Good question. It seems that while many people would like to have a wisteria bonsai, they give up on them when they fail to bloom consistently. After all, it is a rather dull looking plant when not in flower. I’m sure there are many non-blooming wisteria currently being used as umbrella racks.
Wisteria bonsai fail to bloom for several primary reasons. The first is that people tend to repot them too frequently. Keeping the wisteria a bit root bound is important—in fact, you should not repot your wisteria more often than once every five years or so.
That’s the first thing. The second is that wisteria need a lake the size of China to be happy. Give your tree a lot of water by immersion of the lower part of the pot in water during summer. DO NOT leave them soaking year round. Without resting the pot in a basin of water the tree won’t flower much the next year. Oddly, the roots don’t rot. (In Japan they have watering drones called apprentices, who can water in their sleep if necessary. If you don’t own a drone, and don’t wish to water 28 times a day yourself, it is perhaps best to use the immerse technique for the summer growing season.)
These first two practices must be combined: If you don’t repot frequently, the soil will get compacted (which we want for good flowering) but that compaction will make it very difficult to water sufficiently from the top when the tree needs a lot of water, in the summer. Hence the pan underneath.
The third thing to promote consistent blooming is to be careful when and how much you’re fertilizing. It is best to fertilize strongly AFTER FLOWERING (April/May) until about July, and then slack off. That way your flower buds will set for next year, and you will restrain foliage growth over the summer.
Lastly, the wisteria will usually produce some tendril growth, which if encouraged will shift the plant into a vegetative growth habit and reduce flowering. Nip these tendrils back to prevent too much growth.
Keep the darn things in the sun. Put up big fans to keep the clouds away. And then watch them bloom.
Maybe I should have titled this one ‘Preventing the Umbrella Rack!’ —but that would have been only slightly better than last week’s overly ambiguous title.