The subject of wisteria is fraught with frustration for many Canadian gardeners. There are plenty of arbours and pergolas festooned with healthy-looking wisterias that simply refuse to bloom. So what's the secret to getting the magnificent show of fragrant blossoms that are on view each spring at Royal Botanical Gardens in Burlington, Ontario? Gardener Bob May says there's not much to it: don't let your wisteria grow out of control and give it two good prunings a year-once in midsummer and the other in mid-September, or when vigorous growth has ceased. Every three years or so, your wisteria will need a radical renewal pruning to keep it in shape.
Wisteria responds best to stress. Be sparing with fertilizer and wary of high-nitrogen formulas, which will result in too much vigorous growth and foliage. And don't overwater. The phenomenally healthy wisterias that strut their stuff on RBG's pergola pretty much look after themselves and only get watered during extreme drought. What's the most floriferous wisteria on the RBG pergola? It's ‘Lawrence' Japanese wisteria (Wisteria floribunda ‘Lawrence', Zone 5).
It's easy when you know how
After flowering is finished, prune entire plant back, thinning it out well and leaving just one or two buds or nodes per branch. Keep vigorous, strong shoots that have set buds or nodes at their bases. Get rid of any branches that hang down and spoil the shape of the plant. To force the plant to branch more horizontally, make your cuts on a down-facing bud (even if you cut below this bud you will get new branches).
By midsummer, wisterias have put on a great deal of new growth that can tangle into a big, shapeless blob, encroach on nearby plants or weigh down supports. Don't be timid with the pruners: prune entire plant back hard to the desired size and shape and cut thin, overcrowded stems out completely. By summer's end, new shoots will appear and replace most of what's been cut off. This is the time to select vigorous new shoots for training along wires, a trellis or even up a tree.