Hemlock trees are represented by the Tsuga genus, which is in the Pinaceae family. The hemlock tree is different from the herb hemlock, which is poisonous. Although the hemlock is native to many continents, the four major species are found in western and eastern North America. The name hemlock was given to the tree because the leaves have a scent similar to the toxic herb.
The four major hemlock species are the western, Tsuga heterophylla; the eastern, Tsuga canadensis; the mountain, Tsuga mertensiana; and the Carolina, Tsuga caroliniana. The eastern variety has many cultivars, most of which are designated as dwarfs or shrubs: beehive, Bennet, Cole's prostrate, Gentsch white, jeddeloh and pendula.
The average height and width of a mature hemlock varies. The eastern can grow to 40 to 70 feet high and 25 to 30 feet wide; the western can reach 175 to 200 feet high and 4 to 5 feet wide. The mountain averages 60 to 100 feet by 2 to 3 feet, and the Carolina can grow to 40 to 60 feet high and 1 to 1.5 feet wide. A member of the pine family, the hemlock produces foliage in the form of needles. Needles range from one-fourth to three-fourths of an inch long. A hemlock's fruit is a cone, which carries seeds. The cones range from a half inch to 3 inches long. The eastern has brown bark; the western and mountain have brownish-black bark; and the Carolina has reddish-brown bark.
The eastern hemlock can be found in the North American eastern coastal and central states, including Missouri, Minnesota, Michigan, Quebec, Ontario, and New Brunswick. The western hemlock grows in California, Oregon, Washington, Idaho, Montana, British Columbia, Alberta and Alaska. The mountain hemlock is found in the same areas as the western, excluding Alberta. The Carolina is found in North and South Carolina, Tennessee, Georgia and Virginia.
All hemlocks prefer moist soil that promotes water drainage. Poor water drainage can lead to overwatering, which creates standing water around the tree. All types prefer full sun to partial shade, although they can survive in heavily shaded areas.
Hemlock trees do not tolerate car exhaust well, and they might suffer near heavy traffic. Hemlock species have shallow root systems, so high winds can topple trees. They are susceptible to mites and tree borers, which are usually treated with fungicide or removal of the infested tree. The adelgid insect is now specifically targeting hemlocks in their native habitat.