Plant taxonomy classifies eastern or "Canadian" hemlock trees as Tsuga canadensis. They are members of the pine family. Along with their better-known relatives, the eastern white pine trees, they are among the most common evergreen trees growing in the forests of eastern North America.
Moderately slow-growing and long-lived, Canadian hemlock trees in the wild may reach 80 feet tall or higher, with a spread of 25 to 30 feet (for information on compact cultivars developed for landscape use, see below). These are fragrant plants. Crushing the needles releases the aroma.
They are pyramidal or conical in shape. Their small needles give them a fine texture. The needles are dark green on top and light green underneath. The bark of Canadian hemlock trees at maturity may be cinnamon-red or reddish brown.
USDA Plant Hardiness Zones, Sun, and Soil Requirements
These trees require a soil that is moist but that offers good drainage. They prefer a loamy, acidic soil. Shallow-rooted, they also need protection from the wind, else you might return home one day after a storm only to find your specimen lying on the ground. But unlike many large trees, Canadian hemlocks will tolerate (but do not need) quite a bit of shade. Their sunlight requirements thus give you a lot of flexibility with them, as you can grow them as anything from full-sun plants to shade plants.
Canadian hemlock trees can serve as specimen plants or as living-wall privacy screens. Compact cultivars, which are essentially shrubs (see below), are commonly used as hedge plants and/or in foundation plantings. If you begin pruning them when young, they are fairly easy to shape.
Two virtues of Canadian hemlock trees are that they are shade tolerant and make very little mess:
- They offer one of the few options for screening plants in shaded areas.
- Eastern white pine trees, with their large needles and cones covered in sticky pitch, are known to be messy. But the needles and cones of Canadian hemlock trees are much smaller and cleaner.
Socrates, Poison Hemlock, and Canadian Hemlock Trees: Any Connection?
You have probably heard of the ancient Greek philosopher, Socrates, whom the people of Athens sentenced to death for corrupting their youth. Socrates famously yielded to the sentence, drinking from a cup of hemlock to bring about his death. The poison that killed him was not derived from the tree we have been discussing. Rather, it was poison hemlock (Conium maculatum). This is an herbaceous perennial, not a tree.
Another "hemlock" that is poisonous and an herbaceous perennial is water hemlock (Cicuta maculata). As its common name suggests, it is often found in moist meadows, along streams, etc., in the eastern United States.
Warnings About Growing This Tree
Besides being prone to uprooting in wind storms, Canadian hemlock trees have two major drawbacks in the form of two pests that attack them:
- Wooly adelgids
Their susceptibility to wooly adelgids is very concerning. Wooly adelgids (Adelges tsugae) are an invasive species of insect and a type of aphid. They have been a major pest problem for many years now in eastern North America.
"One of the primary coniferous sources of tannins is the bark of the eastern hemlock, Tsuga canadensis, a tree that is widely distributed across eastern North America. The bark of this tree has a tannin content of about 10-12 percent and was used to tan sheepskins and heavy leather for shoes in the United States during the late nineteenth and early years of the twentieth centuries," according to the U.S. Forestry Dept. publication, Non-Wood Forest Products From Conifers.
Cultivars of Canadian Hemlock Trees for Landscape Use
There are many cultivars of Canadian hemlock trees that have been developed for landscape use. Such cultivars have been bred specifically to function in circumstances where a tall tree would be undesirable. Just a few are listed here, to provide some indication of the range of options available:
- Compact cultivars include the dwarf, 'Gentsch White.' This rounded, compact, shrub-like plant attains only 4 feet in height (by about the same width).
- 'Aurea Compacta' (also known as 'Everitt's Golden') is one of those evergreens that is not actually green, instead bearing golden foliage. These Canadian hemlock trees reach 8-10 feet in height, with a spread only about half that.
- The 'Sargent' (or 'Pendula') cultivar has an attractive weeping form. It is supposed to grow 5-8 feet tall (and twice that in width), but it is said to sometimes reach greater heights. Another weeping form suitable for use as a hedge shrub is 'Cole's Prostrate.'