Q. I have three wisteria plants, about 10-12 years old. All look lovely and healthy but not one of them has ever produced a single flower. HELP! I have tried stressing them and adding something called 'Phos something or other', but no luck. No one seems to be able to explain why they are not blooming. They are all in different locations, from part shade to full sun. Hope you can help…
- ---Gaba in Delaware ("Gaba, gaba, hey!")
- ---Sherlene Of Iowa
(Isn't that the title of a Bruce Springsteen song?)
- ---Celeste in Horse Shoe, NC
- ---John in Woodbury, NJ
Although there are several types, most people are growing Chinese wisteria, which can reach 100 feet in length, and produces its awesome flower clusters on the vine's bare branches before the plant leafs out in Spring. The Japanese strain is smaller, topping out at 30 feet in length, and generally bears its VERY fragrant bluish-purple flowers after the plant leafs out. Both are rampant and demand lots of control in the landscape.
Like most plants, wisteria needs sun to bloom; vines that get a lot of shade simply will not flower, say the experts. Some growers feel it also craves phosphorus, the 'flowering nutrient', so folks with no flowers might want to scratch a couple cups of bone meal into the soil now and then add some rock phosphate in the Fall.
But the secret seems to be in the pruning. One of my best general plant books says to always prune after flowering (which, of course, is very cruel to people who don't got no flowers) and only recommends winter pruning to control the size of this often-invasive monster. But another good source say to hack away in winter, removing all but a few buds from about half of the previous year's growth in addition to the summer pruning.
Our old friend Adrian Higgins, long-time Garden Editor for the Washington Post, feels that this second reference is more on the money. In a 2004 piece that has become an Internet favorite, he relates the wonderful tale of a Washingtonian who moved for a time to Germany, where the landlord insisted that his magnificent blooming wisteria not be touched. "In time," writes Adrian, the gardener who cared for the plant "shared his secrets of getting a wisteria to bloom. He gave it a trim in July, after the spring flush of fresh green growth. In January, he cut back almost all of the previous year's growth to leave just three or four buds per stem, one of which would be plumper and become a flower three months later."
The renter returned to Washington, writes Adrian, and "the 20-year-old vine on her deck is distinguished by a pair of gnarled old trunks that rise to about nine feet, and from late April into May, festoon the arbor with pendant lavender blossoms of heady scent."
But the story was three years old, and I often learn a lot more about a subject after I write about it, so I called Adrian to ask if he had anything to add. He said that, if anything, it would be to prune even more. "Some people with magnificent wisteria tell me they cut it back practically monthly throughout the summer," he said, agreeing that such constant trimming would almost certainly rob any other Spring bloomer OF its blooms. This plant just grows so rampantly it defies all the rules.
Pruning expert Lee Reich ("The Pruning Book"; Taunton Press) goes even further; explaining that some bloom lovers cut back the rampant shoots every two weeks during the summer. So if you're already pruning, try a little more lack of tenderness. But if you haven't yet pruned, begin with a July and January schedule.
Adrian and I agreed that such a strategy makes a lot of Darwinian sense. Plants don't really need to flower if they're living in perfect conditions, but they must try and pump out some posies in an attempt to reproduce if they're stressed by something like constant trimming. Or the final straw: The legendary root pruning.
Before they give up the ghost and pull up the plant, wisteria owners who provide enough sun and have exhausted their pruning options might want to drive a sharp shovel eight to ten inches into the ground about a foot and a half out from the trunk to slice into some of the roots and give the plant a shock. Or just to get even.