

slopes experience more rapid mortality than 

those on northeast-facing slopes (Orwig and 

others 2002). Stands deep in ravines seem 

more tolerant of the adelgid than those stands 

on benches at the top of a ravine.

Consider visual impacts.

Trails may be closed by hemlock woolly adelgid.
Rehabilitation Guidelines for Hemlock Stands 11

Tree characteristics

Selection criteria should also consider 

predominant tree characteristics within the 

stand. Desirable attributes include healthy trees 

with medium to large crowns and trees with 

good stem form. When genetic preservation 

is important, a local hemlock population with 

adaptations to regional differences in climate, 

soil, and photoperiod (Olson and others 

1959) should be protected from hemlock 

woolly adelgid to provide a seed source for 

reintroducing hemlock. Chemical control will 

be required to maintain the health and vigor of 

trees in these stands until an effective biological 

control is developed. Therefore, stands should 

be selected that have good accessibility to 

facilitate chemical treatments and future seed 

collection. These stands will also provide a 

reminder of the importance and grandeur of 

hemlock forests.

Rehabilitation planning

The goal of rehabilitation is to speed 

recovery of at least some of the unique 

ecological and esthetic attributes of the 

hemlock forest. Because of the cost 

associated with rehabilitation activities, 

it may be necessary to have a protocol 

established for prioritizing 

hemlock stands as part of the 

planning process. Unique 

stand characteristics and 

the relative importance 

of factors, such as 

riparian buffer integrity 

(water quality), wildlife 

habitat, and esthetic and 

recreational values, should 

be considered. Accessibility 

and the presence (or lack) of 

regeneration might also serve 

as criteria for prioritizing 

hemlock stands.

Control of alien invasive species, such as 

burning bush (Euonymus alatus) and Japanese 

barberry (Berberis thunbergii), should be 

considered because they may crowd out native 

shrubs and herbaceous species. Where alien 

invasive species are a potential problem, 

monitoring and eradication will be necessary in 

the rehabilitation process.

Ideally, selection of replacement species 

will also be made during the planning phase. 

Unfortunately, no native conifer species has the 

combination of unique attributes (evergreen, 

shade tolerant, soft foliage texture) of eastern 

hemlock. Therefore, forest managers will 

have to select a species that best achieves 

management objectives for each particular site 

(e.g., stream shading). Selection criteria should 

consider the importance of 1) using species 

native to Eastern North America, 2) growth 

rate, 3) response to competition, 4) resistance 

to browse damage, 5) site limitations, and 6) 

availability.

Monitoring adelgid populations

Monitoring is an integral part of the process 

to determine when hemlock woolly adelgid 

populations have increased to the level that 

hemlock trees will be impacted. Monitoring 

Prioritize hemlock stands for treatment and restoration.
12 Rehabilitation Guidelines for Hemlock Stands 

hemlock woolly adelgid in a stand need not be 

time consuming. It is helpful to know when the 

stand was fi rst infested and when the infestation 

became generally distributed on most of the 

trees in a stand. Once a tree is infested, it 

takes several years before the tree becomes 

generally infested and damage is visible. The 

lag between initial infestation and widespread 

mortality usually provides the forest manager 

with ample time for developing a rehabilitation 

plan.

Yearly inspection of hemlocks for the 

presence or absence of the hemlock woolly 

adelgid will be suffi cient for many management 

plans. The objective of this inspection is 

to determine if the adelgid is present in the 

stand, and if so, to what extent. Initially, the 

distribution of the adelgid is patchy, occurring 

only on a few trees and limited to small areas 

on those trees. Although the adelgid density 

can be suffi ciently 

high on one or two 

branches on a tree 

to cause needle 

loss and twig 

death, the stand 

will show little 

sign of damage 

until the adelgid 

becomes more 

widely distributed. 

An inspection 

may consist of 

simply walking 

through the stand 

and looking 

closely at a few branches on several trees. 

November through early July is the best time to 

inspect because the adelgid is easier to detect 

by the presence of its woolly masses. A good 

sample size for a grove of a few acres is 10 

to 25 trees and 2 to 4 branches per tree. The 

criteria for determining when to treat will 

depend on a local cost-benefi t analysis. Earlier 

reports noted that hemlock growth dramatically 

slowed or stopped when the proportion of twigs 

infested was greater than 45 percent (Evans 

2002) or there were at least 30 hemlock woolly 

adelgid per 100 needles (Mayer and others 

2002). At these infestation levels, the decline 

in overall appearance and health of the tree 

becomes visible.

