How to prune Wisteria

Wisteria is not exactly a low-maintenance climber, but training it carefully is worth the effort.

IIn the past few years, I have not come across many well-trained or even well-maintained wisterias, here on the French Riviera. Many house owners inherit these versatile climbers from the previous owners, who all too often have already let them become overgrown, twiggy, criss-crossing and generelly out of form. Usually Wisteria becomes less and less flowering the more it overgrows, until it stops blooming completely.

This it the time when many home owners start to think the plant may have become too old, or something is wrong with it. In fact Wisteria can grow quite old. Maybe you have seen photos on Facebook that reappear every year depicting well-trained wisterias from Japan that spread over pergolas for a couple hundred square meters or more. These plants are well over a hundred years old and have been blooming reliably every season under the hands of their careful and knowledgable Japanese gardeners.

Wisteria originates mainly from Asia and northeastern North America. The most commonly sold and planted species is Wisteria chinensis, the violet-blue flowering species from China. Wisteria brachybotrys, from Japan, is the white and intensively fragrant type which is more common in the U.S., less common in Europe. Wisteria floribunda, also from Japan, is the violet or violet-blue species of which there is the highest number of varieties in all kinds of color variations. All of them can be grown as vines, shrubs or trees.

One rarely ever sees them trained into small standards: single-stemmed, trimmed and mostly pruned to a roundish form, of a minor height, maybe up to 2 to 2,50m maximum. As they can be easily kept to this height and a certain width, they are the perfect choice to be aligned along a driveway, to separate a terrace from a footpath or even to visually break up a hedge or property line. Standard trained wisterias are perfect to add structure to a garden or to establish focal points, to highlight a certain area in the distance or to start a formal garden without becoming too formal. Wisteria is very hardy but also takes a lot of heat which makes it suitable for a large span of climates and garden styles. All parts of the plant are poisonous, though there is a debate about how severe possible health issues are; I personally think that parents of small children and dog owners should take care that seeds are taken off from the plant, before they can pose a possible threat.

As unpleasant as it may be, wisteria is not low-maintenance. It needs to be pruned on a regular basis twice a year: once in winter or early spring in order to train it and help it bloom, and once in summer to check lengthy shoots that may weaken the plant. As many other climbers it shares their habit of normally blooming on new wood, growing from 1-3 year old growth. This may sound a little complicated at first, but it really isn’t. Today I’ll try to explain how to prune back wisteria and train it over time so you enjoy its flowers as well as it’s many other pleasant features that make it actually a very useful plant.

The good news is that wisteria, privided it gets pruned twice per year, is actually increasingly easy and quick to prune as long as you carefully keep and maintain an open framework when doing the regular maintenance pruning. So only renovating a wisteria may be a little more time consuming.

This Wisteria had been overgrown over several years. Situation is around mid March.

If you own a specimen that is already overgrown, a thorough renovation will be necessary. Especially given rather tight spaces, where the plant has only a minimal amount of open ground space around its feet, you should sort out only a few main stems, ideally even only one single stem, that will support the future framework. If you have to renovate the plant, don’t be afraid; the harder you cut it back the quicker, healthier and stronger it will come back. Wisterias may display tender blossoms, but make no mistake, they are very strong growers, which makes them the perfect choice for all situations where you need a rather quick coverage for shade.

Stems were tied around supports, no real framework established.

I see multiple stems and long background growth all the time, because Wisteria doesn’t get any training early on, which is very important. Since Wisteria is often planted as an umbrella against the sun, we usually find it around terraces, planted against walls or columns, from where it climbs up toward projecting or main roofs. Singling out stems and taking out background growth is important, because the plant will wind itself around its own stems which can lead to major problems as the plant matures and stems start growing literally into each other. Wisteria needs to be kept tidy. Take out all background shoots if your wisteria is planted against a wall or a column! Wisteria should always be kept away from the background at least 15-20cm, if possible more, because it is very strong and will easily crack various building materials. Establish a very (!) strong support for your wisteria made from strong wood or even strong metal. Don’t let it grow underneath any part of any roofing. If you see it doing that in summer, don’t wait until fall to cut off these shoots as they become woody quite quickly.

