Wisteria needs regular pruning to keep the growth and size under control, but it will also improve the flowering display. Although it seems complicated, wisteria pruning is quite simple if you follow our simple guide. For more information, see this RHS video.
When and how to prune Wisteria
Wisteria is pruned twice a year, in July or August, then again in January or February to keep it floriferous and to prevent it from growing out of its allotted space.
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 from getting into guttering and windows, and encourages it to form flower buds rather than green growth.
Winter pruning (January or February):
Cut back the same growth 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.
Other ways to train Wisteria
The ideal way to grow wisteria against a wall is to train it as an espalier, with horizontal support wires (3mm galvanised steel) set 30cm apart. Over time, and with pruning twice a year, plants will build up a strong spur system. Use new growth that develops near the base of plants as replacement shoots if necessary, or cut out at their point of origin.
On pergolas and arches:
Wisterias with long flower racemes are best admired on structures where they can hang free, unimpeded by branches or foliage. For the best flowers, reduce the number of racemes by thinning out to give those that remain plenty of space to develop.
Growing into trees:
Wisteria can be trained to grow into a small tree, but to the possible detriment of the tree. Growing into large trees can make pruning of the wisteria difficult and flowering may be affected if the leaf canopy is dense. If you choose to grow into a tree, plant the wisteria on the south side of the tree, 1 metre away from the trunk.
Training as standards
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 to 1.5 metre 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’.
- If planting in a pot, John Innes No 3 potting compost is a good choice of compost. Make sure the wisteria is planted to the same depth as it was in its pot from the nursery, spreading out the roots and loosening the root ball before planting. Choose a cheap container that is only slightly larger than the plant needs, potting it on gradually as it grows to fill its final display container.
- Train the stem vertically up the support (this is usually stronger than twining).
- 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 side shoots.
- Prune the side shoots the following winter, shortening them to 15 to 30cm and repeat this process each winter to gradually build up a head.
- 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 these shoots to 2.5cm of their base, just as you would routinely prune a wall trained plant.
The most common problem gardeners have with wisteria is poor flowering. This can be caused by a number of reasons, including:
- Young plants grown from seedlings can take twenty years to flower so avoid disappointment by either buying a plant while it is in flower or choosing a named cultivar.
- Check your pruning technique and timing.
- Look for shredded flowers or teeth marks as telltale signs of bird, mice or pigeon damage.
- Take care to water in dry spells between July and September when flower buds are forming for next year, as drought at this time can result in failure to bloom.
- Be aware that sharp spring frosts can damage developing flowers causing them to drop before they open or to develop in a distorted fashion.
Sometimes a mature and apparently healthy plant will suddenly die and be replaced by a new shoot growing from the ground. This is often caused by wisteria graft failure.
Less common is attack by root fungi like honey fungus or Phytophthora root rot, but wisteria is susceptible to both of these.
Wisterias are also prone to scale insect infestation, including the more unusual wisteria scale.