Wisterias bring timeless elegance to gardens. They are normally vigorous plants that lend themselves to training over pergolas, against walls or being grown as a standard. However they are grown, regular and systematic pruning will help to encourage their spectacular floral displays.
Images: Tim Sandall/RHS Advisory
Wisteria is best known for covering house walls with elegant flowers, but this climber is versatile. It can be trained over pergolas, up tree trunks or even as a standard with a lollipop-head. However, to encourage a fine display of flowers and to keep plants neat, wisterias need regular pruning. While the basic principles are the same, they need adapting to get the right results.
On walls and trellises
Where space allows, wisterias can be left to ramble unchecked, but they usually flower more freely and regularly if you prune to create stumpy spurs. It is on these shortened shoots, at the base of the previous year’s growth, that the larger, plumper flower buds will be produced.
To create spurs, pruning is carried out twice a year.
August: Begin by tying in any new shoots that are needed to extend the existing framework of branches, to fill in any gaps or to replace worn-out limbs. Then shorten the current year's shoots to about 30cm (1ft). Removing shoots at this time of year will restrict the amount of growth, improve air circulation and allow in more sunlight to ripen the young stems. All of these factors help to improve flower-bud formation.
February: Shorten the summer-pruned shoots to within 2.5-5cm (1-2in) of older wood or to two or three buds (left). It should be possible now to distinguish the plumper flower buds from the slimmer growth buds.
Long, whippy shoots that grew after the summer pruning (right) should also be pruned. Cut these back to five or six buds from the main branch, making the cut just above a bud.
There is another school of thought that advocates pruning just once a year in August/September.
Training on pergolas and arches
With its long racemes, Wisteria floribunda is best grown on garden structures, such as pergolas and arches, where the blossoms can hang freely. For maximum flower length, thin out the racemes in mid-spring so that each has room to develop to its full potential. Apart from this, prune as plants grown on walls and trellises.
Using trees as a support
Wisterias can be trained to grow through small trees, although usually to the detriment of the supporting tree. They can also be planted to grow into tall trees but as they need sun to flower may not do so under denser leaf canopies. Plant on the south side of the trunk, at least 1m (3ft) away. Prune in August to shorten the new shoots back to six or seven leaves, and back to two buds in February. However, this can be difficult once a plant reaches the upper branches, so aim to keep all of its growth within easy reach.
Renovation
Older plants may need severe pruning to remove old, worn-out branches growing over windows or protruding outwards from the face of a building. Any such pruning should be done between leaf fall and early February.
It may be necessary to shorten long branches: remove sections from older stems to just above
a strong, young branch or lower shoot, and cut back some sections to a main branch (left) or even ground level. An unhurried, careful approach is needed when larger, thicker branches are removed. Hard pruning will stimulate strong new growth, and the end result should be a framework of reasonably well-spaced branches.
If there are gaps in the framework, new growth can be trained to fill them. Flowering usually resumes in two or three years, especially if normal summer and winter pruning is carried out.
Always think about your safety when you have to prune stems that cannot be reached from the ground. Make sure ladders are securely fastened and get a friend to foot the ladder. Use a sharp pruning saw to make the task easier, and be careful where the prunings drop. If necessary hire a cherry picker or, if you are not competent with this type of machinery, hire a professional arborist.
Growing a standard wisteria
Illustrations: Robin Griggs
Standard wisterias can be grown either as specimens in a border or in a large pot.
Start with a young, single-stemmed plant, and insert a 1.2–1.5m (4–5ft) stout support next to it when you plant into the ground or container. This will be used to create the main stem of the ‘lollipop’ (left).
Wisteria sinensis, which produces its flowers before the leaves appear, has stems that twine anticlockwise. The stems of Wisteria floribunda, which bears leaves and flowers at the same time, twine clockwise. So twine the stem around the support in the right direction for best results.
Allow the plant’s leader to grow unchecked until it reaches the top of the support and then remove the tip in the following February to encourage the formation of sideshoots.
Prune these the following winter, shortening them to 15-30cm (6-12in) and repeat this process each winter to gradually build up a head (left). Weak or misplaced growth can be cut out entirely, as can older branches if the head becomes too dense in later years.
As the head develops, prune in August as well. Cut off above the seventh leaf any shoots that are not needed to extend the head. The following February cut back all these shoots to within a few centimetres (an inch or so) of their bases, just as you would a wall-trained plant.
Leigh Hunt