In short
- The most popular hedging conifer in Central Europe.
- Needs regular watering — sensitive to drought.
- Tolerates formative pruning, but not cuts into old, leafless wood.
- All parts contain toxic thujone.
- Frost-hardy, but can dry out during frosty, snow-poor winters.
- A shallow, wide-spreading root system strongly dries out the soil at the base of the hedge.
- Smaragd and Columna keep their green through winter; Brabant discolors slightly and the species itself most of all — a natural response, not a disease.
- Copes poorly with salt — beside roads treated with de-icing salt it browns on the side facing the carriageway.
Botanical data
- Family
- Cupressaceae (Cupressaceae)
- Height
- 3–15 m
- Width
- 1–4 m
- Habit
- Columnar
- Growth rate
- Moderate
- Position
- Full sun, Partial shade
- Soil
- Loamy, Humus-rich
- pH reaction
- pH 6–8
- Moisture
- Moderate, Moist
- Bloom
- —
- Hardiness
- USDA 3a–8a
- Propagation
- From cuttings
Characteristics
An evergreen tree with a conical or columnar habit and flattened, scale-like leaves with a strong, resinous scent when crushed. Numerous cultivars differ in color (from dark green to golden) and size. In gardens the species and the hedging cultivars reach 3-15 m in height and 1-4 m in width and grow at a moderate rate — hedging cultivars put on anywhere from ten-odd to several dozen centimeters a year, while dwarf and globe-shaped cultivars stop at 0.4-2 m. The sprays are flat and held in a single plane, as in all arborvitae; the feature that distinguishes this species from western red cedar (Thuja plicata) is the distinct, raised resin gland on the upper side of the tightly appressed scales. The plant is monoecious: in spring, tiny reddish-brown male cones appear at the tips of the sprays, while inconspicuous female cones form on other shoots of the same plant and develop by autumn into upright cones about a centimeter long, ripening from green to light brown. In winter the scales of the species and of some cultivars take on a brownish tone, only to color up again in spring — a natural response, not a symptom of disease. The root system is shallow and wide-spreading, hence the low drought tolerance and the rapid browning in response to drying out. It needs a sunny or partly shaded position and fertile, moderately moist soil ranging from slightly acidic to alkaline (pH about 6-8), ideally neutral to alkaline; in the wild it grows both on limestone rocks and along the edges of wetlands, so it copes well with lime and periodic wet, but poorly with salinity in the soil. Hardiness reaches USDA zone 3a, but in snow-free, frosty winters and under strong February sun winter desiccation sets in easily.
Popular cultivars
- ‘Smaragd’
- The most popular hedging cultivar: a narrow, dense cone to 4-5 m, with fresh green scales that do not brown in winter. It grows slowly, so the hedge closes up later, but holds its shape for a long time.
- ‘Brabant’
- Fast-growing cultivar for tall hedges and screens, reaching 4-6 m and, with age, even up to 10 m. Its habit is looser than that of Smaragd, the scales brown slightly in winter, and keeping it in shape calls for more frequent pruning.
- ‘Danica’
- Globe-shaped dwarf cultivar reaching about 0.4-0.6 m in height and width, with dense, vertically held sprays. For edging, beds, rock gardens and containers; its green browns slightly in winter.
- ‘Golden Globe’
- Globe-shaped cultivar with golden-yellow scales that turn a coppery tone in winter, reaching about 1 m. The color is most intense in full sun and greens up in shade.
- ‘Columna’
- A narrow, stiff column with dark green scales, reaching 6-10 m with a width of just 1-1.5 m; it barely browns in winter. For tall screens, avenues and vertical accents where there is no room for a wide crown.
- ‘Rheingold’
- Slow-growing cultivar with a conical to globe-shaped habit and partly needle-like, golden-copper foliage that turns a warm brown in winter. It reaches 1-2 m and in time about 3 m; planted as a color accent rather than for hedging.
Growing and care
Watering
Sensitive to drying out — browning scales are often a result of drought. Needs regular watering, especially in the first years and during heat waves.
Fertilizing
Conifer fertilizers contain magnesium, which prevents yellowing; apply the last feeding by July.
Planting
Fertile, moderately moist soil; for a hedge, plant with a spacing of 50-70 cm.
Pruning calendar
Pruning group: Conifers — minimal pruning
What and how to cut
Remove only dead and damaged shoots. Do any shaping lightly, in late spring or summer, staying strictly within the green, leafy shoots — don't reach deeper with the shears.
