There is a balance between the plant’s needs and the needs of the garden and garden owner that isn’t always reflected in standard pruning textbooks. Its up to you to find the right balance. Managing client expectations may add a level of concern, but it shouldn’t affect your horticultural judgement.
What you need to know is how, and how much, to prune; also why pruning is needed.
Wisteria is a true climbing shrub: it has twining stems which will coil around a support. This may be a trellis or pergola, but it will also use its own older stems. Or indeed, nearby trees and shrubs, fences where there’s a small gap for it to slip a tendril through…Its not described as a vigorous climber for nothing!
But oh so beautiful with those pendulous scented racemes of flowers in spring and often a second, summer flowering.
If you’re lucky, the wisteria you face will be a youngish one that has seen some pruning.
However, if you’re faced with a mature un-pruned wisteria in a client’s garden, the matter is less simple, but still doable.
Why Prune Wisteria?
The reasons are to: –
Remember when you’re feeding after pruning that Wisteria is a member of the Leguminosae family and requires little extra nitrogen. However as a flowering plant it will thank you for potassium-rich feed.
In spring Wisteria flowers from the buds that were formed in late summer the previous year. A second blossoming in summer is more likely if those flowers are removed before they ran to seed. Other summer blooms come from buds which were still juvenile in spring.
Wisteria is hardy, but will not flower as profusely in a shaded site. However, the plant will put out more green leaves in order to maximise its ability to photosynthesise. If possible, the easiest solution is to relocate the wisteria. Pruning would allow more light to the flowers but is likely to reduce the overall health of the shrub in the long term.
To remove the long ‘whippy’ stems and spent flowers.
Wisteria is rampant in growth and removing excess foliage restrains this tendency. Furthermore it allows light and air to reach the flowering spurs (short stems with flower buds) and encourages new flower buds.
In early to mid summer, after the first flowering.
How and What
Prune the long shoots of the current year’s growth back to 15cm, or 5 buds, making the cut just above a bud.
Remove any stems not needed for the main framework of the plant or that are in the ‘wrong place’. For example, those growing outwards away from the fence.
Prune away root suckers, which you may see on grafted varieties.
Remove spent flowers by taking off the stem, not just the blooms.
You may need to repeat this pruning after the second flush of flowers, although it would normally be a lighter prune.
Wisteria is deciduous, which is a great aid during winter pruning as the tracery of twining stems is easily seen.
To prune away long shoots that have grown since the summer pruning and make sense of the overall woody structure which can become very entangled.
Mid-winter, ie January, or early February, but not during a hard frost.
How and What
Considering the shape of wisteria before and during pruning is a critical element of the task; your client wouldn’t want a lop-sided climber! Its made easier as the overall structure is visible in winter.
Firstly do the usual check for dead, diseased and dying branches and remove these. I also look for other signs of pest and disease as I’m inspecting the climber anyway.
Next I would suggest checking the support, whether that’s trellis or pergola. Mature wisterias can be heavy and it’s easier to carry out repairs in winter.
Then cut back the summer-pruned stems to 3 buds form the base to ensure new flowers will not be obscured by leaves.
Pruning Young Wisteria
A pleasant task, best carried out in mid-winter. Stand back and consider not only the shape of the wisteria but whether it has been planted in the best place. If not, then it should be small enough to move without too much damage to the roots.
Depending on the current height of the main stem, reduce it to 75cm – 1m. Untangle the side branches, remove those heading in the wrong direction and reduce the remaining ones by up to a third, always cutting to just above a bud.
Tie into the support, stand back and admire your work.
Renovation aka Hard or Restorative Pruning
Young wisteria or those which have been regularly and correctly pruned are now sorted with the above tips. But what if you’re faced with a towering shrub with stems the sized of your muscled arms?
Wherever possible, hard pruning is best carried out in the winter, or certainly when the plant is leaf-free. Depending on the amount of pruning required, it may need to happen over 2 or 3 years.
Its not a quick prune. You’ll need to trace back twining stems to where you need to cut to be sure of pruning the correct ones. I’ve found marking the one to be cut with twine or pen at intervals along the stem is helpful.
Some older branches may need to be cut back almost to ground level, just above a growth shoot. The wisteria should respond with new growth in spring, but may take 3 years to develop new flowering spurs from such a drastic prune.
If it is a grafted plant, keep all cuts to 3 buds above the graft, giving a safety margin in case of frost or pest damage.
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