My Wisteria is 17 years old. It has had merger blooms( 3-5) over the years ... What I have noticed is that it does not bloom in the Spring...It is Blooming now in August! Why would that be??
Like most plants, wisteria needs sun to bloom; vines that get a lot of shade simply will not flower, say the experts. Some growers feel it also craves phosphorus, the 'flowering nutrient', so folks with no flowers might want to scratch a couple cups of bone meal into the soil now and then add some rock phosphate in the Fall.
But the secret seems to be in the pruning. One of my best general plant books says to always prune after flowering (which, of course, is very cruel to people who don't got no flowers) and only recommends winter pruning to control the size of this often-invasive monster. But another good source say to hack away in winter, removing all but a few buds from about half of the previous year's growth in addition to the summer pruning.