London’s many squares, parks and gardens are planted with a good deal of ingenuity and flair, always with an eye to ease of maintenance and year-round visual value. I have spent quite a bit of time there in recent years, mostly in the colder months, so I have had a chance to observe how much use is made of woody plants that are especially striking in winter. They include winter flowering viburnums and trees and shrubs with distinctive or colorful bark and, of course, evergreens such as Garrya elliptica (an Oregon native) with its long, silvery winter catkins.
Most of the evergreens in small-scale London plantings are broad-leafed types, not conifers. Perhaps that’s because conifers, if not slow-growing dwarfs, would eventually get too big, and most conifers will not regenerate when cut back hard, unlike overgrown broad-leafed shrubs. It’s a loss to skip the density and fine textures of conifers, but there are plenty of textures (and shades of green) available without them. And there’s plenty of drama to be found in combining different broad-leafed plants. In London gardens, contrast in structure, texture and color often takes priority over winter flowers.
One group of evergreen plants that’s widely used in London provides both winter flowers and year-round drama. Mahonias are closely allied with barberries, as the small yellow flowers attest, but mahonias have compound leaves. (Some taxonomists want to lump them with the genus Berberis, but this name change has not stuck yet.) We have our own native mahonias that we call Oregon grape, but the largest and most sculptural species come from Asia.
British gardeners love mahonias much more than American gardeners do. Personally, I think American gardeners should shape up and learn to like them better, because they can contribute so much to the winter landscape. From early fall until mid-spring there’s always a mahonia in bloom, and some are sweetly fragrant. Because of their lack of popularity in the U.S. most of the Asian species can be quite difficult to find, but named cultivars of Mahonia x media (crosses between M. japonica and M. lomarifolia) are readily available and reliably hardy. The cultivar most frequently encountered is named “Charity.”
Roger Gossler, who lists Charity and several other kinds of mahonia in the Gossler Farms Nursery catalog, has called Mahonia x media “one of the greatest groups of plants selected for the woodland garden in the 20th century.” Certainly the leaves are at their most superb in shade, where they may grow as long as 18 inches. But they will also grow in sun, where the multi-stemmed plants remain determinedly upright for many years and bloom heavily. The long spikes of yellow flowers can be upright, drooping or somewhere in between, and the buds are conspicuous and decorative long before they open. Charity blooms in late fall.
Our native mahonias, while less sculptural, are also well worth growing. The most familiar is tall Oregon grape (Mahonia aquifolium; Berberis aquifolium), a work horse of a plant that grows in sun or shade and blooms in time to provide nectar for early-rising bees and birds when little else is flowering. The flowers are bright yellow, on stubby racemes less elegant than those of the Asian mahonias. The leaves are frequently glossy, and they can take on burgundy hues when exposed to winter sun. Tall Oregon grape plants are generally multi-stemmed and can get very tall and gangly. In gardens, they benefit greatly from occasional pruning: stems can be shortened or selectively removed.
There are a few cultivars of tall Oregon grape. The only one I know of in this country is named compact Oregon grape (Mahonia aquifolium “Compacta”), a very useful plant for filling narrow, dry borders. It runs moderately and grows to about 3 feet. On roughly the same scale is Cascade Oregon grape (Mahonia nervosa), also known as longleaf Oregon grape. It is sometimes tricky to establish, but it has the loveliest leaves among the native species, especially in shade. It is usually under 2 feet tall, but can grow taller in many years. It blooms significantly later than M. aquifolium, with flowers of a lighter yellow that are more gracefully displayed.
Lastly, there is creeping Oregon grape (Mahonia repens), which makes a great ground cover and is quite satisfactory in full sun where it will take on deep red tones in winter. Some clones have shiny leaves, some matte. I had always thought of this as a very low-growing plant, until a friend showed me some well-fed specimens that were at least 2 feet tall.