Eastern Hemlock

Eastern Hemlock (Tsuga canadensis) needles and cones
Eastern Hemlock (Tsuga canadensis)

Robert H. Mohlenbrock (USDA)

Scientific Name: Tsuga canadensis
Kingdom: Plantae - Plants
Division: Coniferophyta - Conifers
Class: Pinopsida - Conifers
Family: Pinaceae - Pine family
Genus: Tsuga Carr. - hemlock
Life Cycle: Perennial

Description:

Eastern hemlock is a graceful native evergreen tree. This conifer is unusual in that the terminal leader often droops instead of giving the tree a typical pointed top like that of most trees in the pine family. The 5/16 to 9/16 inch needles (Petrides, 1972) are flat and green above with white, shallow, bands on the underside. They are attached with slender stalks and have a flattened, rather than spiral, appearance on the branch. It takes twenty to forty years for the tree to begin seed production (Elias, 1980). Flowers are produced in spring with cones ripening in the fall and releasing seed during the winter. Although slow growing, a tree can attain a height of 160 feet and trunk diameter of 6 to 7 feet (Harlow and Harrar, 1968). Long lived, they may approach 1000 years of age (Elias, 1980).

These trees are found mostly in cool coves but can also be present on rock outcrops, especially on north-facing slopes. They are shade tolerant and are able to live in fairly acidic soil. Hemlocks have shallow root systems that leave them susceptible to drought, wildland fire, and wind fall.

Habitat:

The eastern hemlock is found from Nova Scotia to eastern Minnesota, south to Maryland and Illinois, and along the Appalachian Mountains to Georgia and northern Alabama.

Hemlock comprises less than 1% of the forest cover in the Park. Hemlock ecosystems are unique. There are several rare species found in hemlock stands. The state rare blackburnian warbler (Dendroica fusca) has been known to nest in the area (Winstead, 1995). Four state rare plants also occur. They are the speckled alder (Alnus rugosa), American fly-honeysuckle (Lonicera canadensis), alderleaf buckthorn (Rhamnus alnifolia), and finely-nerved sedge (Carex leptonervia). There is no other tree that can fill the functional niche of hemlock.

Sadly, since the early 1990s, Eastern hemlocks have been under attack by an exotic sap sucking insect that originated from Asia. This pest, the hemlock woolly adelgid (HWA)(Adelges tsugae) was/is a serious pest that threatens to eliminate all eastern hemlock stands.

To address this threat, the Park began monitoring hemlocks to determine HWA distribution and to assess hemlock health. From 1993-2003, heavy infestations of HWA (along with drought) led to crown health decline and widespread hemlock death. By 2003, roughly 95% of the Park’s hemlocks had succumbed to the adelgid.

In the mid-1990s, the park began HWA suppression by using limited foliar treatments in developed areas with horticultural oil and insecticidal soap. Those treatment were only partially effective, and the HWA kept on killing hemlocks. But in 2005, the park switched to systemic soil treatments of imidacloprid applied every 7 years. Since 2005, staff have been suppressing HWA with imidacloprid in most surviving hemlock areas and treating 2500-3500 trees/year with good results. To date, staff have protected over 30,000 trees! Additionally, we’ve seen crown health improvement at all our treated sites.

Since 2015, the park has been partnering with Virginia Tech and releasing predatory biocontrol beetles (Laricobius spp.) to combat the HWA.  Since 2021, the Shenandoah National Park Trust began supporting additional biocontrol beetle releases in the park. To date, we’ve conducted Laricobius beetle releases at six different sites. Initial surveillance monitoring of the released Laricobius beetles has shown promising establishment and spread of the beetles. We plan to gradually release Laricobius predatory beetles (and eventually predatory silver flies) in the park’s remaining hemlock sites and will begin to phase out the imidacloprid treatments. Additionally, we hope that early beetle releases can establish field nurseries to provide beetles for subsequent park Laricobius releases. The Park’s HWA biocontrol program is intended to provide long-term, sustainable protection to a geographically dispersed portion of the park’s eastern hemlocks without using pesticides.

The hemlock woolly adelgid has the potential to cause additional significant tree mortality in the park. Without intervention, there is a very real possibility that this insect pest could eliminate eastern hemlocks from Shenandoah's ecosystem. Efforts to provide long-term protection for our remaining hemlocks will be accomplished through limited soil treatments and yearly biocontrol releases. The battle to save Shenandoah's hemlocks for future generations continues!

Common Names:

Other common names for eastern hemlock include Canada hemlock and hemlock spruce.

Other Facts:

Hemlock bark was widely used in the tanning of hides (Grimm, 1962) (Elias, 1980). Not to be confused with herb poison hemlock (Conium maculatum), a tea can be made from the leafy twig tips and inner bark. This was sipped for a variety of ailments or used as an external wash. The bark is astringent and was used as a poultice on bleeding wounds in the past (Foster and Duke, 1990). Eastern hemlock is the state tree of Pennsylvania. These lush trees are often planted for shade or as an ornamental.

References

Elias, Thomas S. 1980. The Complete Trees of North America: Field Guide and Natural History. Van Nostrand Reinhold Co. New York, New York. pp. 92-93.

Foster, Steven, and James Duke 1990. Peterson Field Guide: Eastern/Central Medicinal Plants. Houghton Mifflin Co. Boston, Massachusetts. pp. 258-259.

Grimm, William Carey. 1962. The Illustrated Book of Trees. Stackpole Books. Mechanicsburg, Pennsylvania. pp. 69-71.

Harlow, William M. and Ellwood S. Harrar. 1968. Textbook of Dendrology, 5th Ed. McGraw-Hill Book Co. New York, New York. pp. 143-145.

Jones, Gerald. 1993. The Living Earth Book of North American Trees. The Readers Digest Assoc. Inc. Pleasantville, New York. p. 188.

Little, Elbert L. 1980. The Audubon Society Field Guide to North American Trees, Eastern Region. Alfred A. Knopf, Inc.; New York, New York. pp. 299-300.

Petrides, George A. 1972. Peterson Field Guide: A Field Guide to the Trees and Shrubs, 2nd Ed. Houghton Mifflin Co. Boston, Massachusetts. pp. 21-22.

Pollock, George Freeman. (Stuart E. Brown, Jr. Ed.) 1960. Skyland: The Heart of ShenandoahNational Park. Shenandoah Natural History Association. Luray, Virginia. p. 212

Winstead, Randy. 1995. Old Growth Report Shenandoah National Park. Luray, Virginia. pp. 17-22.


Other Databases and Information:

US Forest Service - Hemlock Woolly Adelgid

Integrated Taxonomic Information System (ITIS) Report provides authoritative taxonomic information on plants, animals, fungi, and microbes of North America and the world.

Germplasm Resource Information Network database which is sponsored by the Agricultural Research Service, U.S. Department of Agriculture.

PLANTS National Database, a website supported by the Natural Resource Conservation Service.

Last updated: October 19, 2022

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