Spring and late summer (for hedges).
What not to do
Don't cut into old, leafless (brown) wood — it won't put out new shoots and a bare gap will be left.
Companion plants
Good companions
The green wall of thuja provides a neutral backdrop for flowering perennials and grasses planted in front of it.
Bad companions
A dense thuja hedge heavily shades and dries out the soil at its base, making it difficult for plants growing right next to it to thrive.
The evidence level indicates whether the relationship is backed by research, observation, or gardening tradition.
Diseases and pests
The fungus attacks the roots and stem base of trees and shrubs, causing their slow decline – the plants grow weakly, the crown thins, the leaves become smaller, and eventually they dry up. The diagnostic sign is white, fan-shaped sheets of fungal mycelium forming a dense layer under the bark at the stem base, with a distinct mushroom smell. In the soil and under the bark there are black or dark-brown, bootlace-like rhizomorphs, by which the fungus spreads to further plants. In autumn, clusters of fruiting bodies – honey mushrooms – grow at the base of infected trees. The above-ground symptoms are non-specific and easily confused with drought or overfeeding: some plants die suddenly (the whole tree turns brown within a single season, especially in hot weather), while others weaken over years – producing short growth, discolouring prematurely, and just before death setting an exceptionally heavy crop of flowers or fruit ('stress flowering'). In conifers, resin bleeding at the stem base is common. When the bark is exposed at the root collar, the wood of the roots and stem turns white and softly rots – honey fungus causes a white rot of the wood. The mycelial layer can be cream-coloured and leathery, spreading radially from beneath the dead bark. A biological curiosity, rather than a sign useful in garden diagnosis, is that the mycelium in moist, infected wood can glow very faintly in the dark (so-called 'cold light') – the effect is too weak and requires complete darkness, so in practice it cannot be used to identify the disease. The disease advances in foci – neighbouring plants die off one after another around an infected one.
Spider mitesTiny (0.3–0.5 mm) arachnids, hard to spot with the naked eye, feeding on the underside of leaves. Symptoms: fine, pale speckling (feeding punctures); over time the leaves turn grey, yellow and dry out. Under heavy infestation a fine webbing becomes visible. Warm, dry air favours their development — a common problem for houseplants in winter near radiators. Spider mites are not insects but mites of the family Tetranychidae — adults have 8 legs (larvae only 6). The most common in cultivation is the two-spotted spider mite (Tetranychus urticae): yellowish-green females carry two dark spots on the sides of the body, while overwintering females turn orange-red. In orchards the fruit tree red spider mite (Panonychus ulmi) is frequent, whose red eggs overwinter in clusters on the bark of the shoots. Feeding mechanism: the mites puncture the cells of the epidermis and mesophyll on the underside of the leaf and suck out their contents, and the emptied cells fill with air — hence the characteristic, silvery-grey, fine speckling visible from the upper side of the blade (resembling a mosaic discolouration). At high numbers the leaves turn brown, curl and drop prematurely, and the whole plant weakens and loses its growth. Dense webbing spun between the leaves and shoot tips appears only under an advanced attack and is a sign of the risk of the pest spreading to neighbouring plants. The development cycle is very fast in warmth and drought — at a temperature of 25–30°C a single generation can be completed in 1–2 weeks, hence the numerous generations per season and the rapid, exponential build-up of the population. Females lay spherical, transparent eggs on the underside of the leaves. Fertilised females overwinter (in bark crevices, leaf litter and plant debris) or — in the fruit tree red spider mite — the eggs on the bark. Early infestation is easy to miss; diagnosis is helped by shaking a leaf over a white sheet of paper (moving specks become visible) or examining the underside of the leaf with a magnifying glass.
