This genus has produced a great many good garden varieties and in appearance they are very like some of the false cypresses. There are only three species and all except some forms of Thuja orientalis are hardy in the British Isles. Apart from a dislike of badly drained situations all are reliably easy to grow.
T. occidentalis, the white cedar, originates from eastern North America and is the hardiest of the three species. The species itself is not used as a garden plant but it has given us many forms suitable for individual specimens and for hedging purposes. It has aromatic foliage particularly when crushed.
T. o. danica, as the name suggests, originated in Denmark. It is a very attractive dwarf variety forming a dense rounded bush somewhat like a cushion with erectly held flattened foliage sprays. Bright green in summer, it turns slightly bronze in winter. After ten years it can be expected to be about 1 ft. high by the same width and eventually 2 to 3 ft high by a width of 3 to 4 ft. The variety Holmstrup makes a dense narrowly conical bush of bright green, holding its colour well throughout the year. A first-class reliable plant which was introduced a few years ago in Denmark and worthy of wider recognition, it will grow perhaps 3 to 4 ft. in ten years and eventually 12 to 15 ft.
A similar form but faster growing and more open in habit is T. o. lutea nana which is golden yellow in summer, a colour which is almost intensified in winter — certainly a point to commend it for the garden. It is an extremely easy grower and will withstand exposed situations which cannot be said of some of the golden chamaecyparis varieties. Growing 4 to 5 ft. in ten years, it will probably end up at the 15 to 20-ft. Mark.
The best variety for hedging is probably T. o. pyramidalis which is also known as fastigiata. It has a narrow columnar habit and if used as a hedge would need to be planted at not more than 2-ft. Intervals. Tolerant of a wide range of conditions including some shade, this variety also makes an attractive specimen in its own right. It has densely held sprays of bright green foliage in summer, bronzing somewhat in winter. It of course withstands clipping well. At ten years it will grow 6 to 8 ft. and if allowed to grow to maturity probably 20 to 25 ft.
One of the best garden conifers is T. o. Rheingold, a slow-growing form which turns from golden yellow in summer to rich coppery gold in winter and in very hard winters almost reddish bronze. Quite often one will obtain plants with very soft feathery foliage which appear different to the more typical flattened sprays, or more often both types of foliage will appear on the same plant. Rheingold is an excellent garden plant and associates well with winter-flowering heathers. Like all golden forms the best colour will be obtained by placing it in a sunny position. After ten years it will reach approximately 4 to 5 ft. and eventually will make a broad-based bush 10 to 12 ft. high and as much across.
T. orientalis is known as the Chinese arbor-vitae or tree of life coming as it does from north and western China. The species makes a small tree mostly of conical habit which is not used much as a garden plant, although it has produced several useful dwarf and slow-growing garden varieties.
One of my favourite dwarf conifers is T. o. aurea nana which makes a densely foliaged bush of bright golden yellow in summer, turning yellow green in winter. The flattened foliage sprays are held erectly giving the plant a very neat attractive appearance. It will grow 2 to 21 ft. in ten years and ultimately 5 to 8 ft. There appears to be more than one form in cultivation.
T. o. conspicua is a pyramidal variety of great attraction, a golden-yellow colour through most of the year but particularly bright in summer It is a medium grower reaching 6 to 8 ft. in ten years and eventually as much as 18 to 25 ft. There is a similar but slower-growing variety in T. o. elegantissima which turns a deep bronze in winter.
Lastly a dwarf juvenile-foliaged form of some attraction is T. o. juniperoides, a rounded bush turning from a summer colour of greyish green to a striking deep purple in winter. Unfortunately like a few other forms of similar foliage and habit, it is not reliably hardy and tends to open up with age. Reaching 2 ft. in ten years, it is unlikely to exceed 3 ft. at a somewhat young age of maturity!
T. plicata, the western red cedar, as one might guess originates from western North America and is a very important timber tree. It is also an extremely useful tree for hedging although the selected form atrovirens is much better. T. plicata will grow too large as a garden specimen reaching well over 100 ft. at maturity. The foliage of this species is generally more rounded and glossy than the two so far mentioned and has a very pleasant odour when crushed. The species and all forms used for hedging withstand clipping very well and this is one of the few which
will break new shoots from old wood if cut back hard — a definite advantage for this purpose.
p. rogersii is an unusual but very useful conifer for the small garden. It has very dense foliage, dark green inside the plant and tips of golden, yellow and bronze, particularly pronounced in winter, the whole effect giving a variegated look. Mostly globular in shape it will sometimes assume a leading shoot which alters its habit to conical. In ten years reaching 18 in. to 2 ft., it may ultimately grow to 5 to 6 ft. with a similar width.
A quite choice form which is not generally known is T. p. Stoneham Gold, a slow-growing variety with foliage of a really old-gold colour. The habit is somewhat irregular particularly in the early stages of growth, but the plant develops into a broadly conical bush. The inside of the plant is dark green, accentuating the golden tips of the leaves which turn bright golden bronze in winter. After ten years perhaps reaching 2 to 21 ft., it will eventually grow to 6 to 7 ft. If a more regular-shaped plant is required then a hard trim will not be resented.
Thujopsis is a very similar genus to thuja, but is only represented by one species Thujopsis dolabrata. This makes a medium-sized tree with a broadly pyramidal habit which has large flattened sprays of glossy green. Growing 6 to 8 ft. in ten years, it will eventually reach 30 to 35 ft. in the British Isles. There is a dwarf spreading form in nana of no great attraction.