Taming Wisteria

At a glance

Wisteria only flowers lavishly with two cuts a year: one in high summer, one in late winter.
The summer cut falls in the shoot-growing season around CW 30 to 32: shorten the long whip shoots to about five to six leaves.
The winter cut between CW 6 and 8 takes those same shoots back to two or three buds and so frees up the flowering wood.
A young wisteria may happily grow for three to five years before it flowers reliably. Patience is part of the deal.
The support has to last a lifetime: old plants develop arm-thick, twisting shoots with enormous force.

A wisteria in full bloom is one of those moments you garden for. A whole wall hangs heavy with blue-violet racemes, it smells wonderful, and the bees are hard to miss. Without pruning you almost never get this sight. Instead the plant climbs metres into the neighbour's plot, wraps itself around the gutter and makes plenty of leaf but little flower. The difference between a tangle and a cloud of bloom comes down to two dates a year, and both are quickly done once you know what they aim at.

Why an unpruned wisteria mostly runs wild

The wisteria (Wisteria sinensis) is by nature a woodland-edge climber. It wants to go up, towards the light, and to begin with it pours everything into long shoots. These so-called whip shoots can add three to four metres in a single season. As long as the plant is chasing that growth, it barely sets any flower buds. The flowers sit on short, thickened side shoots, the flowering wood, and that only forms when you stop the long shoots.

Pruning a wisteria is therefore not taming in the sense of punishment but redirecting. Every whip shoot you shorten forces the plant to form short shoots with flower buds at its base. That is the whole trick, and it is behind both pruning dates.

What defines wisteria

Enormous growthUp to four metres of growth a year. An established wisteria can reach ten metres and more and lives for decades.
Flowers on short woodFlowers form on thickened short shoots, not on the long trailing shoots. That is why you prune.
Tough, but bud-sensitiveThe plant itself is nicely hardy. Late frosts, however, can catch open buds.
Poisonous in every partSeeds and pods are toxic. In gardens with small children, remove the seed pods.

Nahaufnahme einer hängenden Blütentraube des Blauregens mit vielen einzelnen Schmetterlingsblüten — Close-up of a flower raceme. · Foto: Soramimi, Wikimedia Commons

The flower sits on the short wood

Look closely at a raceme: it hangs from a short, knobbly shoot, not from a long trailing one. These short shoots are the target of all your pruning work. A well-trained wisteria looks almost bare in winter, with lots of these short spurs on a clear framework. That is exactly where the buds open in May.

The summer cut · CW 30 to 32

The summer cut is the more important of the two and falls in high summer, usually late July to early August. By then the plant has made its long shoots for the season, and you can see exactly what has to go. Take a good, sharp pair of secateurs and a little time on a dry day.

Spot the whip shoots
Find this year's long, whippy shoots. They are lighter, soft and often trail freely into the air or wind around anything they can reach.
Shorten to five or six leaves
Cut each of these shoots back to about five or six leaves, roughly 20 to 30 cm from the base. Cut cleanly just above a leaf.
Spare the framework shoots
Shoots you need to extend the framework, for example along a still empty pergola, you leave standing and tie in to the support.
Free the tangles
Whatever has wound around a gutter, cable or the neighbour's fence you now unravel completely. Old shoots turn woody and later can barely be removed.

Remember one simple rule of thumb: in summer to five or six leaves, in winter to two or three buds. If you remember only that single sentence, you get almost everything right with wisteria.

The winter cut · CW 6 to 8

In late winter, once no hard, lasting frost is forecast, comes the second, finer cut. Now the plant is bare and you can see the framework and the buds crystal-clear. The fat, round buds are the flower buds, the slim, pointed ones the leaf buds.

Take the shoots you shortened in summer back once more, this time to two or three buds. That way the plant puts all its strength into a few strong flower spurs instead of many weak ones. Also remove crossing or inward-growing shoots so the framework stays open. This second cut decides more about the wealth of flower than about the shape.

In summer you take the wisteria's strength away from the trailing shoots, in winter you give it back to the buds.

Old gardener's rule

Support and site: a framework for half a lifetime

A wisteria grows old, often older than the house it climbs on. Its shoots turn woody into arm-thick, twisting cords that work with astonishing force. It simply tears off a thin wire or plastic support after a few years. So plan solidly from the start: galvanised steel cables with tensioners, a sturdy timber or metal frame, at least five to ten centimetres of gap from the wall for airflow.

Blauregen über eine kräftige Holzpergola gezogen, Blütentrauben hängen durch das Gebälk — Wisteria on a sturdy pergola. · Foto: Soramimi, Wikimedia Commons

On a pergola it comes into its own

A pergola is the dream spot for a wisteria: the racemes hang freely through the beams and you walk beneath them. Just make sure the posts really carry the weight. An old wisteria puts many kilos on the scale when it rains, and its grip only grows firmer over the years. You will find more on the right construction in the article on climbing supports.

As for the site, wisteria is grateful: it wants sun, ideally a warm south wall, and a free-draining, not too rich soil. Too much nitrogen, say from the lawn feed next door, drives it into leaf instead of flower. Keep it rather lean, then it flowers more willingly.

Late frosts around mid-May (CW 20) can brown buds that have already opened. That is no drama for the plant, but it does cost flower. In very exposed spots a sheltered wall or a late-flowering variety, whose buds are then usually safe, helps.

Wisteria

Wisteria sinensis

Familie: Legumes (Fabaceae)
Standort: Sunny, warm wall, free-draining soil
Saison: Flowers May to June, summer cut CW 30 to 32
Wasserbedarf: Moderate, water in dry spells
Frosthärte: Hardy to about -20 °C

When the flowers fail to come

A wisteria that does not flower is usually either too young or too well fed. Young seedlings often need five to ten years to their first flower, while grafted plants from the nursery flower early and reliably. When buying, look for a graft and a named variety rather than a nameless seedling plant.

The second most common reason is missing pruning. Whoever just lets it grow gets leaf. The third is too much fertiliser. Stick to the two pruning dates, keep the soil lean, and give the plant time. A wisteria rewards patience like hardly any other climber.

Common questions about pruning wisteria

When do I prune wisteria in summer?

The summer cut falls in high summer, usually from late July to early August, so roughly CW 30 to 32. Then you shorten this year's long, soft shoots to five or six leaves. That steers the plant's strength out of shoot growth and into the flower buds.

Do I really have to prune wisteria twice a year?

For a lavish bloom, yes. The summer cut stops the rampant shoot growth, the winter cut between CW 6 and 8 frees the short shoots to two or three buds. If you prune only once you usually still get flower, but noticeably less and with more tangle.

Why does my wisteria not flower?

The three most common reasons are: the plant is still too young (nameless seedlings can take up to ten years), it is not pruned and only makes leaf, or it gets too much nitrogen. Buy a grafted variety, prune twice a year and keep the soil lean.

How hard can I cut back an old, overgrown wisteria?

An old wisteria takes a hard cut back well. You can take it right into the old wood in late winter and rebuild the framework. Expect less flower in the first year afterwards, because the new flowering wood has to form again. From the second year on it thanks you for it.

Is wisteria poisonous?

Yes, every part of the plant is toxic, especially the seeds and the pea-like pods. In gardens with small children it is best to remove the seed pods after flowering. For bees and bumblebees, on the other hand, the flowers are valuable.

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