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Evergreen Viburnums of UBC Botanical Garden
Author: Douglas Justice
While walking through the garden this winter, I hope visitors appreciate the wide variety of broadleaf evergreen plants that flourish in our maritime climate. Here on the southwest coast, we can grow more kinds of plants without extraordinary protection than anywhere else in Canada. Foremost among our broadleaf collections are rhododendrons. The rhododendron family, Ericaceae, is rich with evergreens, and is especially important to us as well for its Asian and North American gaultherias (salal relatives) and vacciniums (blueberries). The Caprifoliaceae, or honeysuckle family, has fewer evergreens, but they are important constituents in the garden. Among the most familiar of these are the viburnums.
Viburnum is a large genus, with over 150 species of deciduous and evergreen shrubs and trees, the majority originating in China. Many are grown for their fragrant flowers, but the evergreens are grown primarily for their beautiful foliage and berries. The largest evergreen viburnum in the garden is Viburnum rhytidophyllum, the leatherleaf viburnum, a species known for its vigorous upright growth and drooping, dark green, crinkly leaves to nearly 30 cm long. This viburnum is often more attractive at some distance, as at close range, it easily overwhelms the viewer with its crowded, multiple stems (to six or seven metres tall) and coarse texture. Early in the year, masses of tiny, creamy white flowers are borne in terminal, flat-topped corymbs. These are followed by small red berries, which soon turn black. Leatherleaf viburnum works extremely well with other large evergreens, such as big leaf rhododendrons, and can be seen in this complimentary association in places along the southern edge of the Asian Garden.
Viburnum awabuki is another large species, but until recently, one little known outside of its native Japan, Korea and Taiwan. The species exhibits superbly glossy leaves held erect when young and vigorous upright growth to 10 m. In the garden, it performs best with some shelter. The late J.C. Raulston of the North Carolina State University Arboretum collected the form we have in the garden on the island of Chindo in South Korea in 1985. He collected it because of its masses of attractive red fruit, although it seldom produces fruit at the garden.
By far, the most commonly grown evergreen viburnum locally is Viburnum davidii. The species has strongly veined, dark, glossy green leaves on reddish stems; female plants exhibit stunning, porcelain blue fruits. In this species, the sexes aren't strictly separated, but individual plants are usually predominantly one way or the other, and planting different clones ensures cross pollination and better fruit set. Most Viburnum davidii plants you're likely to see are derived from a single Chinese population collected by Ernest Wilson in the early 1900s. While this species enjoys enormous popularity, botanical gardens are often concerned about such genetically "narrow" groups of plants.
In nature, there is generally a free flow of genes between different populations of plants of the same species. This helps to prevent inbreeding and ensures broadly based resistance to pests and disease, for example. In the garden, we have the Wilson-derived davidiis, but also plants from a more recent Chinese collection that look very different. Potentially, these new plants might play a role in some future breeding work, conferring, for example, increased hardiness, resistance to weevils, more reliable berry production, or some other improvement over the existing stock (that is, if we find these traits).
Other evergreen viburnums to see in the garden include: Viburnum cinnamomifolium (another Ernest Wilson introduction), an exquisite large shrub closely related to Viburnum davidii, with large, ribbed leaves and blue-black berries; the gangly, but oddly handsome, red-turning black-berried Viburnum henryi - an uncommon species that eventually takes on an arborescent (tree-like) habit in warmer climates, and Viburnum cylindricum, an attractive, somewhat tender shrubby species with dull, oblong leaves. Viburnum cylindricum is sometimes grown in gardens as a curiosity, since its leaves are covered with a thin waxy material that is easily inscribed with any sharpened object to produce a permanent decoration (of course, this and other horticultural vandalism is frowned upon!).
Another popular evergreen viburnum for general planting is Viburnum tinus, a justly popular Mediterranean species. A number of cultivars of this species can be seen in the Winter Garden and at Nitobe Memorial Garden, as well as throughout the Lower Mainland in residential, park and commercial plantings. Laurustinus (as it is traditionally called) is a bushy, shade-tolerant shrub that grows to five metres or more, although in the past it has occasionally been cut down locally by exceptionally hard freezes. Among its attributes are smallish, deep green leaves, flattened clusters of pink budded, white flowers that are produced copiously from late summer right through to spring, and clusters of blue, eventually black, berries. The form most often seen in the landscape is more compact than the species (to two metres or so) known as 'Spring Bouquet'.
A few worthy evergreen viburnums are missing from our collections, mostly because they've been tried before and were lost, or because we've been unable to obtain material of known wild origin - an increasingly important criterion for us. This category includes the lovely Chinese/Taiwanese Viburnum propinquum (worth growing for its name alone) and the Burmese Viburnum atrocyaneum, both Wilson introductions with small glossy leaves and steel blue berries, as well as Viburnum calvum, a larger tinus relative from western China, and Viburnum japonicum, a robust species from Japan with bold foliage and red berries. Also missing is Viburnum utile, a graceful shrub with arching branches, leaves glossy above and white felted beneath and fragrant flowers. Viburnum utile is the parent of the popular semi-evergreen hybrids known under Viburnum × burkwoodii, but the species itself is not much in cultivation.