A few tricks for growing and flowering Wisteria
Ann Hancock and Marcus Duck
Department of Horticulture, Michigan State University
Our wisterias in the Entrance Pavillion are wowing everyone this spring! Everyone is asking themselves - "How can I do this at home??" A great question - but you may have learned that a bit of patience may be required since there is no doubt that Wisteria vines in Michigan are notorious for taking a long time to flower. Here are a few possible reasons why your vine may not be flowering.
1) Newly planted wisterias seldom bloom right away. Usually they take several years to settle in and begin flowering. Seed grown may take even longer. Jane Taylor told me she waited 20 years for her seed-planted wisteria to flower and finally gave up. If at all possible, buy one in bloom in the nursery.
2) Most wisterias sold commercially are grafted; check the graft union to see if it is healthy looking.
3) Exposure. Wisterias grown in partial shade may never bloom as well as plants in full sun.
4) Water. Wisteria does not like to be dry. A particularly critical period is when the flower buds for next years bloom are being set; usually August-September. Pay particular attention to watering the plant then.
5) Feeding. Wisteria does not need much nitrogen at all; it is a legume and capable of fixing its own nitrogen. Too much nitrogen results in even faster, wilder growth, a frightening scenario. However, it is worth having your soil tested to see if you have adequate levels of potassium. If not, apply superphosphate (at the rate of 2 oz per square yd) and potassium sulphate (1 oz per square yd) in the spring.
6) Frost. This is a factor we really can’t do much about. Late frosts can damage flower buds and cause them to fall. If you have a warmer microclimate in your garden, put the vine there if it won’t endanger anything close by.
7) Pruning is best done on a twice a year schedule. In late August, shorten the current years growth to a foot (about 6 buds). This will help to promote flower bud formation. You can cut these shoots back to 1-2 inches (2-3 buds) in early spring again as we are doing today, if you wish to further shape the plant. In addition, your job will be much simpler if you remove suckers at intervals all during the growing season. Don’t be afraid to remove excess growth at intervals too.
Watch for flowering spurs and familiarize yourself with their appearance. They are stubby shoots, with numerous buds along them.
Tools: sharp pruning shears, loppers and pruning saw. 6 foot step ladder, (taller if your vine is bigger.) Use a bungee cord for bundling clippings into wheelbarrow.
Valder, Peter. 1995. Wisterias, A Comprehensive Guide. Timber Press, Portland, Oregon.