<em>Tsuga</em>, hemlock: planting, pruning and care

Description and botany

Hemlocks or Tsuga in Latin, are native to temperate regions of North America, the Himalaya and eastern Asia and comprise nearly ten species, of which 3 are native to Canada and 3 to China. Most widespread in gardens is Tsuga canadensis, which grows between Lake Superior and Cape Breton Island within mixed forests and maple stands, always in shade, often beneath canopy of other trees on soil carpeted with moss. It reaches average dimensions of 25 to 30 m tall by 8 to 10 m wide, with trunk diameter of 1 m, only 20 m in cultivation. Longevity reaches 600 years. It has given rise to several dwarf cultivars forming graceful mounds of supple branches such as Jeddeloh (80 cm high by 120 cm wide) or Pendula (3.5 m high if staked, up to 5 m wide). Growth is slow in all cases except western hemlock (Tsuga heterophylla).

Young hemlocks have a dense conical or columnar crown ending in an oblique leading shoot inclined in direction of wind and with horizontal or slightly trailing branches. With age, architecture becomes more irregular with branches unevenly spaced, dead branches remaining on trunk for a long time and an uneven crown, giving tree a habit more supple and graceful than that of a fir or spruce. Roots are shallow and spreading. Bark becomes channelled and scaly from a young age and shows violet streaks just after cutting. Initially grey-brown or silvery and smooth in Tsuga canadensis, it later develops broad ridges and furrows turning reddish-brown to rusty brown in heterophylla.

Evergreen foliage consists of flat, flexible needles with rounded or notched tips, fairly short, not exceeding 22 mm long. Needles remain on tree for 3 to 10 years before dropping. Narrow lamina narrows abruptly at base into a short petiole prolonged by a small cushion of 3 mm applied along the shoot as in genus Picea.

Tsuga canadensis – botanical illustration

Needles arranged in a spiral around the shoot appear inserted in two ranks with some short needles on upper side in Tsuga canadensis and heterophylla, whereas in subalpine hemlock (Tsuga mertensiana) they are arranged like a brush all around the shoot. White bands of stomata clearly mark underside of needles, allowing distinction from Taxus (yew) which has pale green bands. Young shoots are slender, flexible and pubescent while buds are small (2 mm), ovoid, unlike very acute buds of Pseudotsuga, and non-resinous.

Male and female cones appear on same tree. Male globose aments of about 3 mm appear in lower part of crown, in axil of needles of previous year’s shoots, and are borne on a surprising scaly peduncle. They release pollen in spring before withering. Pinkish female conelets, slightly larger than males, appear at tips of short shoots of previous year in upper part of crown. Cones are pollinated just before new foliage growth, then mature within the year and hang abundantly at branch tips until following summer while winged seeds are released at year-end. In Tsuga canadensis and heterophylla, cones with rounded scales of a pretty purplish-brown measure only 12 to 25 mm, whereas in Tsuga mertensiana their size reaches 30 to 80 mm.

Tsuga wood, pale orange-yellow, lacks resiniferous canals and is relatively hard compared with most conifers, except wood of species canadensis which is rather soft and whose bark was formerly exploited for its rich tannic acid. Wood gives off a pleasant scent just after sawing. Western hemlock (Tsuga heterophylla), native to west coast and mountains of North America (Sierra Nevada) up to 1500 m altitude, is however an important forest species used for production of timber (interior panelling, railway sleepers), plywood and pulp, etc., and is also exploited in wettest areas of western Europe. Its growth is faster than that of Tsuga canadensis. Tsuga heterophylla and chinensis (with young shoots golden-green and shrubby habit) are also very fine species for garden ornament. Hemlocks are important as source of food (foliage and seeds) and shelter for wildlife.

Main varieties of Tsuga

Original varieties

Tsuga canadensis Pendula - Eastern Hemlock

Moderately sized conifer, slow-growth, forming an opulent, spreading low dome. Green glossy needles set along invariably weeping branches, like drapery, giving bush exceptional ornamental value. Prefers fresh, neutral to acid soil and a position not too exposed to heat, sheltered from wind.

• Height at maturity 3,50 m

See the product sheet

Dwarf varieties

Tsuga canadensis Cole's Prostrate

Dwarf hemlock forming a small prostrate, creeping dome, an excellent groundcover for cool shade. Over time a beautiful contrast forms between white bark of inner branches and dark green needles. Ideal for covering base of a shaded bank, running between rocks in a rockery or spilling over a low wall.

