Q. I planted two wisteria vines three years ago and have yet to see a flower. A friend suggested that I needed male and female plants, but I do not know how to tell one from the other. I did purchase and plant a third one, but they still do not bloom. Do you have any suggestions to help me enjoy those luscious blooms? I think wisteria is one of the loveliest plants.
A. Wisteria (Wisteria spp.) can be a frustrating plant for gardeners longing for those spectacular flowers. If only the solution was as easy as finding a mate for your flowerless vines!
Plants that bear male and female flowers on separate plants are called dioecious. Many familiar ornamental and shade trees fall into this category, including holly (Ilex spp.), black gum (Nyssa spp.), ginkgo (Ginkgo biloba), Kentucky coffee tree, (Gymnocladus dioicus) and bayberry (Morella pensylvanica).
In this case, male and female plants are necessary for the female plants to bear fruit. The easiest way to tell the difference between male and female plants is to examine them when they are in flower. The flowers on male plants bear only male parts that produce pollen, the anthers and filaments. The flowers on female plants bear only female parts that produce fruits and seeds, the stigma and style. It is not necessary to have both sexes present in order for dioecious plants to bloom.
Just to make things really interesting, some dioecious plants change sex at some point in their growth. This phenomena can be frustrating for designers and architects who purposely specify male ginkgo trees to avid the foul-smelling fruit produced by female trees.
Plants that bear male and female flowers on the same plant are called monecious. The majority of plants, including wisteria, fall into this category. One plant is often sufficient for fruit-bearing monecious species to produce fruit (some monecious plants, such as apples, do require the presence of different cultivars for cross-pollination in order to set fruit).
Many woody plants, including wisteria, go through a juvenile growth stage that is strictly vegetative. They put on significant leafy growth without blooming during this stage. Your wisteria vines are likely in this stage of development. This is especially true with seed-grown wisteria vines -- they can take five to 10 years if not longer to outgrow the juvenile stage. Wisteria vines propagated by rooting cuttings from blooming stock plants take much less longer to bloom.
Other factors that reduce bloom in wisteria include too much shade, applications of high nitrogen fertilizer that push vegetative growth at the expense of flowers and pruning at the wrong time of year. Wisteria produces its flowers buds during the previous growing season ("blooms on old wood"). If the plants were pruned from late fall to early spring, those buds were removed.
There are some steps you take to encourage a reluctant wisteria to bloom.
A heavy application of superphosphate (0-20-0), three to five pounds per 1,000 square feet, can promote blooms the following season.
Another trick is root prune wisteria in late fall. Root pruning is just what it sounds like -- cut an edge around the circumference of the vine's root system as though you were going to dig it up, but leave it in the ground. This practice alters the balance of nitrogen to carbohydrates in the plant. Do not dig too close to the trunk -- stay about two feet away from it.
Finally, severely pruning the new growth in late spring or early summer can also change the balance of nutrients in the plant to favor flowering over rampant vegetative growth.
Send questions to Sandy Feather by email at email@example.com or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.