Doublefile viburnum (Japanese Snowball Bush) attracts particular attention at this time of the year. With flat clusters of white flowers spaced in pairs along the spreading branches, the effect of this vigorous shrub is most distinctive and pleasing. Due to this brilliant white effect as well as to a marked horizontal or shelving type of growth, a specimen in bloom, seen at a distance, looks surprisingly like a flowering dogwood. On closer inspection this similarity soon disappears, because the flower clusters are constructed differently.
Leaves and buds of viburnums are always opposite, and the flower clusters are outstanding in their paired arrangement along the branchlets. They grow in flat-topped umbels at the tips of short lateral shoots usually bearing two leaves at the base. These short flowering shoots push out quickly in the spring along the vigorous growth of the preceding year, producing a pleasing regular or somewhat conventional effect, which seems to give a quality especially appropriate in formal plantings.
In doublefile viburnum the clusters are about 4 inches across, with much larger individual flowers set around the margin. These larger flowers lack stamens and pistils; it is the small inconspicuous flowers at the center that are fertile. These produce the berry-like fruits which turn from green to red, and finally to blue-black when mature at the end of the season.
In the showy form of this viburnum first introduced as a garden plant, the clusters of bloom are globose and composed entirely of the large sterile flowers. We know this plant today as Japanese snowball. It is prized as one of the most attractive shrubs in flower, but it produces no berries. This was named Viburnum plicatum in 1794 by the Swedish botanist and world-traveler, Karl Peter Thunberg, because he was impressed by the pleated appearance of the ovate leaves. The outline of the leaves and their veins, which extend nearly straight and parallel from midrib to margin, offer a good means of identification.
Doublefile viburnum is abundant as a wild plant in both China and Japan, especially in open woodland and on brush-covered mountain slopes. It has the same pleated leaves and general growth character as Japanese snowball, and it is known to be the wild plant from which the garden shrub originated. After a singularly troubled course among scientific names, this shrub is now considered a form of Viburnum plicatum. The form name is tomentosum, fortunately the same as has long been considered correct and widely used as a species name. It is likely that in catalogues and lists the name designating double-file viburnum will continue to be Viburnum tomentosum, and its sterile snowball-like form will he called variety sterile or variety plicatum. However confused and inverted the scientific status of these two Oriental shrubs, both give superb accounts of themselves in our gardens.
Several variations of doublefile viburnum have been recorded, but most are of minor importance. In 1940, however, the late Charles Doney of the Brooklyn Botanic Garden noted a specimen in their collection with soft pink flowers.
These shrubs are perfectly hardy and tolerate extreme temperatures without injury. They grow best in a rather heavy loam, particularly in a slightly acid soil that has ample fertility and abundant moisture. Vigorous and dense growth is produced in full sunlight, hut specimens also do well in shade, usually making a less dense framework of branches.
Shrubs of the dimensions of double-file viburnums are best used in backgrounds and border plantings. Ordinarily, pruning is not necessary or desirable as this species has an inherently fine habit and well-balanced pattern of growth. In view of the tendency towards shelving or horizontally-layered branches, a position where one looks down into them from above offers a particularly attractive picture during the flowering season. Groups of this viburnum at the base of a terrace bank or near the lower portion of a slope are dramatic. Wherever they are used. in addition to the ultimate height of about 13 feet. allowance should be made for a total branch spread at least half again as much as the height.
Doublefile viburnums are available from many landscape gardeners and neighborhood nurseries as well as from larger establishments. They can be raised from seeds as well as by stem cuttings. leaf cuttings, and layers. hut all these methods except the last require four or five years to produce plants of any size and would rarely be practical for home gardeners.
Doublefile viburnums should be planted in early spring, if possible, and preferably with a carefully protected ball of roots. Attention to proper soil, mulching, and watering will give good returns in vigorous growth and early blooming.
By B. Blackburn