<em>Thuja</em>: planting, pruning and care

Description and botany

Thuja, whose Latinised scientific name is Thuja, are conifers with scaly branchlets like Cypresses (Cupressus), false cypresses (Chamaecyparis) and some junipers (Juniperus). They belong to family Cupressaceae. Flattened branchlets distinguish them from Cupressus, as do their smaller, non-spherical cones (12–18 mm) reminiscent of a tulip . Leading shoot remains straight whereas that of Chamaecyparis curves. Genus comprises five species if Thuja orientalis (now Platycladus orientalis) is excluded as the sole species of that genus. For simplicity, this profile treats Thuja in the broad sense. The Japanese thuja (Thujopsis dolobrata) is a conifer closely related to Thuja, recognisable by white patches on underside of its scales.

Thuja originate from cool, often well-watered temperate regions of North America and north-east Asia (China and Korea). When giant thuja grows in alluvial wet soils, in dry rich soils or in sphagnum turf moors of western North America, Canadian thuja occupies eastern peat bogs as well as marshy areas overlying calcareous rock or very shallow calcareous soils, notably in Appalachian region. These conifers typically grow mixed with other forest species such as Douglas-fir, Sitka spruce, Tsuga heterophylla, poplars, alders, birches, ashes…

Thuja occidentalis – botanical illustration

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

Thuja leaves are scales 1–3 mm long that persist for several years. These squamiform scales are imbricate and arranged in four ranks along flattened branchlet, lateral scales being folded in two. On erect, cylindrical branchlets, scales are more elongated, measuring up to 10 mm. Branchlets divide laterally like a fan producing a set of flattened branchlets that fall after a few years once browned. Thuja occidentalis bears resiniferous glands on backs of scales, clearly visible unlike on giant thuja. Oriental Thuja shows branchlets grouped on vertical planes giving characteristic vertically striate appearance.

Bark of trunk, smooth and thin, becomes channelled and splits into long thin strips with age. Roots are shallow, very long and sparsely ramified, provided with long, thin reddish-purple rootlets.

Male and female flowers appear on same tree in April or May. Male cones, reddish and ovoid to globose, about 4 mm, form at tips of branchlets generally on different branches from female flowers. Female conelets, green or purplish, also measure about 4 mm before developing into elongated cones of 10 mm. These latter, composed of 4 to 8 coriaceous scales, open in autumn to release small winged seeds during winter. In Oriental Thuja, cones are fleshy with scales bristly with a recurved point, changing from waxy blue to brown. Its seeds lack wing.

Thuja foliage, cones of Thuja occidentalis and cones of Thuja orientalis

Thuja wood is soft, light, aromatic but free of resin ducts and resistant to rot. It splits easily and is used for making shingles, posts, doors, boats, even totemic masts and canoes in Native American cultures. Wood of Chinese thuja is burnt as incense or used in temple construction.

Main Thuja varieties

Our favourite very low vigour varieties

Our favourite medium vigour varieties

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 rockery or large pots. Very hardy, it tolerates well-drained, even poor and dry soil, and a sunny position.

• Height at maturity 70 cm

See the product sheet

Thuja occidentalis Danica - Canadian Arborvitae

Attractive, neat-looking ball with mid-green foliage that deepens to bronze in winter. Suitable for planting in a rockery or large pot on a terrace. Tolerates well-drained but cool soil and a sunny position.

• Height at maturity 1 m

See the product sheet

Thuja plicata Whipcord - Western Red Cedar

Graceful mound of very pendulous, thread-like foliage, showing a glossy green colour from spring to autumn, then mixed with bronze in winter. Well suited to small gardens or to growing in pots, this small conifer is very adaptable, requires no pruning and is easy to grow.

• Height at maturity 1 m

See the product sheet

Thuja occidentalis Mr Bowling Ball - Canadian Arborvitae

This dwarf globe-shaped thuja stands out for its thread-like foliage of very soft appearance, a delightful blue-green. Perfect for rockery or as a specimen in small gardens. Very hardy conifer that tolerates loose, well-drained but not-too-dry soil, even poor and calcareous, in a sunny position.

