Wisteria A Mystery for Most Gardeners
Nothing rivals the beauty of a wisteria arbor in full bloom, but, unfortunately, successfully growing these lovely vines eludes many Midwestern gardeners.
Two types of wisteria are most commonly planted in our area: Japanese wisteria ( Wisteria floribunda ) and Chinese wisteria ( Wisteria sinensis ).
Japanese wisteria is known for its fragrant violet blossoms, which are borne in 8- to 20-inch-long clusters. The individual flowers of a cluster open gradually, beginning at the base.
Chinese wisteria clusters are generally less than 12 inches long, and its individual blooms are slightly larger. Also, the flowers of a cluster tend to all open at the same time. Chinese wisteria is not quite as hardy as the Japanese and also is not as fragrant. There are cultivars of both species that have white blossoms.
Wisteria is a rather vigorous, twining vine and, in fact, can be quite invasive in some areas. The vines require strong support to keep up with their fast growth. Wisteria can grow up to 10 feet a year, especially once it's established in the proper environment. It performs best in deep, moist, but well-drained soils that are neutral to slightly alkaline.
Since most gardeners are drawn to this plant for its blossoms, they are quite frustrated by the plant's notorious tendency to produce only vegetation. There are many potential explanations for this annoying problem, including the plant's immaturity, too much nitrogen, insufficient phosphorus, poor-quality plants and too much shade.
Asian wisterias need to reach a degree of maturity before they are able to produce flowers. In fact, in can take up to 15 years or more before the vines reach blooming stage.
Those who have succeeded in raising wisteria often recommend root pruning, applying superphosphate, rigorous pruning of the shoots and planting in full sun. Most important, you should start with good-quality plants that have been propagated from cuttings of plants known to flower while relatively young. If you know someone willing to share a great specimen, take cuttings of the stem tips in July. Avoid planting seedling vines because the genetic variability of seed reproduction makes it impossible to predict their blooming habit.
These vines produce their flowers on last year's wood in mid- to late May, so wait until late spring or early summer to prune the vine. Severe pruning is often recommended, back to three or four buds, to keep the plant manageable and renewed.
There are a couple of native wisteria species that are a bit more "tame" than their Asian relatives. These native species bloom on current season's growth and reach flowering age sooner than the Asian species. They flower a little later in spring but periodically rebloom through the summer.
American wisteria (W. frutescens) can reach 20-30 feet and bears its flowers in short, condensed clusters about 4-6 inches long. 'Amethyst Falls', the most common cultivar available, features fragrant, lavender-blue flowers. 'Nivea' is less fragrant but has longer clusters of white blooms.
Kentucky wisteria ( W. macrostachys) reaches 15-25 feet and has 8-12-inch-long flower clusters that are packed tight with blooms. Some consider this to be a sub-species of American wisteria. The cultivar 'Blue Moon' is a hardy selection from Minnesota with wonderfully fragrant blossoms that first appear in June and repeat through the summer. 'Aunt Dee' has pale lavender blooms and 'Clara Mack' has white blooms.