Taming Wisteria

In last week’s column we began to take a look at the magnificent and exotic woody vine known as wisteria. It comes in many varieties, but a word of caution about the species Wisteria sinensis, also known as the Chinese wisteria.There has been a move to classify this species (not the entire group, though) as an invasive plant. It’s not that it’s rampant and taking over the Northeast, though it might do that down South, but the vine can easily get out of hand and if left untended or abandoned it can topple trees or do damage to buildings, as it has a tendency to cling, explore and weigh down those objects and structures, including trees, that support it. So beware and when buying this plant check its linage, carefully.

Wisterias do climb by nature but can be controlled best on wires, trellises, arbors and pergolas. However, they will also climb up a gutter, which in time the vines will overpower and destroy. They can be grown on solid, vertical surfaces if proper supports are provided, such as rows of wire attached 4 to 6 inches from the wall. Use sturdy, durable materials such as galvanized wire, steel pipe or tubing or wood. Copper or aluminum wire or tubing are preferred over other metals since these don’t rust. Use pressure-treated or rot-resistant wood for arbors and pergolas. However, do not plant wisteria where the stems can invade and clog building gutters or sneak under shingles or into window sashes.

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 4 to 5 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 6 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 10 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.

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 or pot. 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, and organically, until they fill the allotted space with a low-nitrogen, high-phosphorus fertilizer. Do not expect young vines to bloom, since vegetative growth is being encouraged, but large potted wisteria, which are available in garden centers now, may bloom in only one to two years.

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 the 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.

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 the tip off the main leader when it reaches the desired height.

Summer pruning: 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 1 to 2 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.

Maintenance pruning: 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. With luck, the plant will bloom within two or three years.

Train the main leader to the upper part of the support system and cut it off when it reaches the desired height. Train main side branches as needed so they are spaced about 18 inches apart. Once the vine fills the allotted space, follow summer pruning practices. In winter, prune leader shoots back to only four or five buds. Remove any suckers that appear at the base of the plant. Cut back unpruned leader shoots (from the previous summer) to one-half to two-thirds their length and cut side shoots pruned the previous summer back to only 1 or 2 inches in length.

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 4 feet from the main trunk (in older plants), all around the vine.

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, or it is pruned heavily in winter and spring, which encourages vigorous, vegetative growth, and/or it is pruned improperly.

Also, in severe winters—you do remember 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 (about 3 pounds per 100 square feet of bed), and severe pruning of new growth in late spring or early summer then root pruning in late fall—but again, these tricks won’t help with vines that are just too young to flower. Buy top-quality, named and grafted parent plants. These may have the vigorous W. sinensis as the root stock, but never as the shoot stock. These should flower in three to five years. Otherwise you will just have to wait, and wait and … keep growing.