Monitoring infested stands

When no chemical control will be 

conducted, it may be more effective to monitor 

tree condition rather than hemlock woolly 

adelgid populations. In stands where chemical 

treatments will be implemented, monitoring 

pest densities and tree condition is absolutely 

vital to ensure treatments will be conducted 

before noticeable decline in the health and 

vigor of individual hemlock trees. Examples 

of such areas include exemplary groves of 

mature hemlock, 

campgrounds, 

green belts along 

scenic parkways 

and streams, 

specimen trees, 

and areas that will 

be conserved as a 

local seed source. 

Monitoring tree 

condition is 

essential in stands 

that will have 

a commercial, 

or break-even, 

salvage cutting. 

Although hemlock 

trees generally survive years after the initial 

infestation, sudden mortality is not uncommon 

for visibly declining trees during a period 

of extended drought. To offset costs, it is 

important to harvest trees before they die and 

become unmarketable.

Inspect hemlocks yearly for hemlock woolly adelgid.
Rehabilitation Guidelines for Hemlock Stands 13

Controlling Hemlock Woolly Adelgid

Research and practical experience have 

shown that it is possible to control hemlock 

woolly adelgid in small hemlock groves or 

on individual trees. Treatments are most 

effective if they are initiated before hemlock 

woolly adelgid 

populations 

have reached 

detrimental 

levels and tree 

health has been 

compromised. 

The challenge 

is to develop 

cost-effective 

methods of 

controlling 

hemlock woolly 

adelgid in 

hemlock stands 

and across 

the landscape. In the interim, the following 

measures can be followed to maximize the 

survival of trees in high-priority areas.

Cultural controls

Healthy hemlock trees can tolerate higher 

densities of hemlock woolly adelgid better 

than can trees with low vigor. Therefore, it is 

imperative to maintain the health of threatened 

trees (McClure 1995). Prophylactic steps 

can be as simple as mulching to maintain 

soil moisture and irrigating during periods 

of extended drought. Quarantines must be 

observed to limit movement of hemlock 

products such as logs, fi rewood, and seedlings 

from infested areas into areas that are not yet 

infested. States with hemlock woolly adelgid 

quarantines include Maine, New Hampshire, 

Vermont, and Michigan at this time.

Bird feeders should be removed near 

hemlocks because birds are known to transport 

both egg masses and crawlers for long 

distances. Removing isolated infested trees 

may also help to slow the spread of hemlock 

woolly adelgid. Nitrogen fertilizer should 

not be applied because this enhances adelgid 

survival and reproduction (McClure 1995). 

Recovery of trees treated with insecticides 

may be assisted with moderate 

applications of fertilizers 

that address specifi c nutrient 

defi ciencies.

Chemical controls

While a number of 

insecticides can be effective 

in controlling hemlock 

woolly adelgid on individual 

trees (McClure 1991b), the 

high cost of treatment limits 

insecticide use to protecting 

individual high-value trees. 

Chemical treatment is not a 

permanent solution and applications have 

to be repeated at intervals ranging from 

several months to a few years. Insecticides 

should be chosen that have the least impact 

on benefi cial insect and mite predators. 

Pyrethroids are effective against hemlock 

woolly adelgid (Cowles and Cheah 2002a) 

but are toxic to predatory mites and could 

prevent establishment of specialist predatory 

beetles being released for biological control 

of hemlock woolly adelgid. It is important 

to read and follow the instructions on the 

pesticide label before using any treatment to 

ensure compliance with all laws to protect the 

environment. Many pesticides can only be 

applied by a licensed pesticide applicator. 

Horticultural oil or insecticidal soap 

sprays have been found to be effective in 

controlling hemlock woolly adelgid on 

accessible trees (McClure 1995, Cowles 

and Cheah 2002a). Oil has special value 

Light hemlock woolly adelgid infestation.
14 Rehabilitation Guidelines for Hemlock Stands 

because it is also effective against rust mites, 

spider mites, and armored scales while 

being relatively nontoxic to certain predators 

(Cowles and Abbey 1999). However, the 

need to thoroughly coat the insect renders 

these products impractical for tall trees or 

large areas. 

