Jennifer Schultz Nelson
Extension Educator, Horticulture
Just one look at a Wisteria in bloom reveals why many gardeners want this plant in their garden. Large, pendulous flowers cover this deciduous vine each spring in shades of white, pink, or blue-purple, providing a beautiful display that few other plants can match.
Unfortunately, growing Wisteria is not without challenges both in keeping this vigorous vine under control and then coaxing it to flower.
There are about ten species of Wisteria known worldwide. They are native to the eastern United States and eastern Asia. The two most common species grown in gardens are Chinese Wisteria, Wisteria sinensis, and Japanese Wisteria, Wisteria floribunda. While they have very similar appearing flowers, they have unique characteristics that may make one or the other a better candidate for a particular garden setting.
Chinese Wisteria is generally preferred over the Japanese Wisteria because of its flowering habit. The vine itself can grow to lengths of twenty-five feet or more. The flowers are produced in six to twelve inch long clusters. The flowers open before the foliage is fully expanded in early to mid-May, and all flowers in a cluster open at once. This all-at-once flowering is what makes this the more popular species of Wisteria to grow. It has incredible impact in the spring garden.
Chinese Wisteria flowers are various shades of purple or white. The flowers all have some fragrance, but the white flowered cultivar 'Alba' is more fragrant than the blue-purple flowered cultivars.
Japanese Wisteria also grows to lengths of twenty-five feet or more. The flower clusters are larger than Chinese Wisteria, typically twelve to eighteen inches in length, appearing in late May before the foliage has expanded. However these flowers do not open all at once—they open gradually, beginning at the base of each cluster and extending to the tip. While lacking the all-at-once impact of Chinese Wisteria, the Japanese Wisteria flowers can be enjoyed over a longer period of time.
Japanese Wisteria also has a wider range of flower colors. There are blue-purple and white cultivars just like the Chinese Wisteria, but there are also pink to nearly red flowered cultivars.
Wisteria vines are extremely vigorous growers and can cause damage to buildings if left unchecked. Stems can clog gutters or disrupt siding and shingles. It's highly advisable to grow wisteria on a sturdy trellis or pergola away from buildings.
It's also possible to grow Wisteria into a freestanding tree form. A single-stemmed young vine must be staked upright until the vine is thick enough to support the weight of the side branches. When the single stem is the desired height, the top is cut off. Side branches are allowed to develop on the upper portion of the new "tree", but any branches that develop lower on the new "trunk" are removed. Wisteria "trees" in bloom are stunning, resembling a fountain of blue-purple flowers.
Whether growing a Wisteria "tree" or not, Wisteria need regular pruning to restrain their vigorous growth and promote flowering. One of the biggest complaints gardeners have about Wisteria is failure to flower. Fairly drastic summer and a more gentle late winter pruning of Wisteria is one practice that will encourage flowering.
A major factor determining whether a Wisteria will flower or not is age of the vine. Vines grown from seed may need ten to fifteen or more years in order to produce flowers. Vines grown from cuttings or grafted plants typically flower sooner.
Even if a Wisteria vine is mature enough to produce flowers, it may fail to produce flowers for other reasons such as:
· Wisteria should be planted in locations that are in full sun, receiving at least six hours of sun each day. Without adequate sun flowering may be poor or not happen at all. Wisteria do not transplant well, so in many cases it may be wiser to purchase a new plant rather than attempt moving a large established vine.
· Use of high nitrogen fertilizers should be avoided with Wisteria, as too much nitrogen will encourage foliage growth rather than flowers.
· Heavy pruning during winter and spring will remove flower buds and encourage vigorous foliage growth.
Some methods to encourage Wisteria to bloom include:
· A heavy application of superphosphate (0-20-0) at a rate of three to five pounds per 100 square feet along with heavy pruning of new growth in late spring or early summer and root pruning in late fall.
· Root pruning of Wisteria may limit the vine's ability to take up nitrogen and so encourages flowering rather than foliage growth. It is also a stress on the plant, which can stimulate flowering in that the plant "thinks" it may be dying so its natural response is to reproduce, which is accomplished by most plants in our gardens via flowering.
· Root prune in late fall by inserting a sharp spade about eighteen inches deep at least three to four feet away from the main trunk, all the way around the vine.
Despite their occasional reluctance to flower, gardeners continue to love Wisteria. I think for some people it becomes a showdown of man vs. nature when trying to coax a Wisteria into bloom. But rest assured Mother Nature has the upper hand in this battle. No matter how smart humans think they are, Mother Nature outsmarts us. Maybe in the end a stubborn Wisteria reminds us of how little we really are in the grand scheme of things.