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Wisteria is an attractive vine that can grow to cover almost anything and, if left unchecked, smother other vegetation or lift balconies right off of a house. Gardeners can eliminate overgrowth with wisteria pruning techniques, which require time and attention, but are well worth the effort. Pruning should generally be done two times a year and should facilitate the desired direction of the growth. Whether a gardener intends for the vines to grow horizontally or vertically, largely determines which parts of the plant are removed during pruning. Lattice growth requires a sturdy trellis that can withstand wisteria growth so that pruning is a viable option.
When to prune is just as important as where to prune. Wisteria pruning is a biannual undertaking, once in the winter and once in the summer. While winter pruning can be done around midwinter, summer pruning should be done at a more precise time, about two months after the wisteria has flowered. Pruning at these times ensures that the wisteria does not grow uncontrollably and that it flowers more abundantly.
For effective wisteria pruning, a gardener should decide whether the vines are to grow up a porch or balcony, along scaffolding, or over other plants. The wisteria's location will dictate how much pruning is necessary. Wisteria tends to flower more on horizontal vines than vertical ones, and as most people prize the vine for its flowers, pruning for horizontal growth is a popular choice.
To promote horizontal growth, a gardener can locate the point from which the new wisteria vines should sprout and then sever the main stem. New stems will emerge, and the vines that are to grow horizontally can be tied down to a railing. A vertical shoot will usually continue to grow upward, or it can be trained to do so by tying the stems to a vertical structure. Any unwanted stems that sprout can be further pruned. It is also possible to simply shorten a horizontal stem to about six inches (15 cm) to provide a small, flowering focal point.
Stems growing up a wall should be spaced at least 1.5 feet (45 cm) apart. This practice gives the flowering offshoots more room to blossom by keeping the other vines and stems from obstructing them. In the summer, a gardener can opt to trim off all stems that lie between the two main ones or to shorten these offshoots to no more than four buds. When midwinter arrives, they can be trimmed a second time.
Wisteria is a popular trellis vine, and a common mistake is to buy or construct a weak and unstable trellis. The trellis should be constructed from sturdy material, well grounded, and staked, or the strong vines may overtake, dismantle, and even lift the trellis. No amount of wisteria pruning can make up for poor construction. Pruning successful lattice growth is similar to the techniques used for wisteria on other structures, lopping where needed for horizontal growth. If the trellis is connected to the house, a gardener should take care to avoid planting the wisteria on the side of the trellis attached to the house, or the growth will move up the building.
If wisteria has become overgrown, pruning is an option that can help bring the vine into a manageable state. It is a very resilient plant and difficult to eradicate completely. Removing large portions will not kill it, and the grower should not be concerned if sizable stems or shoots appear to have died. Overgrown wisteria should be cut about 3 feet (1 meter) below the desired length. One disadvantage to doing this, however, is that new flowers will not appear on the fresh stems for several years.