• Height at maturity 30 cm

See the product sheet

Tsuga canadensis Jeddeloh - Eastern Hemlock

Dwarf conifer forming a flattened hemisphere, depressed at centre, comparable to a funnel-shaped birds' nest. Tips of tiered branches droop, barely touching soil.

• Height at maturity 80 cm

See the product sheet

Planting

Where to plant Tsuga?

Very hardy, Tsuga prefer a north or largely shaded aspect, east or within a small wood, benefiting from abundant rainfall or near a pond, stream or marsh. Keep them away from buildings or paths as roots are shallow and very spreading. Plant in deep soil rich in organic matter and kept moist as they do not tolerate drought. Avoid direct afternoon sun as well as urban pollution and wind. Tsuga canadensis Jeddeloh however tolerates full sun in cool soil.

Tsuga can be used as a free-form or clipped hedge as it tolerates pruning well.

When to plant?

Prefer autumn (October–November) to plant Tsuga, or February–March.

How to plant?

This plant is easy to grow in cool shaded conditions. To create a Tsuga hedge, space young plants 1.5 m (clipped) to 3 m (free habit).

• Immerse the pot in a bucket of water to thoroughly moisten it.
• Dig a wide hole at least three times wider than the rootball because roots remain fairly shallow and spread widely.
• Add a few handfuls of sand and gravel to ensure good drainage around roots, which do not like to be waterlogged. If necessary, opt for planting on a mound or within a rockery.
• Add organic matter and a dose of horn meal if soil is sandy.
• Place plant in the planting hole.
• Replace soil and firm lightly.
• Water and mulch generously.

Cones of Tsuga sinensis

Care

• Water Tsuga thoroughly during first three years and in periods of prolonged drought.
• Tsuga needs very little maintenance given its slow growth and its robustness against pests and diseases. If grown in a pot, apply a special conifer fertiliser every year in April and hoe soil in summer to limit evaporation of water from soil, or mulch. Watch for spider mites if air becomes too dry. Spray foliage with water if needles discolour.
• Just remove dead wood that becomes abundant if left alone. Its habit is naturally trailing and does not require pruning, but pruning can be carried out in spring.

Multiplication

Easiest method is to propagate Tsuga by cuttings in summer–autumn or to sow seeds in spring.

Propagation by cuttings

• Prepare a deep pot by filling it with potting compost mixed with sand.
• Take semi-ripe shoot tips about 10 cm long with a heel.
• Remove leaves near base of the cutting and dip the base in plant hormone for cuttings.
• Insert them to two-thirds of their length, ensuring they do not touch one another.
• Keep them covered in shade, for example by placing a cut-off clear plastic bottle over them.
• In spring, separate the rooted cuttings and plant them in buckets which you keep under a cold frame until autumn.

Sowing

Sow seeds in a sandy mix and place them under a cold frame.

Uses and associations

Typical species make very attractive solitary specimens with their dark green, lightly textured trailing branches, a little like Himalayan cedar. The advantage is that the tree will not try to grow tall to find light. If you live near forest, deer will prefer to browse cedar rather than hemlock.

Tsuga form attractive combinations as in nature with birch, beech, sugar maple, Weymouth pine (Pinus strobus), spruces such as Picea sitkensis, red spruce (Picea rubens) or white spruce (Picea glauca), Sequoia sempervirens… There are golden-foliaged forms such as Tsuga canadensis ‘Aurea’ that are more compact than the type and can brighten shaded areas.

Idea for an association in acid soil: Tsuga canadensis ‘Jeddeloh’, Acer palmatum ‘Atropurpureum’, Rhododendron ‘Marcel Menard’, Rhododendron ‘Dream Land’ and Acer palmatum ‘Dissectum’

Specimens such as Tsuga canadensis Pendula make a statement in the middle of a short grass meadow, in a large rock garden or atop a low wall, forming a large weeping dome, spreading and low, or truly creeping if left to grow naturally. With slow growth, if it grows without gardener intervention, it will not exceed 1 m in height for 5–6 m spread at ripeness. If, however, its growth is guided upwards, it will reach 3–4 m in height for 5 m diameter.

Tsuga tolerates pruning well so it can be used as a sumptuous backdrop at the rear of a border or to mark the edge of a small wood, as it appreciates the shelter provided by larger specimens.

Its fine glossy dark green foliage complements the large stones of a rock garden that hosts other dwarf conifers, where volumes and colours can be played with. Hemlock can also contribute to the décor of a natural pond or water feature.

Did you know?

Scientific name of hemlock, Tsuga, originally comes from the Japanese name for this tree.