The world of conifers includes ground covers as well as some of the world’s largest trees. The Washington Park Arboretum is an excellent place to see both ends of the spectrum.
The Russian or Siberian arborvitae (Microbiota decussata) is most often known in the nursery trade and among gardeners by its botanical name, microbiota. Discovered in 1921, it was not officially named until 1923. In nature it is most often found on bald peaks above timberline in the mountains of southeastern Siberia, which may account for its low habit of growth. Reports indicate it can grow up to five feet tall and as wide as twenty-three feet. There are indications that the plant can live at least 250 years. The first arboretum to acquire this plant was the Trompenburg Arboretum in Holland, which did so as late as 1968. This seems to be the source from which all stock available in the Western world has been derived. It is still rare in cultivation.
Microbiota has received accolades from gardeners for its use in rockeries and dwarf conifer collections. It does tend to grow more vigorously in the Pacific Northwest and in England than in the warmer Midwestern and Southern states. In fact, it will not tolerate heat and heavy clay soils, and probably should be grown only under cooler conditions.
In nature, the seeds can remain viable for many years and usually germinate only after a fire. Coming from an alpine habitat, it is not surprising that it is hardy to -40° F.
In cultivation, this spreading, vigorous evergreen shrub reaches only about eighteen inches in height. Occasionally, one or two really dwarf variants are found; one is reported to grow only to a height of eight inches. In the home landscape, microbiota might need light pruning to keep it in bounds, but it is not tolerant of overall heavy pruning.
The foliage consists of flat sprays of short, scale-like, bright green leaves, resembling those of juniper (Juniperus), which confuses many people. Branchlets nod at the tips, giving plants a graceful character. In the shade, the needles are appressed (held tightly to the stem) and soft-textured. On younger plants, the needles are more spreading and needle-like. Most of the plants in cultivation have been asexually propagated and are male, producing inconspicuous cones. The female cones are smallish and round, pale brown, and contain only one seed. Seeds remain dormant for up to two years, but moist stratification at 40° F will hasten germination.
An ideal ground cover, each plant eventually spreads to nine to twelve feet; older specimens may spread farther. As might be expected, it loves full sun, but it will perform well in semi-shade. It prefers a lean (nutrient poor) well-drained soil. It has no diseases or pests.
The dull brown, coppery, or plum winter color of the foliage is distinctive. Some do not care for the color, saying that it looks as though it is suffering from winter burn; others compare the color to winter’s brownish purple of Juniperus horizontalis ‘Plumosa’. It helps to place the plants where this effect will be enjoyed the most, usually next to other, richer green foliage.
In West Coast gardens, microbiota seems to grow better than junipers, often spreading evenly around other low-growing evergreens. It grows moderately fast, and its feathery appearance is an asset all year round.
It is easily propagated by cuttings taken from August through January. Follow a procedure similar to that used for junipers: leave at least one inch of the previous year’s wood at the base of the cutting. The use of a .25% solution of indolebutyric acid (IBA) in alcohol will increase both rooting percentage and uniformity.
A Giant Among Conifers
One of the giants of the conifer world is Taiwania cryptomerioides, a member of the redwood family (Taxodiaceae). There is only one known species, but there are two varieties. The first trees were collected in 1904 in the central mountains of Taiwan. It was introduced into the United States by the Arnold Arboretum from seeds collected by EH Wilson, the great plant explorer in China. A later discovery (1912) was made near the border of Burma and Yunnan, China; first given a new species name, T. flousiana, this tree is now recognized as T. cryptomerioides var. flousiana, differing from the typical variety in the size, shape, color, and scale count of the cones. Trees of this variety are found in several provinces of southwestern China and in the adjacent upper region of Burma (now Myanmar); they are much less cultivated than the Taiwan variety.
Known in Taiwan as the “big tree,” taiwania is one of the tallest conifers of the Old World. It usually tops out over any other associated trees in its native habitat. The largest one found measured over 225 feet tall and more than thirty feet in diameter. In its youth, the tree is extremely elegant, with branches curved gracefully upwards and branchlets slender and drooping. The blue green color of the foliage is distinctive.
Taiwania is sometimes confused with Japanese redwood (Cryptomeria japonica), because the shoots of young trees and the stature of the older trees are similar; the specific epithet, cryptomerioides, alludes to that similarity. Like many conifers, taiwania’s leaves are dimorphic (juvenile leaves are different from adult ones). On younger branchlets, the awl-shaped leaves are arranged equally around the stem, about twenty to the inch; they are spine tipped and stiff. The scaly leaves of older trees are smaller and incurved at the shortly pointed apex; branchlets are held in flat sprays. There are four to five stomatic lines on each side of the scaly mature leaves.
The half-inch-long cones are found at the terminus of a branch or branchlet. The cone scales are numerous, rounded, and overlapping. Inside are oblong seeds with broad wings, usually two on each fertile scale.
The tree bark has a grayish red brown to grayish white color, and is longitudinally fissured. The ridges are flattened, more or less interlacing; the bark exfoliates in thin, narrow, longitudinal strips. The outer bark is thick and fibrous with a dark red brown tiered cross section; beneath, the inner tissue is bright red. The inner bark is thin, finely fibrous, and gradually becomes pale purplish brown after cutting. The freshly cut sapwood is white to pale orange yellow.
Taiwania requires plenty of moisture and summer heat and thrives on acid well-drained soils. It obviously needs a lot of space so is not recommended for most urban sites. It seldom needs any pruning and appears not to be bothered by pests.
Like most conifers, Taiwania propagates best from seeds. Harvest the cones before they open and dry them to force out the seeds. Then store them in a moist medium for three to four months at 40° F. It may take over a year for some seeds to germinate.