Shoots vs. Flowers
Around this time of year, few plants are as dramatically beautiful as a well-grown wisteria, whose chains of lavender flowers drip like little waterfalls from the branches. I’ve always wanted one, and now I have one. But I’ll keep in mind a common complaint people have with wisteria: The frustration when a wisteria plant is all shoots and no flowers!
This problem has some causes and some solutions. The common complaint can often be traced to something as simple as a poor plant or a poor site. Perhaps a wisteria plant is not getting sufficient light, which is six or more hours of direct sunlight.
Wisterias propagated from seed have to be mature enough to bloom. The stage from juvenility to maturity, bringing it to reproductive (flowering) age, can take years. A clonally propagated plant — that is, one with a variety name and propagated from mature wood by a graft, a cutting, or layering — will bloom much sooner.
In occasional years, Ol’ Man Winter snatches away blossoms, killing flower buds even before they open. When this is the case, plump (though dead) flower buds are evident on the plant as it goes into spring. Species of wisteria differ in their ability to face up to that winter cold, Japanese wisteria (Wisteria floribunda) is more cold-hardy, and has more fragrant blossoms on longer chains, than Chinese wisteria (W. sinensis). Years ago I planted a native wisteria, variously known as American wisteria or Kentucky wisteria (W. frutescens). It grew fine its first season, and was stone dead the next spring. It was supposed to be cold hardy here in Hardiness Zone 5.
Wisteria have a tendency to channel their energy into rampant shoot growth at the expense of blossoms. This problem can be exacerbated if you unknowingly lavish care on your wisteria plant in the form of heavy fertilization and winter pruning. Response of wisteria to such treatment — no matter what your intentions — is to send out yet more lanky, flowerless shoots. This type of growth is acceptable only for a young plant still filling its allotted space on a pergola, a fence, or a wall.
Slowing down shoot growth brings an overly enthusiastic wisteria into a blooming habit. Wisteria’s far-reaching roots are well able to seek water and nutrients, so the plan here is to keep a plant on a lean regimen of fertilizer and water. Pruning the roots by thrusting a shovel into the ground all around the plant two to four feet from the trunk further checks growth.
Pruning a wisteria in summer ekes the maximum amount of blossoms from a plant (and keeps it tidy). Ideally, a wisteria vine has been trained to a permanent framework consisting of a trunk with a few permanent arms; summer pruning is directed at side shoots that grow off the arms. (An old plant might need ruthless pruning one winter to bring this framework into view out of the tangled mass of vines.)
The goal in summer pruning is to make side shoots into short “handles” on which to hang the drooping blossom clusters. Do this by cutting off the end of each side shoot as soon as its sixth leaf has unfolded.
Wisteria’s enthusiasm is not so easily quelled, and the plant will push out new growth. Cut it back to two leaves. In winter, go back over all side shoots and shorten them to a couple of inches.
Extension growth from the ends of permanent arms should be treated just like side shoots once arms fill their allotted space.
There are other versions of wisteria pruning: pinching out the tips of all side shoots a few times during the growing season; cutting side shoots monthly back to two or three buds; or, shortening side shoots in July to six inches.
I write about pruning wisteria in my book, THE PRUNING BOOK. I also added, in that section, that If you’re disinclined to pay attention to your wisteria vine once bloom is past, and are willing to sacrifice some bloom, follow the advice written in 1927 by Liberty Hyde Bailey in his Standard Cyclopaedia of Horticulture: “There are several ideas about training wisteria. A good way is to let it alone. This produces rugged, twisted, and picturesque branches and gives a certain oriental effect, but is not the best method for covering a wall-space solidly or for making the best display of bloom.” True, but you do have to start with a good plant and a good site, and put the brakes on fertilization and watering.
Wisteria, No, Yes?
All of which I planned to do with my recently acquired wisteria plant, the Japanese wisteria variety Anwen. But wait! I’ve seen wisteria tear down arbors and threaten to engulf homes. Not only that; it also sends up underground runners many feet from the mother plant. I’ve grown afraid of planting it; experience with bamboo and crocosmia have hardened me to potentially invasive plants, especially with the naturally rich soil here.
It turns out that the native American wisteria is much less invasive than either Asian species. Not only is its growth more sedate; also, it blooms on new shoots of the season so can be pruned back more aggressively to coax growth of new, flowering shoots.
I recently learned that Kentucky wisteria is actually a variant of American wisteria, and is sometimes considered a separate species, W. macrostachya.
Kentucky wisteria is the hardiest of wisterias, to Zone 3, which means my long gone plant was probably not the variant. I might give Kentucky wisteria — the variety Blue Moon, which can bloom three times a season — another try.