1 August 2014
Wisteria for Walls
Wisterias are lovely twining climbers with beautifully scented flowers in shades of white, purple and pink. They are ideal for training into trees and covering walls, pergolas and other garden structures, but can grow quite large, so need keeping under control with regular pruning.
There are two types of Wisteria - W. sinensis produces its flowers before the leaves appear and has stems that twine anticlockwise, while W. floribunda bears leaves and flowers at the same time and has stems that twine clockwise.
When planting Wisteria you should choose a position against a wall or fence that has the roots in a shaded spot, but where the top growth will be in the sunshine. Prepare a large hole and fork the base, digging-in plenty of organic matter, such as garden compost or well-rotted farmyard or stable manure. Top dress your plant every spring with more compost or well-rotted manure. Also feed with a general fertiliser such as Growmore in the spring and then with a liquid tomato feed every 14 days after the end of May.
Wisterias flower from spurs that form on the stems, and the best way to encourage the spurs to develop is to train your wisteria so that the side shoots and branches are horizontal. This is best achieved by training your plant on galvanised wire secured to a wall or fence. In the wild, wisteria plants climb by sending out shoots that look for tree branches to climb on, and once a branch is located the wisteria stem starts turning clockwise, or in some cases anti-clockwise, in a circling motion resulting to secure itself in its new position. Remember this fact when training your plant on a new wire and don't try to train it in the wrong direction as it will undo itself.
With your wisteria trained, you should only give it a light summer pruning during the first three summers to control any wild side shoots that you do not wish to tie in and to reduce any excessive foliage.
Wisteria is normally pruned twice a year, in July or August, then again in January or February. This keeps it floriferous and prevents it from growing out of its allotted space.
In July or August cut back the whippy green shoots of the current year’s growth to five or six leaves, after flowering. This controls the size of the plant, stops it getting into guttering and windows, and encourages it to form flower buds rather than green growth.
In January or February, when the plant is dormant and leafless, cut back the same growths to two or three buds to tidy it up before the growing season starts and ensure the flowers will not be obscured by leaves.
If you have had a wisteria for a number of years that has failed to flower, drastic action is needed. In late autumn, after the leaves have fallen, prune the plant back by 50%, removing all the thinner stems and only leaving the main plant structure. Test the soil to see if it is alkaline and if it is add a small dressing of iron sulphate to help counteract this and help turn the soil slightly acid. Then top dress with some organic matter around the base. Sometimes even this drastic pruning won't result in flowering because the plant was planted incorrectly in the first place, or because it was propagated from a parent plant that also lacked flowers. If this is the case, the only thing you can do is to invest in a new plant which you know has been propagated from a prolific flowering parent.
When buying a new wisteria, always choose one that has been grown from cuttings or by grafting, as seed raised wisterias flower less reliably, and also take longer to flower. Grafted plants will show a visible bulge at the graft union near the base of the stem. Named cultivars are almost always grafted, whereas species plants may not be.