Pacific Northwest Native Hemlocks Bring Grace and Beauty to the Winter Garden

By Corinne Kennedy

Tsuga heterophylla, western hemlock, in the northeastern area of the Garden. (photo: Corinne Kennedy)

Visitors to the Seattle Japanese Garden are sure to notice the flowering trees and shrubs of spring – and later in the year, the fiery fall colors of maples and other deciduous trees and shrubs. Evergreens without showy flowers, including conifers, are often overlooked, despite their myriad shades of green.  Yet they form the Garden’s underlying structure, and are worthy of our attention, especially in the “green” seasons of late fall and winter.

Although our Garden has closed for the winter, now is a great time to look for and appreciate evergreens – in your own garden, in the Washington Park Arboretum, and when you visit the Seattle Japanese Garden again in spring.

Two Pacific Northwest hemlock species are among the native conifers planted here. Like other PNW natives, they ensure that the Garden feels “rooted” in this place. They also harmonize with its other plants, natives of Japan. Western hemlock (Tsuga heterophylla) and mountain hemlock (Tsuga mertensiana) are denizens of the forests and mountains of Western Washington, graceful conifers seen by many of us on summer hikes. Like other Tsuga species, they are evergreen needled conifers characterized by a drooping leader, horizontal branches, and slender branchlets with drooping tips. Their small, papery seed cones fall without shattering.

The genus name, Tsuga, derives from the Japanese tsu-ga, meaning “tree” and “mother,” indicating the importance of Japan’s own native hemlocks, Tsuga sieboldii and Tsuga diversifolia, within its gardens. This interpretation certainly captures the affection I feel for our own.

Tsuga heterophylla (western hemlock):

Tsuga heterophylla is a shade-tolerant conifer common to Pacific Northwest forests, and important commercially for lumber production. In the Seattle Japanese Garden, it appears along the east side fence and in the mountain area at the Garden’s southern end. Both areas are densely planted, with many tall trees, representing the forests of Japan.

Growing at low to middle elevations (up to about 3500 feet), western hemlocks are hardy to USDA Zone 6 (minimum temperature -10 to 0 degrees F). Their range extends from south-central Alaska to northern California. Under ideal conditions – sites west of the Cascades with high rainfall and alluvial soils (deep fertile soils deposited by water flowing over plains or in riverbeds) – trees may reach 250 feet tall.  They can be found growing in almost pure stands or together with most other PNW conifers.  Maximum age is about 500 years.

A slender, pyramidal tree with a narrow crown, a drooping leader and foliage that appears delicate and feathery, Tsuga heterophylla displays a gracefulness unlike that of most conifers. Branches sweep downwards, with needles in flat sprays along the branches. The species name, heterophylla, means “variable leaves,” indicating that the needles vary in length – from ¼ to ¾ of an inch. Short, flat, round-tipped, and twisted at the base, they are irregularly spaced. Their color also varies – from yellowish-green on the upper sides to a lighter green underneath, the result of two broad white bands of tiny pores, known as stomata.

Foliage of western hemlock, Tsuga heterophylla. (photo: Corinne Kennedy)

Western hemlock bark is rough and reddish-brown, becoming thick and furrowed with age. The delicate-looking, papery seed cones are small (¾  to 1 inch long), oval, and borne profusely on the branchlets. Greenish when young, they mature a light brown before dropping to cover the forest floor. Each cone contains about 30-40 seeds. A western hemlock stand in Oregon was found to produce an average of eight million seeds per acre each year.

I know this abundance from personal experience. Seed cones from our neighbor’s western hemlock fall into our gutters and cover the ground in nearby areas of my garden. They germinate successfully in the gutters, and one spring, when conditions were apparently perfect, thousands of tiny seedlings appeared in my garden beds.

Western hemlock is not drought tolerant, so seedlings require consistent soil moisture. However, they will grow into trees on various soil types – including under highly organic conditions, such as on rotting stumps and decaying litter. They are capable of surviving, with extremely slow growth, for many years. When more favorable conditions occur, their growth rate normalizes.

