Plant of the Month: Mountain Hemlock
December and January are dark, cool, and wet in Pacific Northwest lowlands while the high country is buried under a thick blanket of snow. It’s sometimes challenging to bring high-elevation plants down to lowland gardens but Mountain Hemlock, Tsuga mertensiana, is an exception.
Native to west coast mountains from southeast Alaska down to northern California and then inland east of the Sierras, Mountain Hemlock makes a good garden specimen tree. It’s much slower growing than its cousins Western Hemlock (T. heterophylla) and Canadian Hemlock (T. canadensis) so it’s not as likely to get too big for your garden too quickly. It’s hardy to Zone 5 and can handle heavy winter snows.
All of the Hemlocks can be recognized by their short, flat needles, small cones, and drooping leader at the top of the tree. Needles on Mountain Hemlock are up to an inch long, somewhat glaucous (a blueish waxy coating), and wrap around the twig a bit like a bottle brush, in contrast to Western and Canadian Hemlock which hold their needles in a more-or-less straight line across the twig. Mountain Hemlock cones are 2-5 inches long whereas the other two have cones an inch or less long.
Although it grows slowly, it will eventually reach 40 feet or more so be sure to give it room to grow. Like many other conifers, Mountain Hemlock thrives in somewhat acidic soil. It also appreciates moist soil with plenty of humus and good drainage so it doesn’t become waterlogged in the winter. Its roots are shallow so a little protection from high winds is also a good idea. In its native habitat Mountain Hemlock depends mostly on water from melting winter snow. After it’s established in the garden only a little summer water will be needed.
In northwest Washington where I live Mountain Hemlock starts to replace Western Hemlock in our forests around 2,500’ elevation and grows up to the treeline. Old gnarled trees stand sentinel on corniced ridges, crusted in windblown snow after each passing storm.