Systemic treatments, such as the use 

of imidacloprid, are increasingly used by 

arborists to control hemlock woolly adelgid. 

Unlike oil sprays, imidacloprid has little value 

in controlling armored scales or mites, and 

may temporarily lead to greater populations 

of these pests (James 

and Price 2002). 

Various formulations 

of this chemical 

are labeled for soil 

and trunk injection. 

Soil application can 

provide multiple 

years’ benefi t for 

hemlock woolly 

adelgid control 

(Cowles and Cheah 

2002b, Webb and 

others 2003, Cowles 

and others 2004), 

but there is a paucity 

of information 

on the impact on 

aquatic and soil 

organisms. Trunk 

injection may be 

better suited for 

locations where 

there is special concern for minimizing the 

release of imidacloprid in these environments. 

While trunk injection may provide protection 

for several months (McClure 1991b, Doccola 

and others 2003), soil injection provides 

longer protection and does not wound the tree 

(Cowles and Cheah 2002b). 

Biological controls

The hemlock woolly adelgid is not a 

native species and consequently has few 

natural enemies in Eastern North America. 

Restoring and maintaining hemlock as an 

important component of eastern forests 

will ultimately depend on development of 

effective biological controls. Predators 

already established or native to Eastern 

North America are generally not effective in 

reducing populations to nondamaging levels 

(Montgomery and Lyon 1996, Wallace and 

Hain 2000). Several predators known to feed 

exclusively on adelgids 

have been imported and 

are being investigated for 

biological control (McClure 

2001, Reardon and Onken 

2004). It is likely that a 

complex of natural enemies 

(introduced predatory 

insects and diseases), rather 

than a single “magic bullet,” 

will be needed to maintain 

hemlock woolly adelgid 

below damaging levels. 

The fi rst predator to 

be imported and released 

for biological control in 

infested Eastern States is 

a tiny, black lady beetle 

(Sasajiscymnus tsugae) 

from Japan (McClure and 

others 2000). To date, 

nearly a million S. tsugae

have been released in over 100 sites in 15 

Eastern States from Georgia to Maine. 

Several species of lady beetles (Scymnus

sp.) from China and a derodontid beetle 

(Laricobius nigrinus) from British Columbia 

have been imported and the establishment 

of these predators is just beginning. Efforts 

Some insecticides can be injected 

into the soil.
Rehabilitation Guidelines for Hemlock Stands 15

to locate, evaluate, and establish additional 

natural enemies will continue to be a high 

priority for research and forest health 

practitioners in the coming years (Reardon 

and Onken 2004).

Managing Salvage Harvests

Harvesting trees that are dead or dying to 

recover their economic value is classifi ed as 

salvage cutting (Helms 1998). Salvage cutting 

in hemlock stands, especially before complete 

mortality, has sparked public controversy 

in some areas. As mentioned under 

the section on management planning, 

objective salvage standards that have 

included public input will help allay 

concern over cutting “old-growth forests.” 

Failure to salvage hemlocks dying from 

hemlock woolly adelgid has lead to park 

closures in New Jersey (Mayer and others 

2002) and Connecticut. Such parks may 

remain closed for 10 years or more until 

the slowly rotting hemlock snags are no 

longer a public safety hazard.

Although it may be tempting to retain 

hemlocks with some green foliage during 

the salvage operation, it should be recognized 

that hemlock mortality increases sharply when 

more than 80 percent of needles are lost due 

to defoliation (Stephens 1988). Studies in 

Connecticut found that up to 80 percent of 

hemlocks left after the initial harvest died 

within several years (Orwig and Kizlinski 

2002). Hemlocks killed by hemlock woolly 

adelgid near roads and trails must be removed 

to ensure public safety. This removal will 

require additional expenditures that could have 

been avoided if the trees were harvested in the 

Beetles released for biological control (left to right: Sasajiscymnus tsugae from Japan, 

Scymnus ningshanensis from China, and Laricobius nigrinus from British Columbia).

