Issue: November 12, 2005
Wisteria doesn't bloomQuestion:
I have a mature wisteria tree that has never bloomed. I trim it significantly every spring and as necessary during the summer and fall. We also feed it periodically. What could be the problem?
Questions about a wisteria plantís failure to bloom are common in New Mexico and throughout the U. S. In fact, it is the first published garden question I can remember from my childhood. I remember reading this question in a common home and garden magazine when I was about 10 years old. I even remember the answer from that magazine, and find the same answer being given today.
The answer then was to "root prune" the wisteria to "shock" it into growth. The scientific rationale isn't really to shock the plant in the manner that we as humans think of shock. By pruning the roots, we will alter the ratio of nitrogen to carbohydrates in the plant. If you wish to attempt this technique, you will dig a trench or at least push a shovel into the ground around the plant to cut the roots. Do not cut the roots too close to the trunk. Instead, trench or cut roots 2-3 feet from the trunk of your plant.
In the hard, dry soil of New Mexico, you may find it easier to try a different technique to accomplish the same purpose. By applying phosphate fertilizer and eliminating nitrogen fertilizers, you can enhance carbohydrate levels and reduce nitrogen. Phosphorous in plants participates in carbohydrate metabolism and stimulates flowering. Nitrogen collected by the roots stimulates growth at the expense of flowers. Cutting roots reduces the nitrogen absorbed by the roots, but application of phosphate and eliminating nitrogen does the same without as much effort. This method of nutrient management is also much less stressful to the wisteria plant in our harsh New Mexico environment.
Properly pruning the vines can also be an important factor. (Wisteria is really a vine that is often trained to be a "tree".) Like another vigorous vine, the grape, the wisteria will produce excess growth with limited flower (and fruit) production unless the vines are pruned. Many gardeners prune grapes and wisteria in mid-summer (and perhaps again in the fall as you described) to remove much of the excess vine production. Then, in the early spring both of these vines may be cut back so that only 3-5 buds remain from the previous year's growth.
The flower buds of wisteria are produced during the summer, so pruning the previous year's growth too severely will remove the flower buds, but that is rarely the case. Most people do not prune severely enough.
Another consideration is that wisteria grown from seed will not flower for about 7-10 years. Since most plants purchased in a nursery and trained to tree form are already old enough, this may not be a consideration in your case.
Adequate sunlight is also important. In New Mexico, we often have too much sunlight. However, if the wisteria is growing in the shade of a building or under a tree where it receives shade most of the day, insufficient sunlight may be the problem.
As you look at your wisteria, consider the factors described above. If age and sunlight are unlikely culprits and if your pruning is appropriate, try applying phosphate fertilizer (with no nitrogen). Do this by making a ring of shallow holes in the ground a foot beyond the ends of the wisteria branches, filling the holes with the fertilizer, and then watering well to dissolve the phosphate.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: email@example.com, office: 505-865-7340, ext. 113.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!