Wisterias baffle most gardeners when it comes to pruning them. If it's a mature plant then it needs pruning twice each year. The first during August when lateral growths, not the main stem or branches off this but the smaller, thinner ones off those, are cut back to 4 - 6 leaves. Then, in December/January, prune these again back to 2 - 3 plump flower buds, easily recognisable during the winter months. Wisteria can grow 10 - 12 feet in one season, so make sure there's plenty of room for it.