Dead hemlocks should be removed near roads.
16 Rehabilitation Guidelines for Hemlock Stands 

initial salvage operation. Any hemlocks left 

in the hope that they will recover should be at 

least 200 feet (or twice their height) from roads 

and trails.

Special care must be taken to protect 

residual evergreen trees and shrubs during the 

salvage operation for several reasons. First, 

residual evergreens can preserve some of 

the esthetic and ecological values that had 

been provided by hemlock. The presence 

of established 

evergreens will 

also help speed 

recovery. Studies 

have shown that 

smaller trees and 

shrubs can grow 

quite rapidly when 

exposed to the 

increased sunlight 

that occurs after a 

harvest such as a 

hemlock salvage 

operation. Finally, 

remediation 

costs will be lower if residual evergreens are 

protected. Planting and nurturing new trees is 

expensive. It is much more cost effective, and 

ecologically sensitive, to protect those plants 

that are already growing on the site.

Slash standards (height and amount) will 

vary depending on proximity to streams, public 

viewpoints, and whether 

the area will be replanted. 

Although high slash may 

protect regeneration from 

browse damage, low slash 

is preferable in areas with 

high public visitation 

and where new conifers 

will be planted. Salvage 

cutting standards should 

also include the number of 

residual snags (standing 

dead trees) for raptors and species such as 

woodpeckers. Residual snags should be set 

back from trails and roads to minimize risk to 

the public from falling trees and branches.

Best management practices

Salvage cutting, indeed any harvesting, 

requires careful planning before the harvest, 

monitoring during the harvest, and control 

measures after the harvest to protect forest soils 

and water quality. These 

practices are commonly 

called Best Management 

Practices, or BMPs. The 

intent of BMPs is to 

minimize movement of 

soil from a harvested area 

to adjacent wetlands and 

riparian zones.

This section is intended 

to be an introduction to 

BMPs for land managers 

who are unfamiliar with 

forest harvesting operations, 

including salvage cutting. This section is 

not meant as a detailed technical guide, but 

rather a starting point for communication 

between land managers and practicing foresters 

who are familiar with local forests, soils, 

and regulations. More detailed guidelines 

and pertinent State and local regulations 

can be obtained from the State Forester’s 

offi ce, Cooperative 

Extension Service, or 

the Internet (http://

www.stateforesters.org—

follow links to Best 

Management Practices). 

Because hemlock often 

occurs near riparian areas, it 

is especially important to be 

aware of State regulations 

regarding activities near 

watercourses.

Evergreen shrubs should be protected 

during harvesting.

Light slash left after logging.
Rehabilitation Guidelines for Hemlock Stands 17

Before the harvest

Skidding is the hauling of logs from the 

forest to a collection point (landing) where they 

can be loaded onto a truck (Helms 1998). Wiest 

(1998) provides 

a detailed guide 

for forest road 

construction. 

The network of 

skid roads and 

trails by which 

harvested trees 

are removed from 

the forest is the 

largest source 

of soil erosion. 

Erosion on this 

network can be 

greatly reduced by 

carefully locating 

all skid roads 

and trails before 

harvesting. Where 

practical, skid roads 

should be located 

on gentle slopes 

with well-drained 

soils. Road grades 

should be kept as 

low as possible, and 

sediment-control 

structures should 

be installed before 

the road is used, 

especially on steep grades.

Another key to reducing sedimentation is 

to minimize the number of stream and wetland 

crossings. Never allow skidding (hauling logs) 

down or directly across a streambed. A bridge 

or culvert must be used to cross perennial 

streams. In many instances, a corduroy road 

(small trees and culls) may be placed on 

one, or both, approaches to the crossing. A 

corduroy crossing may also be appropriate for 

intermittent streams that do not have running 

water. Placement and construction of these 

structures is beyond the scope of this handbook 

and should be discussed with a professional 

forester. 

The landing is where 

harvested logs are stored 

before being loaded onto trucks 

for shipment to a mill. The 

landing should be located on 

level ground with excellent 

drainage to minimize soil 

erosion. Sediment-control 

systems, such as hay bales, 

may be appropriate to control 

soil loss for landings on gently 

sloping land. Locating the landing 

where it is not immediately visible 

from public roads may reduce 

public criticism. Lastly, require a 

barrel on the landing for oil cans 

and trash.

