Few plants are as extravagant with their blooms as wisteria vines, strung with fragrant, eight- to ten-inch (20- to 25-cm) flower racemes dripping downward. But if your wisteria vine blooms sparsely—or has never bloomed—the best way to trigger flower production is by pruning.
Japanese wisteria (Wisteria floribunda) is a rampant woody plant that requires pruning twice a year in late winter (March) and late summer (August) to encourage flower buds. If your wisteria is a roiling mass of long stems, you’re facing a large and ultimately satisfying job with loppers and secateurs. In March, begin by identifying the thickest and oldest branches (the first-tier wood) that form the central architecture of the vine. Coming off of these are medium-thick branches (second-tier wood). These, in turn, produce lots of long, whippy stems springing outward (third-tier wood).
If the wisteria is congested, with too many medium-sized branches following and twisting around the heaviest first-tier wood, use the loppers to eliminate up to half of the second-tier wood (which will also remove the youngest wood springing from those sections). The second-tier branches tend to twist around older wood and some sections may be too closely bound together to allow removal, so cut as low down and close to the thickest, first-tier wood as you can. This will help to control the size and weight of the vine and improve its appearance when in leaf.
Cut the youngest stems (third-tier wood) back to about eight inches (20 cm) in length, allowing three or four buds to remain. This will encourage the vine to turn these stubs into flower spurs. Be sure to keep any short (six- to eight-inch/15- to 20-cm) flower spurs already present. (You may not encounter any if the wisteria has never bloomed and this is the first pruning.) Buds that lay flat against the wood produce foliage; slightly raised, fatter buds will be flowers. If flower spurs with buds are present, these will bloom and the reduced foliage won’t conceal them. If no flower buds are present, your pruning work this year will pay off in flowers next year.
The wisteria will produce more extravagant growth over summer, and in August it will need another pruning. If the vine is new and not yet covering its structure (such as a heavy trellis or arbour), allow new central growth to remain where you want it to fill in, but shorten the young, whippy stems along the more mature lower sections of the vine so that each stem is left with about five or six leaves. These shortened young sections contain the potential flower spurs for the following year, and are the ones you’ll prune to shorter spurs at the end of next winter.
Pruning twice a year should produce bloom on your wisteria vine. Root pruning (by thrusting a spade down to snap roots 12 inches/30 cm from the trunk) is sometimes suggested as a way to shock the plant into bloom, but may do more harm than good. Feeding the vine with super phosphate (follow instructions on the package) in spring might encourage bloom. But the best way to encourage bloom is to work with the plant’s ability to produce flower spurs. Pruning a large, aggressive wisteria requires good sense and courage—be brave, and reap the rewards!