Patience comes in handy when growing wisteria. "The biggest frustration that gardeners face when growing wisteria is that plants have a longer-than-average juvenile period and sometimes fail to bloom as expected," says Jane C. Martin, horticulture agent for the Ohio State University Extension. The plant's environment and how you care for it also affect blooming.
Try these tips for more blooms:
1. Don't overdo fertilizer: Many gardeners suffer from a desire to fertilize indiscriminately, usually to the detriment of the plant. Juvenile plants should be fertilized only once a year. Use an organic fertilizer with a balanced NPK ratio. Too much fertilizer encourages vigorous vegetative growth at the expense of blooming. Established wisteria needs little fertilizer or additional water. After the plant fills in its trellis, water it only during drought and fertilize if the foliage yellows and new growth is poor.
2. Prune: Wisteria must be properly pruned to promote blooming. Chinese wisteria (Wisteria sinensis) and Japanese wisteria (W. floribunda) are the most commonly planted species, and they produce flower buds on old wood. Don't prune heavily in winter and early spring, because you'll be pruning off flower buds and encouraging new (nonblooming) vegetative growth. You can improve flowering by pruning heavily after bloom time, in late spring or early summer. This practice reduces new vegetative growth and promotes flower production. Since wisteria has very specific pruning needs, get a copy of Cass Turnbull's Guide to Pruning (Sasquatch Books, 2004), which acknowledges the challenges of caring for wisteria and clearly explains how to maintain it properly. (Turnbull is the founder of Plant Amnesty, an organization dedicated to rescuing plants from bad pruning.)
3. Give it sun: Wisteria will grow in partial shade, but it needs at least six to eight hours of full sun to bloom best. You can encourage flowering by pruning heavily in the summer and then root-pruning in late fall: Starting about 4 feet from the trunk, cut 18 inches down into the soil with a spade, circling the trunk. This process tends to "shock" plants back into flowering.
Additional info: Plants grown from seed can take 10 to 15 years or longer to bloom, Martin says. If you know your plant was grown from seed, you may want to take it out and start over. Before purchasing a replacement, check with your nursery about how their wisteria were propagated, and be sure to buy one propagated by grafting or cutting. This will greatly shorten the time to flowering.