First of all, to set your mind at rest, there is no need to fear pruning your wisteria! In fact, they need it to promote flowering. You really cannot harm your plant with pruning!
To promote flowering:
Wisteria need pruning to promote prolific flowering. You can prune all year, but the most important attention is needed after the spring flowering and throughout the growing season. The long green tendrils wisteria throw out (sometimes weekly!) need to be cut back to two buds, ideally when they are still short and can be pinched with the fingers, before the distance between the leaves gets too long. This is the future of your flower spacing – the closer the flowers will be together and the larger the impact when the plant does its spectacular thing in spring. Any bits you’ve missed can be cut off in the winter when the leaves are off and the structure of the plant is fully visible.
It is also vitally important to cut off any shoots at the base of the trunk, again on the more vigorous specimens this is necessary regularly throughout the summer months. Try not to let wisteria branches twist together – eventually after many years it can strangle itself. Also, do not let the plant become too congested – the sun needs to be able to reach all parts to ripen the wood for flowering. If you keep the main trunk clear of shoots this should not become a problem.
Pruning for shape:
Cut off the bits you don’t want, even larger branches with a saw – they will recover albeit perhaps forfeiting flowers for a year.
Pruning for espalier:
Allow a long tendril to grow along a wire, straighten and tie in. The following year, as each bud throws out long tendrils, cut back to two buds, when they are still short. This is creating the new potential flowers.
Pruning wisteria as a shrub or standard:
Wisteria are great as standards, but need support, a sturdy post is best. Without support you can grow them as a shrub; they will scramble as far as you let them, so just cut off the bits you don’t want. Cut off all long tendrils back to two buds and anything else that spoils the shape you want.
Wisteria as bonsai:
Wisteria sinensis particularly does well as bonsai, though not in a shallow pot since the roots should not be allowed to dry out and they need to be cool. Also, wisteria in pots need a lot of sustenance so a lot of little feeder roots are essential to keep them in good health if grown in this way, and a deeper pot gives enough room for them. They also need feeding with a slow release fertiliser and fortnightly boosters with liquid all purpose feed throughout the growing season. They need repotting every couple of years, and root pruned – as with leaf pruning, root pruning stimulates the new growth of the little feeder roots, essential for healthy plants.
n the North of England the floribunda cultivars struggle in small pots, mainly due to big temperature variations from night to day (we can have -3 at night and +20 during the day in the garden)
Remember – a wisteria is a vine, and like all vines, they are vigorous growers and in their natural environment can be found by roadsides and scrambling up trees. They are very resilient, persistent and long-lived plants.