Posted: 16/03/2014 at 21:39
Pruning is the usual solution to this problem.
As this is a FAQ, I keep this info on file.
"Wisterias can be left un-pruned where space allows, but flower more freely if pruned in summer and winter to build up a spur system.
August: Shorten-back current season’s shoots to 30cm (12in). This allows more sun to get to the wood, encouraging flower-bud formation.
February: Shorten-back the summer-pruned shoots to within 2.5-5cm (1-2in) of older wood or two to three buds. It should be possible now to distinguish the plumper flower buds from the slimmer growth buds.
Long, whippy shoots that grew after the summer pruning should also be pruned. Cut these back to five or six buds from the main branch, making the cut just above a bud."