Thuja: planting, pruning and care

Description and botany

Thujas, whose Latinised scientific name is Thuja, are conifers with scale-like shoots similar to the cypresses (Cupressus), false cypresses (Chamaecyparis) and certain junipers (Juniperus). They belong to family Cupressaceae. Flattened shoots distinguish them from Cupressus, as do their smaller, non-spherical cones (12 to 18 mm) that resemble the shape of a tulip. Their leading shoot remains straight whereas that of Chamaecyparis curves. The genus comprises five species if Thuja orientalis — now named Platycladus orientalis, the only species of that genus — is excluded. However, for simplicity this fact sheet treats Thujas in a broad sense. The Japanese thuja (Thujopsis dolobrata) is a conifer related to Thuja, recognisable by white patches on the underside of its scales.

Thujas originate from cool temperate, often well-watered regions of North America and north-eastern Asia (China and Korea). When the giant thuja grows on wet alluvial soils, rich dry soils or sphagnum bogs of western North America, Canadian thuja occupies eastern bogs as well as marshy areas with underlying calcareous rock or very shallow calcareous soils, notably in the Appalachians. These conifers generally grow in mixture with other forest species such as Douglas fir, Sitka spruce, Tsuga heterophylla, poplars, alders, birches, ashes…

Thuja occidentalis – botanical illustration

Giant thuja (Thuja plicata) readily reaches 40 to 60 m in its native range, from southern Alaska to northern California, at altitudes from 0 to 2,000 m, which explains the rapid growth of Thuja hedges even when cultivars are less vigorous, such as ‘Atrovirens’ capable of reaching 20 m. It can live up to 800 years. Species occidentalis (Canadian) and orientalis (Oriental) are much less vigorous, forming pyramidal trees of 15 to 18 m, often conical and compact in isolated or cultivated specimens. The trunk of Thuja occidentalis generally reaches 15 m in height for 30 cm in diameter with an almost columnar habit, but specimens with trunks 25 m high and 90 cm wide are found, as well as small stunted trees over 700 years old growing on the calcareous cliffs of Niagara or in northern Ontario. Platycladus of the Orient, nicknamed “tree of life” by Chinese Buddhists, retains branches down to the ground and a conical to columnar habit for a long time before developing a short trunk and a rounded crown of ascending slender shoots.

Thuja leaves are scales 1 to 3 mm long that persist for several years. These squamiform leaves are imbricate and arranged in four ranks along the flattened shoot, lateral leaves being folded in two. On erect, cylindrical shoots the scales are more elongated, measuring up to 10 mm long. Branchlets divide laterally like a fan, producing a set of flat shoots that brown and fall after several years. Canadian thuja (occidentalis) bears resiniferous glands on the back of the scales, clearly visible unlike those of the giant thuja. Oriental thuja displays branchlets grouped on vertical planes, giving a characteristically vertically striate appearance.

Trunk bark, smooth and thin, becomes channelled and splits into long thin strips with age. Roots are superficial, very long and sparsely ramified, with long thin reddish-purple rootlets.

Male and female flowers appear on the same tree in April or May. Male cones, reddish, ovoid to globular and about 4 mm, form at shoot tips, generally on branches different from the female flowers. Female conelets, green or purplish, also measure about 4 mm before developing into elongated cones of 10 mm. These, composed of 4 to 8 leathery scales, open in autumn to disperse small winged seeds over winter. In oriental thuja, cones are fleshy and bear scales bristly with a recurved point that change from a waxy-blue hue to brown. Its seeds have no wing.

Foliage of Thuja, cones of Thuja occidentalis and cones of Thuja orientalis

Thuja wood is soft, light, aromatic but devoid of resin ducts and resistant to rot. It splits easily and is used for making shingles, posts, doors, boats, even totem poles and canoes in Native American culture. Wood of Chinese thuja is burned as incense or used in temple construction.

Main Thuja varieties

Our favourite varieties of very low vigour

Our favourite varieties of medium vigour

Thuja occidentalis Golden Tuffet - Canadian Arborvitae

Small groundcover conifer, forming a compact, spreading cushion with yellow foliage in summer, turning orange to bronze in winter. Its very slow growth and small size make it suitable for rock gardens or large pots. Very hardy, it tolerates drained soil, even poor and dry, and a sunny exposure.

• Height at maturity 70 cm

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Thuja occidentalis Danica - Canadian Arborvitae

Attractive, neat ball with mid-green foliage that becomes darker to bronze in winter. Suitable for planting in rock gardens or large pots on terraces. Prefers drained but cool soil and a sunny exposure.

• Height at maturity 1 m

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Thuja plicata Whipcord - Western Red Cedar

Graceful mound of very pendulous, thread-like foliage, displaying a lovely glossy green from spring to autumn, then tinged with bronze in winter. Well suited to small gardens or container culture; a small, highly adaptable conifer that requires no pruning and is easy to grow.

• Height at maturity 1 m

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Thuja occidentalis Mr Bowling Ball - Canadian Arborvitae

This dwarf thuja with a rounded habit stands out for its very soft, thread-like foliage of delightful blue-green. Perfect for rock gardens or as a specimen in small gardens. A hardy conifer that tolerates free-draining soil not too dry, even poor and calcareous, in a sunny position.

• Height at maturity 1 m

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Thuja occidentalis Zmatlik - Canadian Arborvitae

Elegant and striking, this thuja forms a small, narrowly conical bush with branches twisted in a spiral. Modest in size with slow growth, it fits small gardens, becoming a focal point or composing an original, refined screen. Very hardy, prefers drained soil that's not too dry. Likes a sunny position.

