How to Prune Wisteria: Five Leaves in Summer, Two Buds in Winter

Why it has to be twice, not once

Wisteria flowers from short, congested spurs on old wood, not from the long whippy growth it makes in summer. Those whips can run twelve feet (3.6 m) in a season and they carry leaves, not flowers. Left in place they shade the older wood, drain the plant's resources into extension growth, and turn the whole thing into a tangle that flowers only at the very top where light still reaches. A wisteria that has never been pruned is the classic case of an enormous plant with a dozen trusses somewhere near the roof.

The two cuts do different jobs. The summer cut, made while the plant is in full growth, removes the leafy extension growth and forces the plant to redirect what it has already made into the buds at the base of each shortened shoot. Those buds swell over the second half of summer and autumn and become flower buds. The winter cut, made when the leaves are off and you can see clearly, shortens the same shoots again to expose the swollen buds, remove any that are only vegetative, and tidy the spur system so light reaches it in spring.

Do only the winter cut and you get a modest improvement, because you never harnessed the summer growth. Do only the summer cut and the plant stays too congested to flower well. Doing both, every year, is the single practice that separates a wisteria smothered in trusses from one that mostly makes leaves.

The summer cut: back to about five leaves

Prune in mid to late summer, roughly two months after flowering has finished, when the long shoots of the current season have hardened slightly at the base and are no longer soft and sappy. In practice you know it is time when the plant has thrown out a mass of long green whips that are waving free of the support and reaching for gutters, other shrubs and each other.

Take every one of those long side shoots back to five or six leaves from its base. Count leaves, not inches; the shortened stub will be somewhere between six and twelve inches (15 to 30 cm) long depending on how vigorously the plant is growing. Do this to shoots coming off the permanent arms and off the trunk. Do not shorten the arms themselves, and do not cut back a shoot you have deliberately chosen to extend the framework or to fill a gap on the support.

Two exceptions are worth knowing. Shoots you want as new permanent arms are left full length and tied in. Shoots emerging from below the graft union or from the ground are removed entirely, flush at their origin, not shortened. Everything else gets the five-leaf treatment. On a large old plant this is an hour or two of work and produces a startling pile of prunings; that is normal and the plant does not suffer for it.

1. Wait until the whips have firmed Mid to late summer, about two months after the flowers finish. The base of each long shoot should have started to go woody and pale rather than soft and green.
2. Identify the permanent framework Trunk and main arms stay untouched. Everything growing sideways off them is this year's extension growth and is what you are cutting.
3. Count five leaves and cut On each long shoot, count five or six leaves from where it joins the older wood and cut just beyond the fifth or sixth. Use pruners; the wood is soft enough that loppers are rarely needed.
4. Leave any shoot you want for the framework If you need a new arm or want to fill a bare stretch of wire, tie that shoot in at full length instead and prune it in later years like the rest.
5. Remove rootstock and ground shoots outright Anything coming from below the graft union or straight out of the soil is cut flush at its base, not shortened. These never flower and they will take the plant over.
6. Retie and check the old ties With the mass of growth gone you can see the ties. Loosen any that are cutting in and replace anything that has perished. Do this now, once a year, and you avoid ring-barked arms.

The winter cut: back to two or three buds

Prune again in the dormant season, once the leaves are down and before the buds begin to move in spring. The plant is bare, the structure is fully visible, and the buds along each summer-shortened stub can be read at a glance.

Go back to every stub you shortened in summer and cut it again, this time to two or three buds from its base, leaving a stub of roughly two to four inches (5 to 10 cm). You are now looking at the buds themselves and choosing where to cut. Flower buds are fat, rounded and often slightly downy. Vegetative buds are small, flat and pointed, pressed close to the stem. Cut so that the buds you keep include the plump ones, and cut just above a bud rather than leaving a long dead snag beyond it.

Over the years each of these positions builds into a knobbly cluster called a spur, and the spurs are where the trusses hang from. Old spur systems eventually become so congested that the buds crowd each other and flower quality drops. When a spur cluster is the size of a fist and producing small trusses, cut a proportion of them out completely each winter and let new spurs form from the resulting growth. Do not do this to more than about a fifth of the spurs in one year.

This is also the winter job for structure. Remove dead, crossing and rubbing wood. Cut out any arm that has grown into the wrong place, and take out one or two of the oldest arms on a very congested mature plant, replacing them with a young shoot tied into the gap. Structural cuts belong in winter, when you can see, and never in summer.

Renovating a wisteria that has never been pruned

A neglected plant is a mound of interwoven stems with a smothered framework somewhere inside it. It can be brought back, but not in one operation. Cutting the whole thing hard in a single winter provokes a forest of vigorous water shoots and costs you several years of flower.

Work in the dormant season over two or three winters. In the first winter, cut out everything obviously dead, then untangle and remove whole stems that cross the plant or run in the wrong direction, aiming to reduce the mass by roughly a third and to expose the main arms. Choose your permanent framework from what remains and mark it in your head, or physically with a tie of a different color.

Through the following summer, treat every long shoot to the five-leaf cut, including the mass of new growth the renovation provokes. In the next winter, take out another third of the unwanted old wood and start the two-bud cut on the retained stubs. By the third year the plant is on the ordinary twice-yearly cycle and flowering should be recovering. Very old main stems can be cut back hard to a stump of live wood if they must be replaced, but expect two to four years before the replacement shoot flowers.

Two cautions for renovation work. Wisteria stems that have wrapped around a support are often holding it together as much as strangling it, so check what will fall before you cut. And all parts of the plant, seeds and pods especially, are poisonous if eaten, so clear the prunings and any pods away rather than leaving them where children or animals reach them.

Never prune a wisteria hard in spring just as growth starts. You remove the flower buds you spent the previous summer creating and provoke a rush of soft shoots. If a plant needs reducing and the buds are already swelling, wait until after flowering and do it as a heavy version of the summer cut.

Reading what the cuts did, and keeping a note

The feedback from pruning arrives ten months after the cut, which is long enough that most people cannot connect a poor year to what they did or failed to do. A short written record closes that gap faster than anything else. Note the week you made the summer cut, roughly how much you removed, and the week you made the winter cut. Then note the following spring how heavily the plant flowered and whether the trusses were spread down the arms or concentrated at the top.

Two patterns emerge quickly from three or four years of notes. Trusses only at the top of the plant means the summer cut is being made too late or too lightly, so extension growth is still shading the lower wood. Plenty of leaf and few trusses everywhere means either the winter cut is not happening, the plant is being fed nitrogen from a nearby lawn feed or border feed, or it is still too young.

Record where on the framework you removed spur clusters for renewal too. A fist-sized spur is unmistakable in winter but easy to lose track of once the plant is in leaf, and the renewal should be spread around the plant rather than repeatedly taken from the same area.