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.
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.
Culture Soil Preparation
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. Copper or aluminum wire or tubing are preferred over other metals since these do not rust. 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 prodroduction.
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.
From: Ohio State University Extension Fact Sheet