Old.

you trim your wisteria twice a year.to control it& direct it.(and it helps it flower better.)Being yours is tree form.you got your main lead comeing up or you may have more than one all together.may have support or may not.(you didn't say...or how old it is)you have side shoots that come off the main lead stems. when you cut them back you want to be sure are at LEAST 6 nodes/spurs/bud thinggys.are left.

My neighbor has a tree form one.she's not shaped it for awhile.Her mom has one growing all down her fenceline& another has a dog run and has covered it with the wisteria.mines being trained up in apple tree.they bloom together.

They form thier buds late winter or very early spring, so old wood. A sever winter can kill the flower buds. I trim mine (where I can reach them) just to keep them tidy and get rid of the long strangly bits. Then in fall I try to cut off all the seed pods so they won't sprout. I can't reach them all though, and about November it sounds like kids having a cap gun shoot out, but it is the pods exploding. First time I heard it, it really startled me!

Not meaning to hijack the post, but will the vine kill trees if it grows up them?

Yes, it eventually "chokes" the tree. It is suggested to never grow wisteria on a living tree.