Wisteria (Wisteria spp., USDA plant hardiness zones 4-9) is a genus of about 10 flowering, woody vine plants with incredible, long-drooping clusters of lilac-hued flowers. They are prolific growers and bloomers, so if you have one, you will need to know how to prune overgrown wisteria. Don't be fooled by their beauty. It is not unusual for these innocent-looking plants to damage buildings and choke out trees.
How to Prune Wisteria
The best way to prevent these plants from growing out of control is to prune wisteria two times a year, keeping in mind that new flowers will form on one-year-old growth. You'll want to do this pruning first when the blooms have faded in early to midsummer and again when the shoots of new growth start to look messy. Prune those new growth shoots down to 6 inches long and remove any shoots that aren't necessary for the vine's primary framework. Also, take off any suckers that you see around the roots.
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Those vines will keep growing after you do this, so prepare your shears for a second wisteria in winter pruning. The winter season is the best time to see which branches are vital for the main vine. Prune the plant's lateral branches from the main trunk down to three to five buds; this includes ones that you may have cut back in the summer. Also, cut off messy and crowded branches that detract from the vine's primary shape. You may need hand pruners, loppers and a saw to complete the job.
Trimming Wisteria Tips
The goal of a wisteria summer pruning is to control growth and allow in more air and light. You must always wait until the flowering ends, even if that means that you need to hold off until the end of the season. Start by surveying the entire plant all the way around, cutting back any leafy and long shoots. You can leave up to five buds and cut the rest off. Then, remove diseased and dead wood and weak growth and make sure that none of the significant branches cross over one another.
As for winter pruning, focus on thinning the plant and keeping it trained. You'll want to attack any diseased or dead wood first, removing any suckers you see around the roots. Leave the larger buds intact and eliminate crossing branches and weak growth. You may want to reshape the wisteria's overall structure, mainly if it grows on a pergola or trellis. Some gardeners remove as much as one-third of the new growth at this time to encourage more flowers and less twiggy growth.
Common Wisteria Problems
Even though wisteria plants are prolific growers, they are not immune to plant diseases, and three of the most common are powdery mildew, leaf spots and crown gall. Powdery mildew is a fungus that presents with a whitish-gray mold that shows up on the leaf surfaces. You may want to spray the wisteria with a protective fungicide before the flowers start to appear in the spring.
Leaf spots are also caused by fungi, but most of the time, you can just pull off and destroy any spotted leaves that you see. Fungicides are not usually needed unless the spotting is taking over the plant. Crown gall is a soil-borne bacteria that leads to swellings on the stems and main roots. There is no cure for this, so carefully inspect wisteria plants before buying them and remove and destroy any plants that succumb to the disease.