• Height at maturity 1 m

See the product sheet

Thuja occidentalis Zmatlik - Canadian Arborvitae

Elegant and architectural, this thuja forms a small bush with a narrow conical habit, supported by spiralled branches. Modest in size and slow-growing, it fits well into a small garden, becoming a focal point or creating an original, refined screen. Very hardy in well-drained, not-too-dry soil. Prefers a sunny position.

• Height at maturity 2,50 m

See the product sheet

Thuja occidentalis Yellow Ribbon - Canadian Arborvitae

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

• Height at maturity 3 m

See the product sheet

Thuja occidentalis Rheingold - Canadian Arborvitae

Sumptuous small thuja with a broad conical habit, displaying through the seasons a wide palette of coppery, tawny or golden shades, vying in splendour. Slow-growing with a moderate size, it allows many uses. Plant in sun in well-drained, not-too-dry soil.

• Height at maturity 1,50 m

See the product sheet

Planting Thuja

Where to plant Thuja?

These very hardy, undemanding conifers are suitable for all regions. The Giant and Canadian Thuja withstand down to −34°C and −28°C respectively, while the Oriental Thuja tolerates only down to −20°C. The latter prefers moist conditions of an oceanic climate, sheltered from strong winds, whereas plicata and occidentalis and dwarf cultivars of Thuja, although they favour a cool soil, tolerate dry situations well where their growth is reduced. Excess moisture can lead to the spread of serious diseases such as Phytophthora when used as hedging. Make sure to plant them in free-draining 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 clipped Thuja hedge, space plants 80 to 100 cm apart and up to 2 m if used as a windbreak left to its natural habit.

• Soak the pot in a bucket of water to moisten it thoroughly.
• Dig a wide hole at least three times the width of the rootball because the roots are fairly shallow and spread widely.
• Avoid planting a Thuja near a bed of perennials or annuals or a vegetable garden unless it is a dwarf variety.
• Add a few handfuls of sand and gravel to ensure good drainage around the roots. In heavy soil, choose raised planting or 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 gently.
• 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 Thuya

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

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

Pruning Thuya

Carry out pruning on trimmed hedges once or twice a year in April and August using a hedge trimmer or a shear. Always leave foliage as old wood does not produce regrowth. Dwarf forms or those with a particular silhouette planted in isolation are simply readjusted if necessary with light pruning. You can also create a hedge with conical specimens of Canadian or oriental thuja without pruning.

Multiplication

Simplest propagation by cuttings consists of propagating twigs in August–September.

Propagation by cuttings

Prepare a box by filling it with a mixture composed of half turf and half sand.

• Take twigs 10 cm long by pulling on them to retain a strip of bark.
• Remove branchings near base of twig.
• Insert these to one-third of their length, avoiding them touching each other.
• Firm gently all around to remove air pockets and ensure good contact between potting soil and cutting.
• Place under cold frame until spring.
• Separate the cuttings and replant in pots or in soil in a nursery for 1 to 2 years or plant directly if soil is favourable.→ Discover our tutorial : How to propagate conifers?

Uses and associations

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

An idea for a mixed planting: 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 coloured-wood dogwoods in background to cheer up winters!

Graphic forms — rounded, weeping, pyramidal — work well with large stones, geometric lines of pools or buildings. But add a touch of whimsy by mixing them with tousled forms of grasses such as Miscanthus or Pennisetum, or on the contrary with a range of other dwarf conifers with prostrate habit (Juniperus horizontalis ‘Blue Chip’), globose (Picea abies ‘Little Gem’), or with the indestructible Microbiota decussata whose autumn colour turns to bronze. True graphic qualities of conifers have a natural place in contemporary garden design, which favours aesthetic of shapes, silhouettes and textures over parade of flowering. These plants, with reassuring permanence, structure a planting long-term, define paths, edge terrace, easily replacing strong presence of clipped box or holly. Key is to play with volumes and colours.

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