• Height at maturity 2,50 m

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Thuja occidentalis Yellow Ribbon - Canadian Arborvitae

Small, regular pyramid with green foliage enlivened by golden highlights in spring and taking pretty bronze tones in cold weather. Modest in size, it is well suited to small spaces, container growing as well as composing low hedges, almost maintenance-free.

• Height at maturity 3 m

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Thuja occidentalis Rheingold - Canadian Arborvitae

Sumptuous small thuja, with a broad conical habit, showcasing through seasons a wide palette of coppery, burnished or golden shades, vying in splendour. Its slow growth and moderate size allow many uses. Plant in sun in drained soil that's not too dry.

• Height at maturity 1,50 m

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Planting Thuja

Where to plant Thuja?

These very hardy, undemanding conifers are suited to all our regions. Giant and Canadian Thuja tolerate down to −34°C and −28°C respectively, while Oriental Thuja tolerates only down to −20°C. The latter prefers the damp conditions of an oceanic climate, sheltered from strong winds, whereas plicata and occidentalis as well as dwarf Thuja cultivars, although they appreciate a cool soil, tolerate dry situations well where their growth is reduced. Excess moisture can lead to serious diseases such as Phytophthora when they form hedges. Ensure you plant them in drained soil.

Exposure can be sunny to semi-shaded.

When to plant?

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

How to plant?

This plant is easy to grow. To create a trimmed Thuja hedge, space plants 80–100 cm apart and up to 2 m if it is a windbreak left in free habit.

• Plunge the pot into a bucket of water to soak it thoroughly.
• Dig a wide hole at least three times the width of the rootball because the roots remain fairly shallow and spread widely.
• Avoid planting a thuja near a bed of perennials or annuals or near a vegetable plot unless they are dwarf varieties.
• Add a few handfuls of sand and gravel to ensure good drainage around the roots. In heavy soil, opt for planting on a raised mound or within a rockery.
• Add a dose of horn meal if the soil is sandy.
• Place the plant in the planting hole.
• Replace the soil and firm lightly.
• Water.

Some Thuja have particularly bright foliage such as T. occidentalis ‘Rheingold’, ‘Golden Tuffet’, ‘Golden Globe’, etc

Care and pruning

Main pests and diseases of Thuja

Thuja planted alone and away from thuja hedges generally requires no maintenance. However, its very common use in single-species hedges sometimes leads to rapid contagion such as more or less severe twig dieback, which can be caused by:

• colonies of aphids (apply an insecticidal treatment quickly such as soap or fern manure),
• juniper buprestid which tunnels into the wood and causes browning in summer. The only solution is to cut and burn the swollen branches.
• diseases such as cryptogamic browning. This appears on lower branches during wet springs. Apply a fungicidal treatment every 15 days based on copper or a decoction of horsetail, or cut and burn affected branches. Phytophthora, which causes progressive drying out of the entire tree, shows necrosis under the bark of the collar tissues and roots. Uproot and burn affected specimens and plant non-susceptible plants (Chamaecyparis is the conifer most susceptible to this decline).

Pruning Thuja

Carry out pruning on trimmed hedges 1 to 2 times per year in April and August with a hedge trimmer or a shear. Always leave foliage because old wood does not produce shoots. Dwarf forms or those with a particular silhouette planted alone are merely adjusted if necessary by light pruning. You can also create a hedge with conical Thuja specimens from Canada or the Orient without carrying out pruning.

Propagation

The simplest way to multiply plants is to take twigs as cuttings in August–September.

Propagation by cuttings

Prepare a box by filling it with a mix made up of equal parts turf and sand.

• Take 10 cm-long twigs by pulling them to retain a strip of bark.
• Remove branching near the base of the twig.
• Insert these to one-third of their length, taking care they do not touch.
• Firm down gently all around to remove air pockets and ensure good contact between compost and cutting.
• Place them under a cold frame until spring.
• Pot up or plant out cuttings in a nursery for 1–2 years, or plant directly in final position if soil is favourable.→ Discover our tutorial : How to take cuttings from conifers?

Uses and companion plants

Foliage of thuja offers varied textures and shades that sometimes declinate to other colours over seasons such as in Thuja plicata Atrovirens which changes from dark green to reddish with cold, Thuja occidentalis Golden Tuffet with tousled yellow shoots in summer that turn orange in winter, Platycladus orientalis Elegantissima that shifts to bright yellow… Use these qualities to brighten or energise a neglected corner of garden.

Idea for a planting scheme for a border: 3 to 5 Thuya occidentalis ‘Smaragd’ surrounded by Crocosmia ‘Lucifer’, Dahlia ‘Karma Choc’, a few clumps of Pennisetum orientale, Sedum ‘Purple Emperor’ and Festuca glauca in foreground. Do not hesitate to add dogwoods with coloured wood in background to enliven winters !

Graphic forms — spherical, weeping, pyramidal — sit well with large stones, geometric lines of pools or buildings. But add a touch of madness by mixing them with tousled forms of grasses such as Miscanthus or Pennisetum, or conversely with a series of other dwarf conifers with prostrate habit (Juniperus horizontalis Blue Chip), globose (Picea abies Little Gem), or with indestructible Microbiota decussata whose autumn colour turns to bronze. The real graphic qualities of conifers assert themselves naturally in contemporary garden design, which prefers aesthetics of shapes, silhouettes and textures to parade of flowering. These plants with reassuring permanence structure a border sustainably, mark paths, edge terrace, easily substituting for strong presence of clipped boxwood or holly. The key is to play with volumes and colours.

With a habit naturally conical and narrow as in Thuya occidentalis Barbant or Thuya occidentalis Yellow Ribbon, you can create an original hedge without pruning work that will become contiguous over time with planting distance of 1 m. Conversely, spacing of 70 cm and repeated pruning will very quickly produce an opaque, uniform screen.