Wisteria is one of those vines that's difficult to get a grip on. "Why doesn't my wisteria bloom?" is a common lament. "When should I prune?" is another common query. There's one thing all the experts agree on: Wisteria is one tough vine and no matter what you do, chances are very good that rather than killing it, you'll shock it into blooming.
"With aggressive vines like wisteria," says Jim Lounsbery of Vineland Nurseries in Beamsville, Ont., "there's no problem beating them up a bit. You won't hurt them." One of the ways to get them under control at this time of year is to cut back the long tendrils, called laterals. "Pruning about 25 per cent of the vine at this time of year is pretty safe," says Lounsbery. Any more, though, and you'll be forcing too much soft-stem growth that won't bear flowers next spring.
Wisteria should also be pruned twice a year. After mid-summer trimming, prune again in late spring or early summer, at the end of the normal flowering period for your region of the country. The goal is to create a strong, but open structure, says Lounsbery, who trains young wisterias much like grape vines. "Take one or two upright main stems and start bending the laterals against the trellis horizontally, or at a 30-degree angle, cutting back the laterals to about three to five buds, " he explains. "This early in the season, you can almost tie the laterals in knots, they're that flexible." This gives the vine some semblance of order that will keep it from getting out of hand later.
To coax wisteria to flower, Lounsbery recommends another trick that can be done in late fall. Use a spade to cut into the soil deep enough to reach the roots, cutting all around them. "This promotes fibrous root growth," he explains. This "root pruning" is a common practice for stimulating flowers in many vigorous ornamental vines and shrubs, such as dogwoods and fruit trees.
When buying wisteria, look for grafted varieties. These will have a better chance of flowering. "Whether they're grafted or not, make sure you can see some flower buds forming," he says. Otherwise, you may be waiting six or seven years or more for flowers.
To promote flowering in a wisteria that has been declining in blooms or has yet to bloom, Vineland Nurseries' Jim Lounsbery recommends making a spring application of fertilizer high in phosphorous and potash. Never use one that's rich in nitrogen, which will force stem and leaf growth at the expense of flowers.