How to grow and prune wisteria

When spring meets summer one of the most glorious garden climbers is the wisteria, a member of the pea family.

The dangling, fragrant flowers come in shades of blue, pink and white so consider your background carefully for these climbers are happiest climbing a wall, or reclining on a sunny pergola. A white wall would flatter one of the stronger blues for instance, rather than a pallid colour. However on warm, sunset-toned brick you may wish to go for a paler blue or lilac.

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Buying your wisteria

Always buy your wisterias in flower because flower colour and length of raceme can vary. Look for good flowers and the blues are superior to the whites in most situations because blue flowers fade far better than whites which tend to brown rather quickly. 

Named wisterias are grafted on to rootstock, so look at the bumpy bit above the stem and check that it’s sound. If it’s split walk away and if it isn’t grafted don’t buy it. Wisterias can be expensive, but a good, well-chosen plant will outlive you.

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Where to plant

If you’re planting against the wall opt for a western or southern aspect because wisterias will not flower on shady, cool north walls. Avoid eastern aspects too, because these are exposed to morning sunshine and, should there be a frost as the buds break, a quick thaw can and does kill them. 

Late frosts should be avoided so careful positioning is important for wisterias need warmth and good light to do well.

How to plant

Prepare the planting hole well, at least three feet from the wall ideally, add some loam-based compost such as John Innes no 2 or 3 and remove the plant from the pot. If the roots are going round in a circle tease them downwards with your fingers. 

If they resist, make some cuts with the secateurs. Make sure the graft union (the bumpy bit ) is above the ground. Dig a hole as deep as the root ball and two to three times as wide and top up with compost. Place your root ball in the hole - the top of the pot should be level with the top of the soil when you’ve finished.

When planting your wisteria it will need watering for it’s first season and dribbling a hose over it will do no good at all. It’s far better to get a bucket of tepid water and gently tip it round the plant every week in its first season. 

This is vital for wall-planted shrubs, because they will get little rainfall on their roots. Once the first growing season is over, you don’t have to be so diligent, although regular watering in warm weather produces good growth.

Training wisteria

If you're growing wisteria on a wall, strong wires are much more suitable than wooden trellis work which is likely to rot. It's worth investing in a trellis with self-tensioning stainless steel wires and these can be expensive. S3i sell functional, stylish kits. You can also use strong wire and vine ties, although the wire is difficult to tighten once it sags.

Gripple is another extremely good product for supporting climbing plants, because the thick wires can easily be tensioned.

A young wisteria will need to put on height when it’s young so allow it two or three years to get its feet down and grow. Once the required height has been reached, and not before, pinch out the top and begin to train the side branches so that they are horizontal enough to almost defy gravity.

This will slow the flow of sap down and encourage more flower buds. If you allow your wisteria to go straight up forever you’ll get a few flowers at the very top. You can also coil new leaders round a support - again to slow up the flow of sap - before training them horizontally. This is a good technique on a strong pergola.

Pruning wisteria

Wisterias are generally pruned twice, once in July and once in February or March depending on the weather. You need to prune before the buds break so those with warm gardens in the south of England may prune in January in good years. Never prune in severe weather, always choose a clement, still day. 

Wisteria sap stains horribly, leaving a rusty mark, so wear your oldest garden clothing and do watch your step on ladders. If you aren’t steady on your feet get someone else to prune your wisteria for you.

Pruning wisteria will reduce the weight of the wires and encourage more flower.

Summer pruning of wisteria

July is the time to cut back the long whippy new growth that’s been produced this season. Take it back to 12 inches ( 30 cm), leaving four to six leaves intact. If there’s another growth spurt before autumn you may need to do it again, depending on the season. Summer pruning allows the sunlight to ripen the wood and concentrates the plant’s energy into producing more flower buds for next year. So it makes more flower rather than lots of leaf.

Winter pruning of wisteria

Take all the side branches you trimmed in July back to two buds in late February or March - whilst the plant is still dormant - to reveal a bare, neat skeleton. You can also remove damaged or unwanted branches then. This regime will produce a masses of hanging flowers.

If tackling an overgrown wisteria, do it in February when it’s dormant.

Feeding wisteria

Feed wistera in the spring, with Growmore or Fish, Blood and Bone at the rate recommended on the packet. In sandy soils (which may have low potassium levels), also apply sulphate of potash at 20g per sq m (1/2oz per sq yd). You could also use rose or flowering shrub fertilisers. Don’t overfeed though, leguminous plants resent it.

Wisteria varieties

Wisteria brachybotrys 'Okayama'
Long, pendulous racemes of beautiful, dark violet purple flowers in early summer once established. Wisteria brachybotrys 'Okayama' is reasonably vigorous and an ideal for climbing up wires or trellis against a sunny wall or fence.

Wisteria floribunda 'Honbeni (Pink Ice)'
Long panicles of lightly scented pink flowers in late spring or early summer up to approx 60 cm long once established. A popular plant for training along wires or up trellis against a sunny house wall or fence and Wisteria floribunda 'Honbeni' is also extremely attractive when grown through established trees or shrubs.

Wisteria frutescens 'Amethyst Falls' (PBR)
Wisteria frutescens 'Amethys Falls' is an American species and is more compact, climbing to 5m (16ft) and the densely packed rich-violet flowers are held in shorter racemes.

Wisteria sinensis ‘Amethyst'
Red-flushed, violet-blue flowers in late spring to early summer. The foliage emerges soon after the flowers, and often has an initial bronze flush, which fades as the leaves mature.

Did you know?

Wisterias are named after Caspar Wistar (1761 - 1818), a professor of anatomy at the University of Pennsylvania. It’s an early botanical typo and they should really be Wistarias although we stick to Wisteria.

Wisterias are legumes, as you can see from the pea-like flowers, and there are ten species found China, Japan and the Eastern USA. In the wild they are often found close to streams, near wet woodlands and close to damp cliffs.

The differences between Chinese and Japanese wisterias

Wisterias are twiners, but they don’t all twine the same way.

Chinese wisterias (labelled Wisteria sinensis) twine anticlockwise and an easy way to remember this is to make a letter C ( for Chinese ) with your finger - beginning at the top - and you’ll make an anti clockwise movement.

Now make a J starting from the top for the Japanese wisteria (Wisteria floribunda) and your finger will move in an clockwise direction - the same way the Japanese wisterias twine.

Another way of telling is that Chinese wisterias produce flowers on bare wood, whilst Japanese wisterias have leaves and flowers at the same time.

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