July and early August are the best times to give the Wisteria it's summer prune. Over the summer months the Wisteria has put on a lot of vigorous growth and it needs a good prune . This prune needs to take off long wippy growth, and where the Wisteria has outgrown its allotted space. In the image it is impinging on the windows and needs cutting back.
The other reason for pruning Wisteria in the summer is to ensure it keeps flowering. The pruning regime, Winter and Summer is an intrinsic part of the routine to ensure it flowers well.
These quick before and after images show just how much Wisteria can be happily cut back. Advise on growing and pruning Wisteria, video advise on how to make Wisteria flower including the summer prune, and a reminder how lovely the Wisteria looked earlier in the year.