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New York State Hemlock Initiative

Keeping the legacy alive

Hemlock Trees

Hemlocks are a valuable resource in New York State for many reasons, and conserving hemlock trees is vastly important for ecosystem health and for the many benefits they provide to humans and wildlife. Here’s what makes hemlocks so special:

What are eastern hemlocks and why are they important?

hemlock stand
Hemlock stands provide shelter to wildlife. Photo by Mark Whitmore.

The eastern hemlock (Tsuga canadensis) is a foundation species in the areas of the Northeast forest where it is found.  It provides vital ecosystem services that benefit humans and wildlife.  If it is removed from the landscape, the effects can be devastating to other species and costly to remediate.


The eastern hemlock is a slow-growing coniferous tree with an attractive, lacy crown that can reach up to 175 feet tall. Hemlocks have small, flat needles that are deep green on the top and pale on the bottom. In the spring, hemlock twigs have bright, almost lime green tips indicating new growth. The bark is roughly textured and can have a reddish hue to it. Hemlocks can live up to 900 years!


Hemlock trees prefer cool, moist sites and are often found along streams, lakes, and on the steep slopes of gorges.  Their shallow roots make them vulnerable to drought stress and windthrow.


Hemlocks fulfill a variety of essential ecosystem roles, making them a necessary component of a healthy landscape.  They provide food and shelter for wildlife across the landscape, and they protect and filter water sources that are valuable to animals and humans alike.

Deer, moose, porcupine, grouse, squirrel, and mice all eat hemlock, especially during the winter when other food sources may be scarce.  Hemlock groves also provide excellent sources of cover because of they have a dense canopy and open understory.

The cool shade created by the overhead canopy provides a sanctuary during the hot summer months and keeps snow from melting rapidly during spring thaw, creating a slow steady influx of water into streams and lakes.  Hemlock shade directly over streams is also important for maintaining the temperatures necessary for many aquatic species, including native brook trout.

Hemlocks make up much of the forest cover along shorelines and streams, and are key for preventing erosion and filtering out pollutants.  New York City and other cities in the state rely on the hemlocks surrounding reservoirs and lakes to keep municipal water supplies clean.

How to identify a hemlock tree

Not sure you can identify an eastern hemlock?  The Arbor Day Foundation’s What tree is that? can help with some easy guided questions.  The Brandeis Conifer Field Guide allows you to view selected species side-by-side as you learn to differentiate them.  For auditory learners, Cornell’s ForestConnect has a great dendrology crash-course video to help you identify and understand the ecology of Northeast conifers.

What is hemlock woolly adelgid (HWA)?

HWA is easily recognized by woolly egg sacs.
HWA is easily recognized by woolly egg sacs. Photo by Connecticut Agricultural Experiment Station,

The hemlock woolly adelgid (Adelges tsugae) is an aphid-like insect native to Asia that has devastated hemlock populations in the eastern United States.  It was first discovered in the US and established as a pest in the 1950s.  HWA is easily recognized by white woolly egg sacs, which can be found on twigs in late winter through early summer.  Once hatched, adelgids attach to the twigs near the base of the needles and feed on the tree’s sap.  An infestation can slow or prevent the tree’s growth, eventually resulting in the death of the tree over the course of several seasons.

You can learn more about hemlock woolly adelgid on our HWA: Basics page.

Why is HWA a threat?

HWA has caused widespread mortality in eastern and Carolina hemlock populations in the United States due to a lack of predators and low resistance among the trees.  This drastic decline in hemlocks poses a problem for forest composition, as many species of wildlife rely on hemlocks for food and shelter. It also may cause a decrease in water quality for human drinking water supplies.

Where is HWA in New York?

HWA has moved through southern and western New York.  It was discovered in the Adirondacks in late July 2017 on Prospect Mountain near Lake George.  Click here to view a time-lapse map from the New York State Department of Environmental Conservation showing the spread of HWA in New York since 1985.  Originally, it was hoped that cold winters would prevent HWA from spreading north or into the Adirondacks, but the insect has demonstrated resistance to cold, reaching as far north as Nova Scotia.  Even if there is a temperature limit to HWA’s range, it will continue to spread and cause widespread mortality among hemlock populations of New York unless effective identification, treatment, and prevention protocols are adopted.

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