Eastern arborvitae

Identification and origin

Thuja occidentalis belongs to the Cupressaceae. It is native to central and eastern Canada and the northeastern United States, from the Gulf of St. Lawrence to Manitoba and the Great Lakes, with isolated populations along the Appalachians as far south as Tennessee and North Carolina. Common in swamps, it nonetheless thrives best on well-drained soils. A young plant brought back from the St. Lawrence by Jacques Cartier, probably in 1536, is said to have lived for many years at Fontainebleau: it was one of the first North American trees grown in Europe, under the name “tree of life.”

In the wild it grows 40–60 ft tall and 10–15 ft wide, with a trunk 2–3 ft across. In gardens it rarely exceeds 20–30 ft. Growth is slow to moderate. The outline is dense, conical to pyramidal; the often twisted trunk frequently divides into several stems. The gray to reddish-brown bark peels in narrow, fibrous strips.

The leaves, scales about 0 in long, form flattened sprays held in horizontal planes. Dark green above and paler below, each carries a small resin gland, and when crushed they give off a strong scent reminiscent of tansy. In winter the species’ foliage often turns bronze or yellowish brown. Unlike western redcedar, the undersides of the sprays show no whitish patches.

The oblong cones measure 0–0 in and have four to five pairs of scales. Yellowish green at first, they turn pale cinnamon brown in late summer, open one to two weeks later and release winged seeds.

Among the many cultivars, ‘Smaragd’ (‘Emerald Green’) forms a narrow column of 10–15 ft that stays green in winter, and ‘Holmstrup’ reaches 6–8 ft. ‘Danica’ and ‘Golden Globe’ are dwarf globes; orange-yellow ‘Rheingold’ turns coppery in winter.

Light

Eastern arborvitae grows best in full sun, with at least six hours of direct light a day. It tolerates partial shade, but in heavy shade it becomes loose and open. Young plants dislike drying winds: a sheltered spot on the east or northeast side helps limit winter burn.

Watering

This is a tree of moist soils that copes poorly with drought. Watch the soil rather than the calendar: when it starts to dry below the surface, water deeply at the base rather than with a sprinkler. During dry spells even an established plant needs water; a recently planted one, whose roots are still shallow, needs more. The species tolerates wet soils, but not dry, windy, exposed sites.

Keep checking soil moisture in fall until the ground freezes: a well-moistened root ball before hard freezes reduces winter drying of the foliage. A mulch layer of 2 in on clay or 4 in on sandy soil, kept 4 in from the trunk, slows evaporation and evens out soil temperature swings.

Potting mix

It prefers deep, loamy, moist, well-drained soil. It also grows in clay, sandy or chalky soil, at a pH of about 6 to 8, with a preference for neutral to alkaline conditions. It tolerates poor, rocky ground, urban air pollution and black walnut. In a container, use a free-draining mix that stays evenly moist, and choose a dwarf form.

Temperature

Very hardy, it suits USDA zones 3a to 7b; cultivars such as ‘Danica’ withstand cold below -4 °F. It does poorly with extreme heat combined with drought. Its weak point is winter burn: on sunny, windy days the foliage loses water that roots in frozen soil cannot replace. On exposed sides, stretch burlap over stakes 4–5 ft tall, about 2 ft from the foliage, without wrapping the plant tightly, and take it down in spring. In a pot, insulate the container for winter.

Repotting

It transplants easily, whether container-grown or balled and burlapped. Plant in early spring or between late summer and early fall. Its roots are shallow and spreading: in a hedge, give each plant enough room. Small globe forms suit containers, provided the pot is protected from hard frost and watering is closely watched.

Pruning

Outside a hedge it needs no pruning. It tolerates shearing well: one light trim each year is enough to keep a hedge in bounds. Avoid pruning in late summer or early fall, since regrowth would not have time to harden off. After winter burn, wait until mid-spring: scratch the bark; if the tissue underneath is green, the branch can recover; if it is brown, cut back to a live branch, sometimes deep inside the canopy.

Propagation

Cultivars are propagated by semi-hardwood cuttings or by layering; in moist places the species layers itself naturally. For seed, collect the cones as they turn from yellow to brown, before they open, and let them dry for one to three weeks. Some growers recommend one to two months of cool, moist stratification at around 34–41 °F for more even germination. Fall sowing is preferred; barely cover the seeds, 0–0 in deep at most.

Common problems

Browning can have several causes: winter burn, drought, salt, shade, pests or disease. Some interior browning in fall is normal. Look at where the damage is before acting.

• Deer: they browse the foliage readily in winter; ‘Holmstrup’ is less affected.
• Pests: bagworms, the most common (pick the bags off by hand), leafminers that dry out the branch tips, spider mites, scale insects and conifer aphids.
• Diseases: cankers, including Coryneum canker, thuja blight, tip blight and honey fungus.
• Physical damage: heavy snow and ice easily break the branches of the species; cultivars are less prone to it.

Toxicity

Eastern arborvitae is toxic. Its branches and cones contain an essential oil rich in monoterpenes, including thujone. Swallowed, they cause abdominal pain, vomiting that is sometimes bloody, and diarrhea; in very large amounts, especially as an infusion, they can cause impaired consciousness, seizures, and liver and kidney damage. Repeated contact irritates the skin, and sap in the eyes causes burning and redness: wear gloves when pruning. It is also toxic to animals, even though it is missing from some veterinary plant lists: do not let pets chew the branches. If a large amount is swallowed or symptoms appear, contact a poison control center or a veterinarian.