Toxicity
| For whom | Level | Notes |
|---|---|---|
| Humans | Moderate | Contains thujone — the essential oil and shoots are toxic if ingested, and the sap can irritate the skin. |
| Dogs | Moderate | — |
| Cats | Moderate | — |
| Horses | Moderate | — |
History and origin
Western arborvitae was the first North American tree brought into cultivation in Europe, in the 16th century. The name "arborvitae" (tree of life) comes from the supposed cure of scurvy in French explorers using a decoction of its leaves. That account comes from Jacques Cartier's expedition to the St. Lawrence River in 1535-1536, when Indigenous people gave his scurvy-stricken crew an infusion made from the twigs of a tree called annedda — whose botanical identity is still disputed today. Such infusions are no longer prepared: the shoots and the essential oil of arborvitae contain toxic thujone, a neurotoxic compound that is dangerous in pregnancy (see the toxicity section). Plants soon reached France and the species spread quickly through European gardens; its formal scientific description was published by Carl Linnaeus in Species Plantarum in 1753. In North America the wood of arborvitae — light, soft and exceptionally rot-resistant — has been used for centuries for shingles, fence posts and canoe ribs, and its foliage is among the ceremonial plants of the Great Lakes peoples. In Europe thuja first became widespread in parks and cemeteries, and in the second half of the 20th century, with the fashion for clipped evergreen hedges, it became one of the most frequently planted conifers in home gardens.
Uses
Primarily for clipped hedges and green walls, as well as a specimen plant and for group plantings. Dwarf cultivars for small gardens and containers. For a hedge, plants are usually set every 50-70 cm — closer (40-60 cm) for the slow-growing Smaragd, wider apart (60-80 cm) for the fast Brabant — with the root ball level with the surface of the ground; planting too deep is one of the more common causes of browning. A clipped hedge is usually kept 0.6-1.2 m wide, but plants left unpruned spread to 1-4 m, so along a property boundary the row is set back by half the planned hedge width. After planting, a watering basin and a bark mulch help, as does regular watering through the first two or three seasons. The base of the hedge creates dry shade in which only a few ground-cover perennials will succeed — common periwinkle (Vinca minor), Japanese spurge (Pachysandra terminalis) or big-root geranium (Geranium macrorrhizum) — while sun-loving and moisture-loving species will be starved there. Narrow columnar cultivars serve as vertical accents by entrances and along avenues, while globe-shaped dwarfs suit bed edging, rock gardens and containers — in a container the root ball freezes through more easily than in open ground and needs protection for the winter. Thujas sold in pots as seasonal decoration are not houseplants: they need a cool, bright position outdoors.
Trivia
- The Latin name "arborvitae" means "tree of life."
- Despite its huge popularity as a hedging plant, thuja has low value for native wildlife compared with native deciduous shrubs.
- Despite the English name "northern white cedar," arborvitae is not a cedar — it belongs to the cypress family (Cupressaceae), whereas cedars (Cedrus) belong to the pine family (Pinaceae).
- On the limestone cliffs of the Niagara Escarpment in Ontario grow stunted, twisted arborvitae over a thousand years old — among the oldest trees in eastern North America; in gardens the species lives many times shorter.
Frequently asked questions
Why does thuja turn brown?
The most common causes are drying out (especially during frosty, snow-poor winters and hot summers), planting too deep, or cutting into old wood. Regular watering is key.
How closely should thuja be planted for a hedge?
Usually every 50-70 cm for the 'Brabant' cultivar and every 40-60 cm for the slower-growing 'Smaragd', depending on how quickly you want the hedge to fill in.
Is thuja poisonous?
Yes — all parts contain thujone, which is toxic if ingested by people and animals. The sap can also irritate the skin, so it's worth wearing gloves when pruning.
How does western arborvitae differ from western red cedar (Thuja plicata)?
Western red cedar grows faster and taller, has darker, glossier green foliage, a sweetish scent when crushed and white, butterfly-shaped wax markings on the underside of the sprays, and it discolors less in winter. Western arborvitae, in turn, is distinctly hardier (zone 3a against about 5-6 for T. plicata) and has clearly visible resin glands on its scales. Nurseries sell mainly the cultivars Smaragd and Brabant (T. occidentalis) and Martin and Atrovirens (T. plicata) for hedging.
Can you cut the top off a thuja to limit its height?
Yes, the leader can be shortened, but you have to cut into green, leafy growth and do it before the plant outgrows its intended height. After topping, a thuja will not rebuild a regular cone: instead of a single leader it usually sends up several competing tops, which splay apart under the weight of wet snow and split the crown, so on tall specimens they are either tied together or thinned every few years down to one. Hedge height is then maintained by trimming this year's growth each year — in spring and in late summer; cutting into old, leafless wood leaves permanent bare patches.
Sources
- Plants of the World Online (POWO)Database (GBIF, POWO…)
- RHS — Thuja occidentalisInstitution / botanical garden
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