Hemlock trees are conifers in the genus Tsuga, and four hemlock species are native to North America. Canadian hemlock, or Eastern hemlock (Tsuga canadensis), is hardy in U.S. Department of Agriculture (USDA) hardiness zones 3 through 7, western hemlock (Tsuga heterophylla) in USDA zones 5 through 7, mountain hemlock (Tsuga mertensiana) in zones 5 through 8 and Carolina hemlock (Tsuga caroliniana) in zones 6 and 7. Douglas fir (Pseudotsuga menziesii Franco), a conifer hardy in USDA zones 4 through 6, is not a true fir but rather a member of the genus Pseudotsuga, so named because of its members' resemblance to hemlocks. Despite the trees' resemblance, however, the cones of hemlock and Douglas fir are very different.
Conifers are gymnosperms, a taxonomic classification that includes all cone-producing trees. Gymnosperms produce seeds inside structures made from specialized leaves called bracts, as opposed to angiosperms, which produce their seeds inside fruits that develop from flowers. The bracts of many conifers dry and toughen as they mature, forming the distinctive cones commonly associated with conifers. The trees of the Pine (Pinaceae) family, of which both hemlock and Douglas fir are members, have cones that open as they mature, exposing the seeds inside them.
The cones of all hemlocks are among the smallest of any trees in the Pine family, and the cones of Eastern hemlock are the smallest of all hemlock cones, ranging from 1/2 to 3/4 inch in length. In comparison, the seed-producing, female cones of a Douglas fir are much larger, ranging from 2 to 4 inches in length; Douglas fir's male cones are smaller.
Bracts and Color
The small cones of hemlocks are thimble-shaped with scalloped, overlapping bracts that change from yellow-green to brown as they mature. The bracts toughen and become woody as they dry and open. Douglas fir cones are purplish or reddish green when young and become brown and woody as they mature. The woody scales of a mature Douglas fir cone, which has a smooth outer margin, are covered by distinctive, three-pronged bracts that extend from between the rows of scales. These bracts distinguish Douglas fir cones from all other conifer cones.
Douglas fir cones mature in a single season and then fall from their tree with the seeds still inside them. Hemlock cones grow to full size by the end of summer, but they do not open fully until well into autumn. The hemlock seeds disperse while the cones are still on their tree, usually into winter, and the cones may remain on their tree for more than one year after they begin to develop.
- Exploring the American West: Douglas Fir
- Silvics of North America, Vol. 1 -- Conifers; U.S. Department of Agriculture, Forest Service
- Missouri Botanical Garden: Tsuga Canadensis
- Washington State University: Mountain Hemlock -- Tsuga Mertensiana
- North Carolina State Cooperative Extension Service: Large Trees for North Carolina
- Arbor Day Foundation: Douglas Fir
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