Hemlock

Hemlocks are evergreen conifer trees in the genus Tsuga, part of the pine family. They are graceful trees with soft, short needles and drooping branches, and they grow in cool, moist forests in North America and eastern Asia. Despite their name, hemlock trees are not poisonous.

Scientific Classification & Quick Facts

Common name	Hemlock
Scientific name	Tsuga
Kingdom	Plantae
Phylum	Tracheophyta
Class	Pinopsida
Order	Pinales
Family	Pinaceae
Type	Evergreen conifer tree
Habitat	Cool, moist, dense forests and mountains
Native range	North America and eastern Asia
Height	10 to 60 meters
Special feature	Needles have two white bands underneath

A Confusing Name

The hemlock tree got its name because its crushed needles were said to smell a bit like poison hemlock, a completely unrelated plant in the carrot family. Poison hemlock is extremely toxic, but hemlock trees are not. Over the years these trees have also been called hemlock spruce and spruce pine, even though they are not spruces or pines.

Kinds of Hemlock

There are about eight to ten species of hemlock. Four grow in North America:

• Eastern hemlock (Tsuga canadensis) – found across eastern Canada and the eastern United States.
• Carolina hemlock (Tsuga caroliniana) – a rare species of the southern Appalachian Mountains.
• Western hemlock (Tsuga heterophylla) – a tall, important timber tree of the Pacific Northwest.
• Mountain hemlock (Tsuga mertensiana) – grows high in the mountains of western North America, from Alaska to California.

The others grow in eastern Asia, including the Japanese hemlock (Tsuga sieboldii), the northern Japanese hemlock, the Chinese hemlock and the Himalayan hemlock.

What a Hemlock Looks Like

Photo by: Sten Porse

Hemlocks are evergreen, so they stay green all year. Most have a cone or pyramid shape, with gently drooping branches and a top shoot (leader) that usually nods over to one side. That drooping tip is a handy way to recognize a hemlock from a distance.

Sizes vary by species. Eastern hemlocks often reach 20 to 30 meters (65 to 100 feet), while western hemlocks can grow more than 50 meters (165 feet) tall. Hemlocks can also live a very long time; some eastern hemlocks are hundreds of years old.

The bark is reddish-brown to gray, scaly and deeply furrowed on older trees. In the past, the bark was an important source of tannin, which was used to turn animal hides into leather.

Needles and Cones

Hemlock needles are flat, soft and short, usually about 1 to 2.5 cm (half an inch to an inch) long. Each needle grows from a tiny woody peg on the twig. On most species, the needles are dark green on top and have two white stripes underneath. Mountain hemlock is different: its needles are the same blue-green color all around and stick out in all directions.

The cones are small and woody, hanging from the tips of the branches. On the eastern hemlock they are only about 1.5 to 2.5 cm (around an inch) long, while mountain hemlock cones are longer.

Habitat

Hemlocks love cool, damp, shady places. In the wild you will find them in moist forests, along streams, in ravines and on north-facing slopes. They are very shade-tolerant, so young hemlocks can grow slowly for many years in the shadow of bigger trees.

Hemlock forests are important for wildlife. Their thick branches give deer shelter in winter, many birds nest in them, and their shade keeps streams cool for trout and other fish.

Growing a Hemlock at Home

Photo by: Sten Porse

Choosing a Spot

Hemlocks can be fussy about where they grow. They do best in partial shade or morning sun, sheltered from drying winds and hot afternoon sun. They dislike hot, dry sites, polluted city air and road salt.

Soil

Plant hemlocks in moist but well-drained, slightly acidic soil that is rich in organic matter. A layer of mulch, such as shredded bark or pine needles, helps keep the roots cool and moist.

Planting

Dig a hole about twice as wide as the root ball and about as deep, so that the top of the root ball sits level with the ground. Place the tree in the hole, fill in with soil, water well and add mulch around the base, keeping it from touching the trunk.

Watering

Water is the key to growing a healthy hemlock. Water young trees deeply and slowly at the base, for example by letting a hose trickle for 15 to 20 minutes, so the water soaks down to the roots. Keep the soil evenly moist during the first few years and during any dry spells, even for older trees.

Pruning and Feeding

Hemlocks can be left to grow into their natural shape or trimmed into a dense hedge. Eastern hemlock is one of the few conifers that handles regular trimming well. Prune in spring or early summer, when the tree is growing actively and heals quickly. Avoid pruning in late summer or fall, because new growth may not harden before winter.

Do not fertilize a newly planted tree; let it settle in first. Healthy established hemlocks usually need little feeding. If growth is poor, a light, balanced, slow-release fertilizer in spring is enough.

Pests and Problems

The biggest threat to hemlocks in eastern North America is the hemlock woolly adelgid, a tiny sap-sucking insect from Asia. It shows up as small, white, cottony tufts at the base of the needles and has killed huge numbers of eastern and Carolina hemlocks. Other pests include the elongate hemlock scale and caterpillars such as the hemlock looper.

Small rodents such as mice and voles sometimes gnaw the bark of young trees in winter, and deer may browse low branches. If you spot signs of pests, ask a local tree expert or garden center for the safest treatment.

Hemlock FAQs