The Eastern Hemlock

Issue: 
July-August 2013

The eastern hemlock is abundant throughout the Maritime Provinces, along the St. Lawrence, and into Ontario as far north as the northern shore of Georgian Bay and Lake Huron. To the south, it is associated with the Appalachian Mountains, but not occurring at sea level as in Canada. It is uncommon in southern Ontario’s Carolinian forest remnants, and those trees growing there on cool, moist north-facing slopes and ravines may be remnants of more extensive populations. 

Naturally, the eastern hemlock has a graceful, conical form. Its slender and flexible branches spread more or less horizontally from the tapering bole and support heavy foliage. It grows slowly and can live hundreds of years. It can attain a height of about 30 m and a girth of 3.5 m (i.e. about 1 m in diameter). Those characters make it a prized ornamental tree, but old trees with dead branches tend to take on a ragged appearance. The terminal leader tends to droop and flag away from the direction of the prevailing wind. Thus, the leaders’ orientations towards the east make a natural compass in the wood. The sound of the wind in a stand of eastern hemlock, or passing through a single tree, is soft; the branches flex, rise and fall as if gently brushing the restless air. 

As a timber tree, it is not especially valued. In growth, the wood may split between annual rings (called “ring-shake”) or along radial lines (“star-shake”). When cut, the wood splits and warps easily. It is brittle with flinty knots hard enough to quickly dull saws, chip axe blades and deflect nails. Despites those drawbacks, the lumber is used for coarse construction, making boxes, pallets, crates and for railway ties (the ties have the reputation of gripping spikes well). Eastern hemlock is used for pulp and paper manufacture. Young trees can be trimmed easily and used for hedges. When burned, it is sparky so must be used with care. 

There are also hundreds of horticultural selections of eastern hemlock. Dwarf and weeping growth forms are used extensively in landscaping. Propagation is by taking semi-hardwood cuttings from the growth of the year and rooting them. I don’t know how so many have come to be propagated. Perhaps horticulturalists found mutated stems (somatic mutations) on otherwise normal trees and made cuttings. One cultivar, named ‘Jeddeloh’ has Award of Garden Merit status from the Royal Horticultural Society of the UK. Gerry Waldron (author of Trees of the Carolinian Forest (2003)) indicates that Sargentii is his favourite. Most cultivars grow to less than 3 m high and take on shrubby, bushy growth forms. In fall and winter, their greenery contrasts beautifully with leafless deciduous ornamental shrubs, bushes and treelets and also with gleaming garden snowscapes. 

The leafy twigs, shaped like flat sprays, can be used to make tea and impart aroma to sauna steam baths, as in First Nations’ traditions. The inner, bright red-purple bark can be used in traditional medicine to make a tea prescribed for colds, fevers, diarrhea, stomach ailments and scurvy. The bark itself, which tends to dark brown and from scales in younger growth to deep furrows in age, was used when shredded or pounded to make poultices to staunch bleeding from wounds and to treat burns and sores. At one time, the bark of hemlock trees was used extensively for tanning leather. Trees were stripped of bark and left to die. 

The leaves of eastern hemlock are flat and narrow, with rounded or slightly notched tips. Above, they are dark green, but below are paler and with two parallel white stripes. They may remain on the leafy twigs for three years or more. 

The mature seed cones are ovoid and have rather few scales. They are greenish to purple when mature. They are produced on short (2-3 mm) stalks, open and shed their tiny, winged seeds in fall and winter. Seed collection is best when the cones are changing colour. The depleted, brownish-grey cones fall to the ground in spring. The much smaller receptive seed cones, single at the tips of twigs, are pink at pollination time in spring when the yellow, pollen-producing male cones are produced in clusters on the same tree. The spherical pollen grains of eastern hemlock are quite large, about 70-80 µm. They do not have the characteristic double sacs of pine pollen, instead they have an encircling frilly fringe that probably aids in the grains’ aerial buoyancy and capacity to disperse on the wind. Pollination takes place early in summer. Mature trees have heavy seed yields every 2-3 years.

Eastern hemlock grows well under shade and may persist as a sapling for decades. It colonizes sites where the boles and stumps of dead trees are rotting, as does yellow birch. Thus, soils that are slightly acidic to neutral, moist but well drained, humic and rich in nutrients are prime sites for this tree. Seeds germinate the year after they are shed, but the seedling are tiny (about 1 cm tall) and must be nurtured for artificial propagation. It can seed into rocky crevices and grow and develop formidable rooting systems, often cracking the rocks still further. Bare-root transplanting is difficult to achieve once seedlings are older than 3 years. There are numerous fungi that have been recorded associated with eastern hemlock. Some of those are no doubt important as mycorrhizal symbionts involved in the trees’ roots obtaining mineral nutrition from the soil. Root rot fungus (Armillaria mellea) can be a problem. 

In forests, stands dominated by eastern hemlock make a dense canopy that impedes snow accumulation on the floor. That environmental effect of an almost snow-free forest floor is taken advantage of by wildlife such as white-tailed deer, turkey and ruffed grouse during periods of heavy snowfall. Various small birds and mammals eat the seeds. The branchlets are browsed by deer and eaten by porcupines, the leaves and buds by ruffed grouse, and arrays of sapsucker holes perforate the bark.

The fossil pollen record for eastern hemlock indicates that it was hugely abundant following the last ice age, but between then and now (about 10,000 years), it went through an unexplained major decline some 5,000 year ago but has since rebounded in abundance and distribution to some extent. Was it some disease (as suggested by some scientists) or climatic change that changed the genetic structure of the population to allow survival? The diversity of cultivars suggests great genetic variability. 

Until recently, eastern hemlock was thought of as being quite free of insect pests and diseases. Hemlock looper caterpillars feed on foliage and the grubs of hemlock borer, a beetle, leave holes in the bark. Neither is considered a major pest. However, today eastern hemlock, especially in the US, is threatened by the hemlock woolly adelgid (Adelges tsugae). This pest is aphid-like in that it feeds by sucking sap from the phloem. The woolliness of the adelgid protects its body and eggs from cold, heat, drying, natural enemies and pesticides. This insect entered North America on the west coast from its native Asia in the 1920s, but was not found in the east until the late 1960s. It has spread rapidly through the range of eastern hemlock in the US, but to date has not been considered a major pest in eastern Canada. Perhaps with climate change and the warming trends predicted for eastern Canada, this easily dispersed insect will become problematic.  

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