Wisteria is a member of the Pea family, Fabaceae (formerly Leguminoseae).
Wisteria are vigorous, twining vines with wide landscape usage where space permits and gardeners are committed to keeping them in bounds. Among their attributes are hardiness, vigor, longevity and the ability to climb high. They are greatly valued for their large, pendulous flower clusters that occur in the spring. Flowers are pea-like and may be white, pink, lilac-blue, bluish-purple or purple in color. The fruit is a long, green flattened pod that is not particularly ornamental. The plant climbs by means of twining stems and has alternate, pinnately compound leaves. Older,established plants may have a twisted, woody trunk several inches in diameter. Plants that have been grown from seed remain in a long juvenile stage and often do not bloom for 10 to 15 years or longer. Plants that are grafted, and plants grown from cuttings or layered from a flowering plant will usually begin flowering earlier than seedlings.
Two species of wisteria are typically grown in home gardens: Wisteria sinensis or Chinese wisteria, and Wisteria floribunda or Japanese wisteria. The Chinese wisteria is the more popular plant due to its flowering habit. It grows to a height of 25 feet or more and has flower clusters six inches to a foot in length, which open before the foliage has expanded. Individual flowers in the clusters open all at once for a very showy display. Flowers are violet-blue and slightly fragrant. Plants are most showy from early to mid-May in most seasons. There is also a white flowering form of Chinese wisteria, W. sinensis ‘Alba,’ which is very fragrant. Two cultivars include: ‘Black Dragon,’ which has double dark purple flowers and ‘Plena’ with double, rosette-shaped, lilac flowers. Chinese wisteria may bloom within three to four years after planting; however, the juvenile period may be much longer.
Japanese wisteria grows to a height of 25 feet or more and has violet-blue, fragrant flowers that bloom as the foliage is also expanding. Individual flowers open gradually from the base of the cluster to the tip. Clusters can be 12 to 18 inches in length and are effective in late May in most seasons. The plant has yellow fall foliage color.
In order to bloom well, wisteria require full sun (six or more hours of direct sun per day) and a deep, moderately fertile, moist soil that does not dry out excessively. They will adapt to most soils, though they prefer a neutral to slightly acid soil pH of 6.0-7.0 for best results. Some type of support will be necessary as mature plants can be quite heavy.
Good site preparation will help ensure plant establishment. Begin with a soil test to determine if the soil pH or the phosphorus level need correction. If so, make additions of materials as you are preparing the soil. Prepare soil in an area two to three feet in diameter and 18 to 24 inches deep. Mix into the native soil either peat moss, compost or well rotted manure, one-third by volume, to improve soil aeration and drainage.
Wisterias climb best on wires, trellises, arbors and pergolas. They can be grown on solid, vertical surfaces if proper supports are provided, such as rows of wire attached four to six inches from the wall. Use sturdy, durable materials such as galvanized wire, tubing or wood. Use pressure-treated wood for arbors and pergolas. However, do not plant wisteria where the stems can invade and clog building gutters. Wisteria can also be grown as a single trunk standard or a tree-form. To accomplish this, the plant must be staked in an upright position. When it has reached four to five feet in height its top is cut off. Side shoots are allowed to develop on the upper part, but are continually removed from the lower stem. Side shoots are pruned each winter to six inches to a foot in length until the top is as large as desired. Future pruning consists of cutting summer shoots to the sixth or seventh leaf as soon as it expands and of cutting off secondary shoots that develop just beyond the first or second leaf. In winter, these secondary shoots are cut back to within an inch of their base. Living trees are often used as support but this must be done carefully. Trees less than ten inches in diameter can be quickly killed by girdling of the twining wisteria stem. Larger trees can also be damaged. If trees are used they should be inspected every several years to prevent girdling. If a tree is being girdled, the wisteria can be cut back to the soil line and allowed to grow back. The old girdling stem must be physically removed from the tree to avoid future damage.
Planting and Establishing
Once the soil is prepared and the support system is in place, the vine can be planted. Place the root ball of the plant in the hole so it is no deeper than it originally grew in the nursery. If your wisteria is grafted, set it so the graft union is slightly below the soil surface. Fill in the hole with the prepared soil mix and firm it around the root ball. Water well after planting, soaking the entire area. New plants will require an inch of water per week applied either through irrigation or as rainfall. Young plants should be fertilized annually until they fill the allotted space. Do not expect young vines to bloom since vegetative growth is being encouraged. Once the plant is established and has filled the allotted space, do not fertilize unless shoot and foliage growth and color is not good. Water only if foliage wilts as it might during drought conditions. Both of these practices stimulate vegetative growth and limit flower production.
Some annual pruning is required to maintain plant quality; it is not advisable to allow the vine to grow randomly and take over surrounding plants and structures. Pruning will help reduce the vigor of the vine and promote flowering.
Training New Vines
To train plants on a wire trellis or an arbor, select a vigorous, upright stem to serve as the main leader and attach this to the support. Remove other side shoots. As the main leader grows, it will develop side branches that will then produce more shoots and the flower buds. Continue to train the main leader upward and the new side branches as needed to form a framework to fit the allotted space (allow about 18 inches between side branches). Pinch off the main leader when it reaches the desired height.
Allow only one strong leader to develop from the end of each main framework branch without pruning it and stretch and attach this shoot along the support. Cut off the ends of all new side shoots just beyond the sixth or seventh leaf as soon as that leaf develops. New shoots will form as a result of these cuts. As they do, cut them back as soon as only one or two leaves develop.
Late Winter Pruning
Cut unpruned leader shoots back to one-half or two-thirds their length. Cut side shoots pruned the previous summer back to only one to two inches from their base for short flowering spurs.
This method of pruning allows permanent framework branches to extend each year by half the annual growth and side shoots to become short flowering spurs.
Once the vine has been trained into the allotted space, follow summer pruning practices as outlined above. In winter, prune leader shoots back to only four or five buds. Remove any suckers that appear at the base of the plant.
Pruning Neglected Vines
In winter, prune away all but a framework of branches. Shorten these drastically, removing crowded and poorly spaced branches. Then follow pruning and training as described above for a new vine. Hopefully, the plant will bloom within two or three years.
Root pruning is sometimes done in late fall to stimulate young plants to bloom or to restore blooming on older plants. It serves to check top growth and favor flower production and must be combined with summer pruning to be effective. Use a spade to cut vertically into the soil (about 18 inches deep) and about four feet from the main trunk, all around the vine.
Failure to Bloom
The biggest frustration gardeners face when growing wisteria is that plants have a longer than average juvenile period and sometimes fail to bloom as expected. Start with grafted plants or those produced from cuttings rather than those grown from seed. A plant will also fail to bloom if: it does not receive full sunlight; there is excessive vegetative growth that may have been stimulated by excess nitrogen fertilizer; it is pruned heavily in winter and spring, which encourages vigorous, vegetative growth; and/or it is pruned improperly. Also, in severe winters, flower buds may be injured or killed. The following practices may help induce non-blooming vines to flower: a heavy application of superphosphate (0-20-0) in early spring, severe pruning of new growth in late spring or early summer root pruning in late fall.
Wisterias do not transplant well and usually suffer a severe setback if moved. Large specimens sometimes do not recover.
Wisteria may be attacked by insects or plant disease, though neither is especially common. Should plants show symptoms of insect or disease damage, check with the staff at Port Kells Nurseries for diagnosis and management ideas.