Robert H. Mohlenbrock (USDA)

Eastern Hemlock (Tsuga canadensis)

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. Athough 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 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. The Limberlost area is probably the most famous hemlock area in the Park. According to legend, George Freeman Pollock (then owner of Skyland) wanted to save the trees. His wife, Addie Nairn Pollock bought 100 hemlock trees at $10 a tree to save them from the logger’s ax (Pollock, 1960). Some of the trees in this area were more than 3 centuries old. Hemlock ecosystems are unique. There are several rare species found in this area. 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.

Eastern hemlocks are currently under attack by an exotic sap sucking insect that originated from Asia. The hemlock woolly adelgid (Adelges tsugae) is a serious pest in Shenandoah National Park that threatens to eliminate all eastern hemlock stands. First observed in 1988, it has since been found in all sections of the park, at all elevations, and on all aspects where surveys have occurred. Hemlock woolly adelgid has caused significant decline in hemlock crown health and tree mortality has increased Park-wide. Without intervention, there is a very real possibility that this insect pest could directly or indirectly eliminate eastern hemlocks from Shenandoah’s ecosystem. Efforts to extend the lives of our remaining healthy hemlocks in 2005 will be accomplished through soil and stem treatments with a systemic pesticide that remains at effective levels to kill adelgids for over a year. Some vehicle accessible areas will also be sprayed with an insecticidal soap. The battle to save a lasting remnant of Shenandoah’s hemlock gene pool 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, 2^nd 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.

US Forest Service Hemlock Woolly Adelgid website

Hemlock Woolly Adelgid Related Bibliography

Further information can be found:

Integrated Taxonomic Information System (ITIS) Report U.S. Department of Agriculture.

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.