Wisterias can be left to ramble unchecked where space allows but will usually flower more freely and regularly if pruned twice a year. The removal of growth in summer allows better air circulation and more sunlight to reach the base of the young growths, encouraging better ripening of the wood and improving the chances of flower bud formation. Restricting the amount of vegetative growth and encouraging short, flowering spurs will result in more flowers.
Summer pruning (July or August)
Cut back the whippy green shoots of the current year’s growth to five or six leaves after flowering in July or August.
This controls the size of the wisteria, preventing it getting into guttering and windows, and encourages it to form flower buds rather than green growth.
Winter pruning (January or February)
Then, cut back the same growths to two or three buds in January or February (when the plant is dormant and leafless) to tidy it up before the growing season starts and ensure the flowers will not be obscured by leaves.
Renovation or hard pruning
With older plants severe pruning may be needed to remove old, worn-out growths, or branches growing over windows or protruding outwards from the face of the building. Likewise, hard pruning maybe required where maintenance needs to be carried out on the structure supporting the plant.
Drastically shortening back long branches, removing sections of older stems to just above a strong young branch or growth shoot lower down, or cutting completely back to a main branch, or even to ground level may be necessary. A careful, unhurried approach is needed if larger, thicker branches are to be removed and where a branch is twining it may be necessary to trace back and mark it at intervals with string before removing it. The end result should be a skeleton frame work of reasonably well-spaced branches.
Other points to consider when hard pruning;
- Hard pruning will stimulate strong, new growth so it is better to avoid feeding in the first spring after hard pruning
- If there are gaps in the framework suitably positioned new growths can be trained in to form replacement branches, with flowering usually resuming in two or three years’ time. Often there is strong basal shoot growth
- If unwanted for replacement branches they can be removed. Any such pruning can be done during the period from leaf fall to early February
- Other new growths can be pruned back summer and winter as for normal routine pruning