There are several different reasons why wisteria fails to bloom.
Don't let your wisteria grow rampantly without any pruning. Pruning your wisteria will help reduce vigorous vegetative growth and direct the plant's energy into producing flowers. In addition, it will keep the vines at a manageable size and prevent them from becoming overly aggressive.
If your wisteria aren't blooming, try this method recommended by Mark Viette.
Mark has found that pruning your wisteria during the initial period of flower bud production (from August through September depending on the species*), will help ensure flowering the following season. He has found that pruning in this way forces nutrients into the remaining buds much like you try to achieve when pruning grapes.
- Cut back side shoots by 1/2 in August.
- Prune side shoots from trunk to 6" - 8" (leaving 6 or 7 buds) the first week in September.
- In mid September or February, prune each shoot to 3 buds.
- When cutting side shoots leave 1/2" to 1" of the stem outside of the buds.
- Some growth can be removed completely back to main laterals.
- You can continue to perform minor pruning throughout the season as necessary.
This late pruning helps reduce excessive growth and each pruning focuses energy into the remaining buds which helps convert vegetative buds into flowering buds.
Once wisteria begins to flower, it will usually continue to flower each year.
* Japanese wisteria (Wisteria floribunda) blooms in May and produces flower buds from August through September.
* Chinese wisteria (Wisteria sinensis) blooms in June and develops flower buds from September through October.
Wisteria frutescens 'Amethyst Falls' is an excellent cultivar of the American wisteria. Unlike the Chinese wisteria and the Japanese wisteria, it blooms at a young age and is not overly invasive.