Q My wisteria has never flowered. Why is this and what can I do about it?
A Where a plant has never flowered the likely causes are:
The plant is too young, or is not a named variety: The two most common species grown in gardens are Chinese wisteria (Wisteria sinensis) and Japanese wisteria (W. floribunda) Seedlings of these species may take 20 years to mature sufficiently to start flowering. Most cultivated wisterias are named varieties grafted onto an appropriate rootstock. For example W. sinensis 'Prolific' or W. floribunda 'Multijuga', 'Rosea' and 'Yae-kokuryu' ('Black Dragon'). These varieties should all flower within a few years of planting.
If you’re buying a wisteria, always make sure it’s a grafted, named variety. If you inherit a non-flowering wisteria it could be a seedling, or it could be a grafted plant where the graft has failed, allowing the rootstock to take over. There is little you can do to encourage earlier flowering, though pruning may help a little. If you’re impatient, buy another plant while it’s in bloom.
Incorrect pruning: If wisterias are left unpruned they are not only likely to get out of hand, but will take longer to come into flower. Side-shoots should be cut back to about 30cm in August. This will encourage flower buds to develop near the base of the shoots. These shortened shoots should be cut back further, to about 5cm in winter.
Too much shade: Wisterias need a sunny position to flower well. If shade seems to be the problem, then reduce overhead cover, move the plant or train some branches out into a sunnier spot.