Buffer zones around wetland 

and riparian areas should be 

delineated and clearly marked 

(Welsch 1991, Welsch and others 

1995). Use of heavy equipment 

in these zones may be fully or 

partly restricted depending on soil A well-designed road system can 

provide a trail network.

Loading logs at the landing.

Temporary bridges protect streams.
18 Rehabilitation Guidelines for Hemlock Stands 

type and the amount of saturation. Whenever 

possible, trees should be felled away from 

streams to minimize damage to streambanks 

and prevent debris dams. Many States also 

require that a minimal level of residual trees 

(expressed as number of trees or basal area 

per acre) be left near watercourses to protect 

streambanks. Felling and leaving trees parallel 

to the slope (along the contour) on steep slopes 

may also help reduce erosion.

Before the salvage cutting operation, 

accessible trails should be rerouted to maintain 

public safety from falling branches and trees. 

It is also important to keep all-terrain vehicle, 

motorbike, and mountain bike traffi c off skid 

trails in salvage areas. These stands are already 

vulnerable to erosion because of dead root 

systems.

During the harvest

The primary focus during the salvage 

harvest is to ensure that the cutting plan 

and all BMPs are followed. Doing this will 

require periodic inspections during the harvest 

operation. A primary concern during these site 

visits will include ensuring that skidding stays 

on designated 

roads and trails 

to minimize 

sediment 

movement. 

During periods 

of extended 

rainfall or a 

thaw during the 

winter, it may 

be necessary to 

halt skidding 

in some 

sections with 

saturated soils. 

Inspection will 

determine that 

equipment 

entrance into the buffer zones is in accordance 

with the cutting plan. The skidder and other 

equipment should be examined for leaks that 

would contaminate wetlands and streams. 

Inspection 

will also 

determine that 

those trees 

marked for 

cutting, and 

only those 

trees, are 

harvested. 

This part of 

the inspection 

will examine 

the residual 

trees (those left 

after cutting) to 

determine that 

reasonable care 

is being taken 

to protect them from damage during felling 

and skidding operations. Because existing 

evergreen trees and shrubs will help speed 

recovery of the unique attributes that had been 

provided by hemlock, special attention 

should be paid to their protection. Lastly, 

the inspection will ensure that slash and 

stump heights are within the guidelines of 

the cutting plan to speed esthetic recovery 

of the forest. This is especially crucial for 

those areas with public access.

After the harvest

The primary focus after the harvest has 

been completed is to minimize sediment 

movement by stabilizing exposed soil. 

This involves smoothing the skid roads, 

installing water bars (if needed) to divert 

water, and then seeding the roads. Brush 

can also be placed on skid roads to slow 

waterfl ow and discourage their use by 

motorized vehicles and mountain bikes. In 

Professional loggers cause little 

damage to live trees.

Trees should be protected 

from logging damage.
Rehabilitation Guidelines for Hemlock Stands 19

meadow, thus increasing local habitat diversity 

and beauty (Jones 1993). It is important to 

keep motorized vehicles and mountain bikes off 

the landing and road network at least until the 

new vegetation has become established.

Rehabilitation

Rehabilitation is the restoration of some of 

the ecological or esthetic attributes associated 

with hemlock forests before they were damaged 

by hemlock woolly adelgid. As noted earlier, 

hemlock forests destroyed by hemlock woolly 

adelgid will often be replaced by hardwood 

species, such as maple and birch. The resulting 

forest will have quite different ecological 

attributes and habitat diversity. In some 

cases, it may be possible to begin restoration 

before salvage operations by encouraging 

regeneration of desirable species, such as white 

pine (below). Prescriptions for increasing 

regeneration could include thinning, prescribed 

burning, or underplanting with seedlings 

from a tree nursery. Fencing may be required 

where seedlings may be damaged by deer 

browsing. Destruction of new seedlings by 

trampling in parks and along trails with high 

public use can be reduced by a combination 

of fencing, improved trail delineation, and 

educational signs. More detailed information 

areas with soils that are susceptible to erosion, 

it may be advisable to use additional sedimentcontrol measures, such as staked hay bales or 

silt fences.

