The State Tree of Pennsylvania: Eastern Hemlock

Eastern hemlock (Tsuga canadensis) became the state tree of Pennsylvania by law in 1931. Its native range stretches from Nova Scotia in the north, westward to Wisconsin, and down along the eastern mountainous regions from New England through New York, Pennsylvania, and as far south as Georgia and Alabama. It is found throughout forests, public gardens, and home landscapes in our state. This graceful tree was important in Pennsylvania's history as a source of lumber and for the tannins produced by its bark, which were used extensively in the 19th-century leather industry. 

The eastern hemlock is now considered a keystone species, a designation that signifies its importance in creating unique microclimates and supporting over 100 species of vertebrates. The dense canopy it provides creates shade that cools the forest floor as well as streams. Hemlocks provide shelter for birds and mammals of all sizes. Many aquatic species exist because of the cool, steady water temperatures that hemlocks help maintain. Hemlock is also a food source. Small mammals and some birds eat its seeds. White-tailed deer (Odocoileus virginianus) sometimes browse the foliage in the depths of winter, and ruffed grouse (Bonasa umbellus) eat its buds.

Since 1967, this tree has been attacked by the invasive hemlock woolly adelgid (HWA, Adelges tsugae) in Pennsylvania. Whole stands have been decimated as this insect spreads from east to west throughout Pennsylvania. By 2023, HWA had spread to all 67 counties. Yet treatments are available for the home gardener to combat this invasive insect. Many additional pests and diseases are potential problems for hemlock, including elongate hemlock scale (Fiorinia externa), hemlock borer (Melanophila fulvoguttata), spruce spider mites (Oligonychus ununguis), and needle rust (Melampsora abietis-canadensis). While this tree has many benefits, choosing to plant this splendid tree comes with risks and potential costs that the homeowner must assess before planting.

Identification

In the Pinaceae (pine) family, eastern hemlock is a beautiful, conical, needled evergreen tree with drooping branches. Its acicular (needle-shaped) needles are opposite and grow up to one-half inch long in spiral planes along its branches. The leaves generally lie flat along the branch. For identification purposes, the leaves are glossy green, but on the underside are two parallel, glaucous (pale bluish) bands. The leaf's margins (edges) are considered minutely dentate — the ridges can be felt. The needles also have a small indentation on their tip.

The ovoid cones of this hemlock are small, less than one inch wide and long. They begin green but mature to brown, hanging on twig tips.

The species grows at a slow-to-moderate rate to 40 to 70 feet tall and 25 to 35 feet wide. They are long-lived, reaching maturity at 250 to 300 years. Some trees are known to have lived longer than 800 years.

Growing Conditions and Maintenance

Eastern hemlock prefers acidic, moist soils with good drainage, though it will grow in sand, loam, or clay. It will grow in full sun to partial shade and will even tolerate full shade. This tree can be grown in all areas of Pennsylvania, as it thrives in USDA Plant Hardiness Zones 3a through 7b.

Eastern hemlock is readily available at reputable nurseries as either balled-and-burlapped (B&B) or container-grown specimens. B&B has been preferred as the root system is very fibrous. Plant in early spring or fall. Dig a hole twice the diameter of the root ball, deep enough so the root flare is slightly above the surrounding soil. Water well, then mulch with 2 inches or so of organic mulch, such as pine bark. Continue to water it well during its first year, and during dry spells until the tree is well established. This tree generally needs no pruning as it grows, except to "shape" it to keep it balanced. Observe it periodically for the presence of insect or disease damage.

Use in the Landscape

A mature eastern hemlock is a graceful, beautiful tree and would be a welcome specimen tree in any large landscape. It can be a great addition to a woodland and makes a good understory tree. It also responds well to pruning and can be used as a privacy screen. Because it is large, many home gardens can instead plant one (or more) of the following smaller cultivars.

• 'Abbott's Pygmy' is a mini conifer growing into a tiny globe with very small leaves, eventually reaching 2 feet tall and wide.
• 'Betty Rose' is a dwarf, maturing to 2 feet tall and wide. It creates a somewhat irregular mound and has white-tipped foliage that shows a little pink in the spring.
• 'Gracilis' is a gorgeous low-growing spreader, preferring shadier sites. It tops out at 2 feet tall and 4 feet wide.
• 'Jeddeloh' tops out at 2 feet tall and 4 feet wide. It is considered the bird's nest hemlock because its center is depressed and funnel-shaped.
• 'Cole's Prostrate' is a slow-growing prostrate form that spreads, weeps, and creates a low groundcover. It looks stunning cascading over a wall. Its mature size is 6 feet tall and 4 feet wide.
• Emerald Fountain® 'Monler' has the traditional shape of the species but tops out at 6 to 10 feet tall and 2 to 3 feet wide. It grows best in partial to full shade.
• 'Gentsch White' is vigorous and can top out at 10 feet tall and wide, but can be kept smaller and tighter by pruning, which might be desirable as its new growth tips are silvery-white.
• 'Golden Splendor' tops out smaller than the species at just 25 feet tall and 15 feet wide. It looks like the species in shape, but it is bright gold in color. It is fast-growing and is more golden in a sunnier spot.
• 'Sargentii' is a larger, dense, mounding, weeping form growing up to 10 to 12 feet tall and 20 to 30 feet wide. It is a stunning specimen in a larger landscape.

And many, many more cultivars of eastern hemlock are available to fit just about any spot in the home landscape as a rock garden addition, a screen, a specimen, or an understory plant. Be sure to weigh all the benefits against the risks, as should be done with all major purchases for your garden.

For further information on hemlock woolly adelgid and other problems of hemlocks, please review the resources below:

Integrated Approach to Hemlock Woolly Adelgid Mitigation

Hemlock Diseases

Eastern Hemlock Health Update