Wisteria naturally grow as twining, climbing vines but proper and regular pruning trains them as weeping mounded shrubs, or even as small trees if lowest foliage is removed. The main concern with wisteria is choosing the appropriate species as some, particularly Chinese wisteria, can seed itself and become a beautiful but disruptive weed. All parts of the wisteria plant cause stomach pain and upset if eaten.
According to the "A-Z Encyclopedia of Garden Plants" there are nearly ten species of wisteria, all native to woodlands or sunny stream banks in China, Korea, Japan and the central and southern United States. Four species most often encountered in garden settings include the silky wistera (Wisteria brachybotrys), Japanese wisteria (Wisteria floribunda), Chinese wisteria (Wisteria sinensis) and the American wisteria (Wisteria frutescens).
All wisteria plants naturally grow with long, woody stems and feathery green leaves upon twining stems with tendrils. All are winter deciduous. In spring to early summer, they produce pendent clusters of pea-shaped flowers that are sweetly fragrant and range in color from white to lavender, sky blue or blue-violet. Afterwards, once pollinated by insects, the flowers develop into drooping, bean-like seed pods.
Plant wisteria in a fertile soil that is moist but well-draining. Tolerating partial shade, the plants must receive at least four hours of direct sun daily to flower well; full sun exposures work nicely. Usually trained to grow upon a sturdy arbor, pergola or sometimes wildly growing up a tree trunk, wisteria needs frequent, scheduled pruning to keep it smaller as either a small tree or as a tidy mounding shrub. Do not allow prolonged droughts to stress wisteria. Compost applications over the root system keeps plants growing well.
The key to creating and maintaining the wisteria vine as a shrub is proper and timely pruning across most of the calendar year. In winter when the plant is dormant, the main vine stem is severely cut back to a desired overall plant height--the height of the manipulated shrub. Then, all side stems are pruned back so that only two or three buds remain. These buds, that were created the previous growing season, will yield flowers the following spring.
After flowering, trim back the vigorous new growth stems back monthly to keep the plant in roughly the same shape and size as a shrub. The following winter, the same severe pruning is employed, reducing main stems back to the original shrub height and side shoots with about three dormant flower buds on a nicely shaped framework. Little by little the wisteria shrub will get larger as the pruning cuts on the side shoots move to include three buds each year. On large plants, consider retaining flower buds on two-thirds of the side shoots and removing the other third for full rejuvenation the next summer.
Flowering numbers may be diminished if there is too much pruning. During the winter pruning, consider leaving some side branches with four or five buds so more flowers occur that spring. This should not be a problem if amply light reaches plants. In summer, only tip prune new leggy stems, not fully cut them off from their base. Also consider avoiding the use of the very aggressive and large Chinese wisteria and even the Japanese wisteria as their seeds germinate and become noxious weeds; choose the "better behaved" American wisteria instead.