Family: Cupressaceae
Common Name: Arbor-vitae

Native to temperate North America and eastern Asia, this genus consists of five species of conifers, columnar to pyramidal in shape. Their straight trunks are clothed in deeply furrowed, fibrous bark. They have aromatic, flattened, scale-like leaves and small, oval cones covered in overlapping scales. Several of the species are cultivated for their timber.

Species

T. occidentalis, American arbor-vitae, zone 3, from eastern North America, grows to about 20 m (65 ft), with a pyramidal crown and a thick trunk covered in reddish brown, peeling bark. The yellow-green leaves, blueish under­neath, colour bronze in autumn. A vast number of cultivars is available. 'Ellwangerana Aurea' grows only 2-4 m (6-13 ft) high, and is broadly pyramidal to rounded in shape. The golden yellow foliage colours bronze-gold in winter. 'Ericoides' is a dwarf plant, growing to 50 cm (20 in), with loose, soft, crowded branches and soft, needlelike young foliage, maturing to brownish green. 'Globosa' has a dense, com­pact habit, growing to around 1 m (3 ft), with bright green foliage. 'Lutea' is very slow-grow­ing, to about 3 m (10 ft), with a conical habit and golden yellow foliage, becoming a paler shade of yellow in winter. 'Pyramidalis' has a symmetrical, narrow, compact habit and is useful for formal plantings. 'Rheingold' is simi­lar to 'Fllvangerana Aurea', but more rounded, tapering to a slight point at the centre. The foliage is rich gold throughout winter.

T. orientalis, Chinese arbor-vitae, zone 6, is a native of China, but has long been cultivated in Japan. When young, it is a conical shape with dense foliage, maturing to a rather open, untidy shape, with protruding side branches. It is dis­tinguished by its vertical, flat foliage sprays. This species has also given rise to many culti­vars. 'Aurea Nana' has a neat, rounded habit, growing to I m (3 ft) high. The dense branch-lets are green with gold tips. Beverleyensis' has a columnar habit, growing to around 3 m (10 ft). The gold-tipped foliage becomes a bronze-yellow in winter; Juniperoides' (Synonym: 'Decussata') has a rounded shape, growing less than 2 m (6 ft) in height. The dense, gray-green summer foliage matures to a rich purple in winter. 'Rosedalis' is a dwarf, oval form, grow­ing to 1 m (3 ft), with small, soft, dense leaves. It changes colour from pale yellow in spring to a bright light green in summer, and then to brownish purple in winter.

T. plicata, western red cedar, zone 5, from western North America, grows up to 60 m (200 ft) in its habitat, but gen­erally to around 25 m (80 ft) in cultivation. It is well known for its valuable timber. It is fast growing, with a broad, pyramidal form over many years, becoming more columnar with age. The foliage is a rich green colour and highly aromatic. There are numerous cultivars, includ­ing 'Stoneham Gold', a cone-shaped plant to 2 m (6 ft), with rich gold young foliage turning to deep green, and 'Zebrina', cone-shaped to 15 m (50 ft), whose leaves are banded with light yellow.

Cultivation

Thujas will grow in any well-drained soil, provided it holds adequate mois­ture and is reasonably deep. They must have a position in full sun (particularly the coloured-foliage cultivars), and a spot that is sheltered from wind. The majority of species are suitable for growing as formal hedges, and these should be trimmed in spring and again in late summer. The species can be raised from seed sown in spring, in a garden frame. All thujas can be propagated from semi-ripe cuttings, from late summer to mid-autumn, rooting them in a heated propagating case.

Climate

Wide moisture and temperature range, but most prefer cool, moist climates.