When to Prune Wisteria Depends

Q. I read your blog on pruning flowering shrubs but want to know if this information also pertains to wisteria vines? I’ve read that flowering occurs on new growth, so I am not sure about trimming it.

A. In my blog I was telling people to differentiate between plants that flower early in the spring versus those that flower later in the spring. Bottom line, always prune flowering trees and shrubs any time after they finish flowering if flowering is important to you.

Let me first give you a textbook answer. Wisteria falls into two major groups; the Asian varieties and the western varieties. The Asian varieties of wisteria flower on last year's growth (early spring flowers) while the American or western varieties flower on current season growth (late spring or summer flowering).

Something interesting about wisteria is the amount of time the plant needs to begin flowering. It's usually quite a while. Some people say 10 years to initiate flowering, others say 15 years and I have even seen some people say 20 years! That's actually quite old for a woody plant to start flowering. Typically unimproved woody plants begin flowering in about 6 to 8 years. 

Plant breeding and producing hybrids that flower earlier can address this problem. When you're pruning try to leave as much older and larger wood as possible. Prune its structure the way you want it as early as possible. A few things that can affect how soon or at what age the plant begins flowering can be removing too much larger wood, using a lot of high nitrogen fertilizers, how much shade it's growing in to name a few. It will flower when it's good and ready to flower. Just remember prune after it finishes flowering and you won't go wrong. Fertilize once a year in the very early spring with a “rose fertilizer” and not a “lawn fertilizer” or apply compost in the early spring.