<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 include nearly a dozen species, of which three are native to Canada and three to China. The most widespread in gardens is Tsuga canadensis which grows between Lake Superior and Cape Breton Island, within mixed forests and sugar‑maple stands, always in shade, often under the canopy of other trees on ground carpeted with moss. It reaches average dimensions of 25 to 30 m tall by 8 to 10 m wide, with a trunk diameter of 1 m, only 20 m in cultivation. Its longevity reaches 600 years. It has given rise to several dwarf cultivars forming graceful mounds of flexible branches such as Jeddeloh (80 cm high by 120 cm wide) or Pendula (3.5 m high if staked, 5 m wide). Growth is slow in all cases except in western hemlock (Tsuga heterophylla).

Younger hemlocks have a dense conical or columnar crown ending in an oblique leading shoot leaning in the direction of the wind and a horizontal or slightly trailing branching habit. With age the architecture becomes more irregular with branches unevenly spaced, dead branches that remain on the trunk for a long time and an uneven crown, giving the tree a more supple, graceful habit than a fir or spruce. Roots are superficial and spreading. Bark becomes channelled and scaly from an early age, showing purple streaks immediately after cutting. Initially grey‑brown or silvery and smooth in Tsuga canadensis, it later forms 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 in length. Needles remain on the tree for 3 to 10 years before falling. The narrow lamina narrows abruptly at the base into a short petiole extended by a 3 mm small cushion applied along the shoot as in genus Picea.

Tsuga canadensis – botanical illustration

Needles arranged in a spiral around the shoot appear to be inserted on two ranks with some short needles on the 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 the underside of the needles, which helps distinguish them from Taxus (yew) with pale green bands. Young shoots are thin, flexible and pubescent while buds are small (2 mm), ovoid, unlike very acute buds of Pseudotsuga, and not resinous.

Male and female cones appear on the same tree. Male aments, globose at about 3 mm, appear in the lower part of the crown at the axils of the previous year’s needles and are borne on a remarkable scaly peduncle. They release pollen in spring before withering. Female conelets, pinkish and slightly larger than the males, appear at the tips of short shoots of the previous year in the upper crown. Pollinated just before new foliage flush, they ripen within the year and hang abundantly at branch tips until the following summer, while winged seeds are released at the end of the year. In Tsuga canadensis and heterophylla, cones with rounded scales of a lovely 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 that of canadensis which is rather soft and whose bark was formerly used for its tannin content. The wood gives off a pleasant scent just after sawing. Western hemlock (Tsuga heterophylla), native to the west coast and mountains of North America (Sierra Nevada) up to 1500 m altitude, is an important forest species used for production of timber (interior panelling, railway sleepers), plywood and pulp, etc., and is also exploited in the wetter areas of western Europe. Its growth is faster than Tsuga canadensis. Tsuga heterophylla and Tsuga chinensis (with golden young shoots and a shrubby habit) are also excellent species for ornamental gardens. Tsugas are important as a source of food (foliage and seeds) and shelter for wildlife.

Main varieties of Tsuga

Original varieties

Tsuga canadensis Pendula - Eastern Hemlock

Modest-sized conifer, with slow growth, forming an opulent, spreading, low dome. Green, glossy needles are borne along invariably weeping branches, resembling draperies, conferring the bush exceptional ornamental value. Prefers cool, neutral to acidic soil and a position not exposed to strong sun, sheltered from the 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, making an excellent groundcover for cool shade. Over time, a pleasing contrast develops between the white bark of the branches at the centre and the dark green of the needles. Ideal for covering the 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 with a depressed centre, comparable to a funnel-shaped bird's nest. Tips of the tiered branches droop, barely touching the soil.

• Height at maturity 80 cm

See the product sheet

Planting

Where to plant Hemlock?

Very hardy, Tsuga prefer a north-facing or heavily shaded aspect, an east-facing position or a spot within a grove, benefitting from abundant rainfall or the edges of a pond, a watercourse or a marsh. Keep them away from a building or a path as roots are shallow and very spreading. Plant them in deep soil rich in organic matter and moist as they do not tolerate drought. Avoid direct afternoon sun as well as urban pollution and wind. Tsuga canadensis Jeddeloh, however, accepts full sun in cool soil.

Tsuga can be used as a free-growing or clipped hedge because 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 plants 1.50 m (pruned) 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 root ball as roots remain fairly shallow and spread widely.
• Add a few handfuls of sand and gravel to ensure good drainage around the roots, which do not like to be waterlogged. Opt for planting on a raised mound or in a rockery if necessary.
• Bring in organic matter and a dose of horn meal if soil is sandy.
• Place the plant in the planting hole.
• Replace the soil and firm lightly.
• Water and mulch generously.

Cones of Tsuga sinensis

Care

• Water Tsuga well during first three years, then during prolonged drought.
• Tsuga needs very little maintenance given its slow growth and robustness against pests and diseases. If in a container, apply conifer fertiliser every year in April and hoe the soil in summer to limit evaporation of water from soil, or mulch. Be careful however with mites if air is too dry. Mist foliage if needles discolour.
• Confine yourself to removing dead wood, which becomes abundant if left unchecked. Its habit is naturally trailing and does not require pruning, but pruning can be carried out in spring.

Multiplication

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

Propagation by cuttings

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

Sowing

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

Uses and companion plants

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

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

An idea for a planting combination in acid soil : Tsuga canadensis ‘Jeddeloh’, Acer palmatum ‘Atropurpureum’, Rhododendron ‘Marcel Menard’, Rhododendron ‘Dream Land’ et Acer palmatum ‘Dissectum’

Specimens such as Tsuga canadensis Pendula make a statement in the middle of a short grass meadow, a large rock garden or atop a 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 to 6 m spread at ripeness. If, however, its growth is trained upwards, it will reach 3 to 4 m in height for 5 m diameter.

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

Its fine, glossy dark green foliage pairs well with large stones in a rock garden that hosts other dwarf conifers, allowing play with volume and colour. Hemlock can also contribute to the setting of a natural pond or water feature.

Did you know?

Hemlock’s scientific name, Tsuga, originates from the Japanese name for this tree.