All temporary stream- and wetlandcrossing structures should be removed and 

the approaches should be graded to nearly the 

original condition. All graded areas should 

be quickly reseeded. Planting shrubs native 

to the area can also help stabilize the soil. 

Because these areas are of special concern, it 

is important to err on the side of caution and 

install sediment-control measures immediately 

after the crossing is no longer needed. It may 

be necessary to fence off streambanks adjacent 

to parks, picnic areas, and trails with high 

visitor use to protect the site from trampling 

until new vegetation has become established. 

The landing should be smoothed, seeded, 

and have sediment-control structures installed 

where needed. The landing also provides an 

excellent opportunity to install a wildfl ower 

White pine regeneration after hemlock mortality.

Seeded skid trail (above) and 

log landing (below).
20 Rehabilitation Guidelines for Hemlock Stands 

on regenerating forests can be obtained from 

the State Forester’s offi ce, the Cooperative 

Extension Service, or Ward and Worthley 

(2003).

An important step in restoring damaged 

stands is to limit the spread of alien invasive 

species. A combination of an ability to grow 

in shade and high resistance to browse damage 

allows several alien species to form dense 

patches that can exclude native shrubs and 

wildfl owers. Controlling alien invasive 

species is best done by eradicating established 

plants before hemlock mortality to limit 

vegetative growth and seed production. 

Continual monitoring and eradication may be 

necessary to eliminate new seedlings.

In the forest, two alien invasive species 

are especially common in areas of high deer 

density: burning bush and Japanese barberry. 

Both species have been listed as widespread 

and invasive throughout much of the Eastern 

United States. Other alien invasive species of 

concern include Asiatic bittersweet (Celastrus 

orbiculatus), Japanese stiltgrass (Microstegium 

vimineum), Norway maple (Acer platanoides), 

and various honeysuckles (Lonicera sp.).

Species selection

Planting alternative evergreen species to 

replace hemlocks that were killed by hemlock 

woolly adelgid can restore some of the 

ecological or esthetic attributes that were lost. 

Do not replace hemlocks with single species 

plantations. The unfortunate reality is that other 

exotic insects and diseases will probably be 

introduced. Replacing eastern hemlock with a 

monoculture may well create a similar problem 

for future resource managers. A properly 

planned planting with a diversity of species will 

look like and function as a natural stand. Using 

evergreen shrubs during the initial planting will 

increase their survival and growth, and have the 

additional benefi t of increasing habitat diversity. 

New seedlings can be obtained from 

tree nurseries.

Prevent alien invasive species from 

becoming established.

Plant mixed species where hemlock was lost.
Rehabilitation Guidelines for Hemlock Stands 21

Irregular planting, as opposed to planting on a 

grid, will also enhance the natural appearance 

of the stand as it develops.

Potential native species for planting 

include eastern white pine (Pinus strobus), 

red pine (P. resinosa), pitch pine (P. rigida), 

eastern redcedar (Juniperus virginiana), 

northern white-cedar (Thuja occidentalis), 

balsam fi r (Abies balsamea), Fraser fi r (A. 

fraseri), Atlantic white-cedar (Chamaecyparis 

thyoides), red spruce (Picea rubens), and 

white spruce (Picea glauca). Several native 

broadleaf species could 

be used to provide low 

evergreen cover. These 

include American holly 

(Ilex opaca), inkberry (Ilex 

glabra), rhododendron 

(Rhododendron maximum, 

R. catawba), and mountain 

laurel (Kalmia latifolia).

Non-native conifer 

species that may be 

planted include Norway 

spruce (Picea abies), 

Colorado spruce (Picea 

pungens), Douglas-fi r 

(Pseudotsuga menziesii), 

sawara (Chamaecyparis 

pisifera), cedar of Lebanon 

(Cedrus libani), English 

yew (Taxus baccata), and 

western hemlock (Tsuga 

heterophylla). There are also three Asian 

hemlock species that are resistant to hemlock 

woolly adelgid and have been grown in arboreta 

and ornamental settings in the Eastern United 

States: northern Japanese hemlock (Tsuga 

diversifolia), southern Japanese hemlock 

(Tsuga sieboldii), and Chinese hemlock (Tsuga 

chinensis). 