One of Wisteria’s advantages is that it’s an ideal screen against the summer sun, while it lets through all the light in winter, as it is decidious. But that’s also kind of the reason why it does not get the proper attention in winter. People often worry that if they take out to much of the growth they will not have enough shade in summer. That’s completely wrong; the opposite is true, really. If you imagine a wisteria that is trained along a pergola with about 50 cm between the main established stems that run along the pergola’s supports, you can easily leave squares 50 by 50 centimeters, that are cut free in fall to let some sunlight through. Come April/May these fourth of a square meter wide blanks will be filled completely with new growth, while at the same time letting the air through.

Situation after a complete clean-up, around mid April, same year.

Between January and late March, take on a renovation of your overgrown Wisteria.

Start at the bottom and follow along the thinnest and longest shoots and single them out. Everything that’s thinner than a good centimeter but runs for several meters all the way from the base can be cut right back to the ground and can then be pulled out. You will also notice that these long shoots, last year’s streamers, often have very little numbers of buds. Tidy up the complete plant by taking out all dead wood, all criss-crossing and longish background growth and growth without any buds, as well as any congested growth. You don’t need many long shoots, choose the thickest and healthiest and get rid of most of the rest. Only where you find larger gaps in your framework, either in height or in length, you can leave some of the younger and thinner shoots. But take them back this time about 50 to 70 percent of their total length and keep them in mind for next winter, so you don’t forget that you wanted to keep them. You will then cut them back by 20 to 30%.

Dead wood in a wisteria will be darker and brownish while wood that will produce new growth will be light and greyish. Often you see wrinkles in the bark signaling it’s dead. You can always test if there’s still life in a shoot by cutting bits of at the end: if you see a light green area right underneath the silvery-greyish bark, the shoot is alive. Dead wood will be a lot harder when you cut it, even when of a small diameter; it will have a dark yellowish-sandy to brownish color. Dead buds that will not produce any growth will be a lot smaller, darker and closer to the stem so to speak than the buds that will produce new growth. If you’re unsure just cut slightly before a bud and you’ll know.

Without cutting back last year’s growth you will not receive a sufficient amount of flowers. Flowers will emerge from new growth growing from clusters of already existing buds on 1-3 year old growth. You can identify this by its diameter which will be around between 5 and 10 mm. If you cut back shoots from the stems that will remain as the frame work, no matter how long the shoots may be, take them back all the way to 3 buds only, not more, or 15-20 centimeters in length maximum, unless you want to establish a new framework stem. As with roses or vines, cut close to a bud, about 5mm is perfect. Cut off any dead ends if they are longer than this and always cut into healthy wood. If dead wood starts right behind a bud, take the twig back to the next bud, even if you will have left only two or one bud on that individual stem. It is more important to have healthy buds on a healthy shoot than the right amount of buds.

Where possible a framework was established, situation around mid to end of April, same year.

In rainy regions I would also recommend cutting on angles away from the buds for a better water run off. On the French Riviera this is of no importance as it will stop raining soon after the new growth season begins anyway. If you have a rather large framework with gaps beyond 50 x 50 cm, you can leave a variety of lengths, depending on the number of buds, for interest. Rub off with your fingers any buds on the main or leading upward growing stems (trunks), and get rid of any suckers and thin shoots coming from the base or growing on the leading stems.

With old and established wisterias on pergolas you often get a lot of vertical growth out of the framework that will grow toward your windows and shutters, leaving an untidy display in summer when you look down on the pergola from above. When you train a new wisteria on your pergola or arbor, choose buds that grow horizontally to the framework’s level and rub off any buds that are on top or underneath the framework. If you renovate an established specimen it will be harder to correct this vertical growth, but you can try to achieve this over more than one season by taking back vertical growth to one or two buds above the framework and by trying to establish new framing stems from this season’s streamers. You can also try to take vertical growth back all the way to the horizontal leaders of your framework, by cutting below the last bud of a vertical shoot, but new buds on wood that’s older than three years may take a while, maybe more than one season. If in doubt, choose the method mentioned above.

In summer, all you need to do really is to cut back all the long streamers that may get tangled up in the framework (or in windows and shutters). Save those though that you want to use to extend your framework (see above) and tie them to your supports. Don’t tie them on to close and don’t take a shortcut by winding them around the supports, you’ll regret this in a couple of years!

When planting a new wisteria, don’t worry too much about the type of soil, just make sure it has good drainage (add enough compost) and the future leading stems have plenty of room to become large enough for supporting the future framework. Always plant on the side away from your house, training the climber toward your house. Wisteria does grow in spots that are not 100% sunny, but it does best on south facing sides.

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