Q. I have had a wisteria in my yard for 10 years that has never bloomed. It is in full sun. The foliage is lush, it appears healthy. Although I no longer remember what kind it is, I recall that it was suitable for planting in USDA Zone 4, where I live. In the past, I have tried pruning the roots, as some books suggest. - R.Q., Wausau, Wis.
A. The usual remedy for a non-blooming wisteria, as you mentioned, is to root prune, according to Judy Lowe, an experienced gardener in Chattanooga, Tenn.
For gardeners who wish to try this, wait until after flowering time, just to be sure, then cut vertically with a spade into the ground in a circle out from the plant but in the root zone.
In your situation, there may be other reasons that a mature plant doesn't bloom.
1. You may have inadvertently pruned off the flower buds, which are usually formed in summer.
2. Overfertilization, especially too much nitrogen (the first of the three numbers on the label of the fertilizer container). This usually causes lush leaf growth at the expense of flowers. In reasonably good soil, wisteria needs little or no fertilization.
3. Flower buds are killed by low temperatures or late frosts.
No. 3 is probably the problem here. While Japanese, Chinese, and American wisteria are vegetatively hardy in Zone 4 (meaning that the leaves and stems are able to withstand temperatures as low as minus 20 or minus 30 degrees F.), the flower buds aren't.
A horticulture expert at the University of Minnesota says it is rare for any wisteria to bloom in the Minneapolis area (which is also in Zone 4). The exception is Kentucky wisteria (Wisteria Macrostachya), a native species that is hard to find in nurseries.
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