Alison Marsden, Gardening by Design

Wisteria

Pruning mature Wisteria

Wisteria, like any other plant, is happy to grow and flower without any human intervention by way of pruning and the gardener should understand that he or she prunes a plant to change its natural growth or flowering/fruiting habit. If left unpruned, Wisteria will readily cover a house or tree with winding stems up to 30m long and will flower every year. However, pruning can induce a greater density of flowers as well as keeping the size of the plant under control.

Wisteria flowers early in the season, typically May in SE England, from buds on short twiggy spurs that mature the previous year. The object of the pruning regime described below is to remove long winding immature growth and encourage the formation of flower buds on these short spurs growing from the main structural branches (called laterals).

August

Cut back shoots that have grown in the current season to within 30cm of the lateral. This should leave 4-6 leaves on each side-shoot and allows more sun to get to the wood, ripening it and encouraging flower-buds to form. Any side shoot required to extend the plant should be trained as required and not pruned; this will form a new lateral.

February

Cut back the summer-pruned shoots again to 5-10cm leaving only 2 or 3 flower buds on the side-shoot (creating a spur). At this time of year it is possible to tell the round flower buds from the flat growth buds.

More flowers are obtained if the laterals are trained and tied in to the support more or less horizontally. This restricts the flow of sap, carrying nutrients, and encourages flower buds to form along the horizontal stems. A plant that is allowed to romp upwards will produce flowers mainly at the top.

Rescuing an overgrown specimen

If you have an overgrown Wisteria that needs to be drastically cut back and retrained, it is best to do this in sections over a number of years to keep it flowering in the meantime. Each year in the late winter pruning you can cut one main branch back and train a new one its place. A mature plant cut back all at the same time will regrow, but will take several years to produce flowers again. Wisterias are usually propagated commercially by grafting a cultivated variety onto a more vigorous rootstock. You will be able to see the point of the graft (called the union) as a bulbous knobbly bit above soil level. Do not cut the main stem below the union or all new growth will be from the rootstock instead of from the specially selected variety and flowering will be disappointing.

General cultivation

Wisterias grow naturally in forests and climb their way up trees from shade at the roots to reach the light at the forest canopy. The soil is rich in organic matter and slightly acid built up over the years from the decay of fallen leaves. The ideal garden situation mimics this and if your soil is alkaline (primarily clay or chalk) then it is doubly worth digging in a large amount of any well-rotted organic matter. The huge amount of growth put on each year makes Wisteria a ‘hungry’ plant and annual topping up with a mulch of compost or a general purpose balanced fertiliser will keep it healthy and flowering. Do not use a high nitrogen fertiliser or you will get even more green leafy growth at the expense of flowers.