Trees grow well in sun as well as shade so long as moisture remains adequate. However, seedlings do not compete well with more sun-loving trees. Unlike those pioneer species, Tsuga heterophylla is shade-tolerant. It seeds prolifically, producing seedlings that will grow in the shade of the “parent” trees. Eventually it becomes the dominant tree species, initiating the final or climax stage in the process of forest succession. This stability persists, ending only when a fire or other disturbance eliminates the climax species and the process begins again with pioneer plants.

Tsuga mertensiana (mountain hemlock):

As indicated by its common name, mountain hemlock is a higher-elevation species. Its species name, mertensiana, acknowledges the German botanist who discovered it, Karl Heinrich Mertens. Many specimens are planted in the southern, “mountain” area of the Seattle Japanese Garden – including a tall tree just past the south entrance gate and two younger ones next to the west side fence.

Tsuga mertensiana, mountain hemlock, in the southwestern area of the Garden. (photo: Corinne Kennedy)

Tsuga mertensiana is widespread in the Cascades and Olympics of Washington state, usually at elevations of 4,000 – 7,000 feet. Its range, however, extends from southeastern Alaska to California’s central Sierra Nevada mountains. Hardy to USDA Zone 5 (minimum temperature -20 to -10 degrees F), it thrives in areas with long winters and deep, wet snow-packs. Hence its tolerance of heavy snow loads and massive runoff – and its preference for moist soils and cool environments.  Trees grow in the upper reaches of forests and in the open meadows of subalpine parkland, in pure or mixed stands. In the latter, they most often appear together with subalpine fir, Pacific silver fir, and/or Alaska cedar. The species also grows well at sea level, where relatively slow growth makes it an excellent choice for small residential gardens. I love its irregular and strongly picturesque habit, which calls to mind its dramatic mountain origins.

Mountain hemlock is characterized by a slightly drooping leader and branches of unequal length that spread horizontally or angle downwards, then bend upward at their tips. Its needles, unlike those of western hemlock, are nearly equal in length and displayed all around the twigs. With stomata on both surfaces, their color is an attractive bluish-green.

Foliage of mountain hemlock, Tsuga mertensiana. (photo: Corinne Kennedy)

The bark is dark reddish-brown, thicker and more deeply furrowed than that of other hemlocks. Brown seed cones, often purplish when young, are cylindrical in shape, with tapered ends. They’re also larger than the cones of other hemlocks – usually about 1½ to 2 ½  inches in length. Like western hemlocks, trees seed prolifically, and their seedlings are often the first trees to colonize timberline meadows and moraines (accumulations of soil and rock deposited by moving glaciers). They also reproduce extensively through layering, where lower branches touch the ground, take root, and develop into upright trees.

Mountain hemlock’s height and habit vary with location.  Wind-exposed timberline specimens are generally under ten feet tall, sometimes with a shrublike multi-trunked form often referred to as “alpine scrub.”  At lower elevations, trees may exceed 100 feet in height. They grow well in sun or part shade, and at varying elevations, so long as the soil remains moist. Intolerant of urban pollution, they are less shade-tolerant than western hemlocks, sometimes forming the overstory above a Tsuga heterophylla understory.

Although the Seattle Japanese Garden is closed until spring, I encourage blog readers to look for our two native hemlock species on your winter walks – perhaps even in your own neighborhood. Notice the birds that eat their seeds – including juncos, siskins, chickadees, finches, and crossbills. Look for tall specimens of western hemlock in the Arboretum and at Carkeek, Discovery, Lincoln, Schmitz and Seward Parks. Several dwarf cultivars have been developed and are appropriate for residential gardens, but they’re not easy to distinguish from the many dwarf cultivars of Tsuga canadensis, native to the East Coast. Look for mountain hemlocks in the Arboretum, at the Locks, on the UW campus, and in private gardens, where they usually reach only 20-30 feet in height.

Corinne Kennedy is a Garden Guide, frequent contributor to the Seattle Japanese Garden blog, and retired garden designer.