Marin Master Gardeners
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Coaxing My Wisteria to Flower

October 6, 2008
Marilyn Geary

When I first caught a glimpse of the little bungalow I now call home, I was captivated by its arbor of wisteria and grape vines. What could be more delightful than sitting in the shade in the springtime under fragrant, cascading wisteria blossoms? I imagined an abundance of pendulous flowers and dangling grape clusters. Since then I have harvested enough grapes to make cases and cases of Concord jelly each October, but I have seen just a few straggly wisteria blossoms in spring.

I’ve discovered that many gardeners have been similarly frustrated. Inducing wisteria to flower is a common problem. It’s been said that growing wisteria is like getting a pet dog. It will be with you for years, and you’ll spend a long time training it. My wisteria seems more like a wild critter than an untamed puppy. I set out to determine more about the blossomless beast taking over my arbor.
Wisteria is a member of the five-petaled pea family, Fabaceae (formerly Leguminoseae). The genus, named for the anatomy professor Caspar Wistar (1761-1818), includes ten species of deciduous climbing vines, two native to the southern United States and the others native to eastern Asia. Chinese wisteria (Wisteria sinensis) and Japanese wisteria (Wisteria floribunda)are the species most commonly found in nurseries and gardens.
Chinese wisteria is grown for its showy racemes, clusters of individual flowers on a single axis. These plump clusters range from six inches to a foot long. Its blossoms open all at the same time, usually before the leaves appear on the plant. The compound leaves usually have 7-13 leaflets.
Japanese wisteria has longer blossoms, from 12 to 18 inches in length. It blooms later in the spring with the expanding foliage, and its flowers open gradually, from the cluster base to the tip. The compound leaves usually have 13-29 leaflets. American wisteria (Wisteria frutescens)is less fragrant than Chinese wisteria, and its racemes are much shorter, about six inches long. Each species is available in cultivars with colors ranging from white to pink, lilac, and purple.
Since I could not recall the blossoms on my wisteria, I looked for other identifying features. Tracing the twirling vines until my head spun like a top, I determined that my wisteria vine twined counterclockwise as seen from above. This characteristic identifies it as a Chinese wisteria; the Japanese wisteria twines clockwise around its support.
Wisteria will grow in relatively poor-quality soils, but it does better in a moist, well-drained soil. It blossoms best in full sun. My Chinese wisteria enjoys sun all day long, so its lack of blossoms must be due to another factor.
Wisteria’s fruit is a long, flat pod that turns brown as it dries on the vine. Hot days cause the mature pods to split, releasing their poisonous seeds in an explosion that flicks them far from the pod shell. Wisteria grown from seed may take decades to bloom, so it’s best to purchase a plant propagated through root cuttings or grafts. Chinese wisteria may take up to twenty years to mature. My wisteria, planted by a prior owner, is well established with a strong, thick trunk. It was planted in the Seventies, so immaturity can’t be the cause of my wisteria’s failure to flower.
Too much nitrogen fertilizer also results in a lack of blossoms. Like other legumes, wisteria hosts Rhizobia soil bacteria in its root nodules. The Rhizobia name comes from the Greek rhiza for roots and bios for life. When established in the root nodules of legumes, Rhizobia bacteria fix nitrogen, converting atmospheric nitrogen into compounds enriching the soil. Too much nitrogen encourages leaf growth at the expense of blossoms, so mature wisteria should not be fertilized with nitrogen. Wisteria rarely requires any fertilizer, and I have never fertilized my wisteria. Too much nitrogen fertilizer is not its problem.
Wisteria, especially Wisteria sinensis and Wisteria floribunda, are hardy and aggressive, growing so fast that they are considered invasive species in the southern United States.  Both the Chinese and Japanese species can grow up to 25 feet tall and work best twining up and over a sturdy, durable arbor or pergola away from your house. They can damage weaker structures and tear shingles off your roof. They have long lives, some vines surviving 50 years or more.
The prime example, a Chinese wisteria in Sierra Madre, California, is over 100 years old, weighs more than 250 tons, and covers more than one acre. The Guinness Book of World Records names it the world’s largest blossoming plant, one of the seven horticultural wonders of the world. This wisteria produces more than 1.5 million blossoms every year with 40 blooms per square foot and is at the center of Sierra Madre’s annual Wisteria Festival.
Clearly, pruning is essential to keep wisteria under control. Proper pruning also encourages wisteria to bloom. Wisteria plants produce numerous stolons, above-ground stems that develop roots and shoots at short intervals. If not restrained, wisteria will produce excess vegetation and limited fruits and flowers.
I have snipped off shoots of my wisteria frequently, but not thoughtfully, to promote blossoms. Wisteria should be pruned at least twice a year. In summer, after the blooming cycle is complete, the long wiry tendrils should be pruned back to within four to six leaves from where they join the main stem. This pruning reduces the amount of growth and allows more sunlight in to foster flower budding.
In the late winter dormant period, when all the stems are bare and you can see what you are snipping, these side shoots should be cut off even further, to within 1-2 inches of the older wood, leaving only 2-3 buds. It’s important to avoid cutting off the woodier spurs, modified branches bearing the flower buds. The flower buds are larger and plumper than the leaf buds. They are usually fuzzy, particularly in early spring when they begin to grow. Proper pruning focuses the plant’s energy on the flower buds rather than stem growth at the expense of flowers.
If careful, heavy pruning still does not produce blossoms, root pruning is recommended. This technique involves cutting the roots 2-3 feet from the trunk of the wisteria and applying phosphate fertilizer. Pruning the roots alters the ratio of nitrogen to carbohydrates in the plant by reducing the nitrogen absorbed by the roots. The phosphate fertilizer stimulates flowering.
Now that I’ve discovered how to encourage my wisteria to bloom, the thought of springtime blossoms gives me an Edward Scissorhands-like urge to prune it for optimum flowering. I hope we’ll be together for a long time, so the sooner I start coaxing my vigorous creature to blossom, the happier we’ll both be.

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