Studies are underway to develop hemlocks 

with resistance to hemlock woolly adelgid 

that are suitable for eastern climates. Hybrid 

crosses between the Asian species and T. 

canadensis have not been successful, but 

hybrid crosses between T. caroliniana and T. 

chinensis have provided viable hybrids (Bentz 

and others 2002). Other studies seek to identify 

the chemical nature of resistance to hemlock 

woolly adelgid in hemlocks (Lagalante and 

Montgomery 2003). Hopefully, hemlock with 

characteristics of the eastern species that are 

resistant to the adelgid will be available in the 

future.

Planting operations

Planting operations require careful 

orchestration of personnel, planting stock, 

planting equipment, and browse protection (if 

needed). Seedlings are perishable and should 

be planted as soon as possible after they are 

delivered from the nursery. Ensure that the 

planting site is accessible and that the amount 

of slash will not impede operations prior to 

receiving planting stock. Volunteers used 

to assist in planting should have appropriate 

Norway spruce (left) and eastern white pine (right) are evergreen 

species that can be planted following hemlock mortality.
22 Rehabilitation Guidelines for Hemlock Stands 

effectiveness of using larger hemlock seedlings 

should be fully examined.

Lastly, a plan should be developed to 

monitor and control hardwood competition. 

Conifer seedlings are often overtopped by 

hardwood regeneration. These competitors can 

limit the amount of available light (Salonius 

and Beaton 1997). Height growth and survival 

of overtopped seedlings will be greatly reduced. 

A machete or brush hook can be used to remove 

larger hardwoods until the conifer 

seedlings have become well established 

with vigorous crowns.

Reintroducing hemlock

In the short term, the reintroduction 

of eastern hemlock should only be 

considered if the trees will be monitored 

and protected from future adelgid 

infestations. Controlling adelgids on 

small trees can be easily accomplished 

using horticultural oils applied 

with backpack sprayers, but annual 

inspections should be conducted to 

ensure timely applications. 

Once effective biological controls 

have been developed, the need for 

continued chemical applications should be 

minimal or unnecessary. Seed could be 

collected from those hemlock trees that were 

protected to produce seedlings adapted to the 

region. The expertise gained from planting 

other conifer species can then be applied to 

reintroduce eastern hemlock. In areas where 

deer densities are high, the experience gained in 

establishing other evergreens will be especially 

crucial in deciding on effective barriers to 

prevent browse damage.

training. Unless the planting will be contracted, 

it will be necessary to provide suffi cient 

planting bars, shovels, and planting bags for all 

personnel.

Reestablishing an evergreen component in 

forests that were once dominated by eastern 

hemlock will be especially diffi cult in areas 

with large deer herds. Eastern hemlock and 

other conifers are preferred browse species 

(Alverson and others 1988). Deer browse 

damage on nursery 

seedlings is higher than 

on naturally established 

seedlings. 

Planting evergreens, 

especially white pine 

and hemlock seedlings, 

in areas with high deer 

populations is a waste of 

material and labor unless 

deer browse protection 

is provided (Ward 2002). 

The decision to use 

browse protection, and the 

type of browse protection 

used, should only be made 

after a careful analysis 

of the costs. In some 

circumstances, it may be more cost effective 

to use larger seedlings; however, the cost 

Rigid mesh tubing can 

reduce browse damage.

Planted eastern white pines protected 

from deer browsing.
Rehabilitation Guidelines for Hemlock Stands 23

Summary

• Develop a management plan before adelgid infestations and hemlock damage occur. 

• Identify those stands that are at risk and prioritize them for treatment according to ecological, 

esthetic, and economic values. 

• Monitor the prevalence of hemlock 

woolly adelgid and hemlock health. 

• Preserve valuable hemlocks with 

appropriate cultural and chemical 

controls and encourage the 

establishment of biological controls 

where possible.

• Protect public safety by removing 

dead and dying hemlocks near roads 

and trails.

• Conduct harvest operations using 

Best Management Practices to protect 

forest soils and water quality in 

adjacent wetlands and riparian areas.

• Establish evergreen cover, where 

possible, with mixed species plantings 

to restore some of the ecological and 

esthetic characteristics that had been 

provided by hemlock.
24 Rehabilitation Guidelines for Hemlock Stands 

References

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