Why Wisteria Won't Bloom

Wisteria is slow before it is anything else

Before diagnosing a problem, establish whether the plant is simply too young. This is the most common answer and the hardest to accept.

University of Maryland Extension puts the principle generally: “Most flowering trees and shrubs must be physiologically mature before they are capable of blooming. During the juvenile stage of growth, plants do not produce flower buds.”

For wisteria that juvenile period is long. A seed-grown plant may take five to ten years, and sometimes longer, before it flowers at all. There is no treatment for juvenility. The plant is not failing; it is not finished growing up.

How the plant was propagated matters enormously

This is the detail that separates a three-year wait from a ten-year one.

Most named cultivars are grafted or produced from cuttings taken off an already-flowering plant, and those flower far sooner. Seedlings do not.

If you bought an unnamed, inexpensive wisteria, there is a reasonable chance it is seed-grown. The standard advice is to buy plants in flower, which is the only way to be certain the plant is old enough and of a clone that blooms.

Too much nitrogen

The second most common cause, and the one gardeners inflict on themselves.

Maryland lists excess nitrogen fertiliser as promoting vegetative growth at the expense of blooms. A wisteria that puts on many feet of lush growth a year and produces nothing is being fed too well — often by lawn fertiliser applied nearby, since wisteria roots range widely.

What to do: stop feeding it. Wisteria fixes its own nitrogen and needs very little help. If it sits over or beside a fed lawn, that is your source.

Pruning

Maryland notes that “improper timing of pruning spring-flowering trees and shrubs will result in the removal of flower buds,” and separately that removing more than a third of a plant at once “stimulates vegetative growth and results in little or no flower bud formation.”

Both apply. Hacking a rampant wisteria back hard in winter is satisfying and counterproductive — it removes the short flowering spurs and provokes a surge of leafy regrowth.

The traditional approach is two cuts a year: shorten the long whippy summer growth back to a few buds in midsummer, then tidy those shortened shoots again in winter. The aim is to build up short spurs close to the main framework, because that is where the flowers come from.

Frost, shade and roots

Late frost is, in Maryland’s words, “a very common reason” for flowering failure generally. Wisteria buds swell early and a hard frost can destroy a season’s display overnight, on a plant that is otherwise perfectly capable.

Shade. Wisteria wants full sun. A vine that has grown up into a tree canopy or is shaded by a building will grow enthusiastically and flower poorly.

Root pruning is sometimes recommended for stubborn non-flowering plants — severing roots with a spade in a circle around the plant in autumn to check vegetative growth and push it toward flowering. It is a traditional technique with reasonable support, and worth trying on an established, well-fed, well-lit plant that still refuses.

Before you replace it: check which wisteria you have

If you have reached the point of replacing the plant, this is worth knowing first.

The two wisterias sold in most US garden centres are Chinese wisteria (Wisteria sinensis) and Japanese wisteria (Wisteria floribunda). The USDA PLANTS Database records both as introduced in the lower 48, and flags both as having invasive status listings.

They earn it. University of Maryland Extension describes vines that “can shade out native trees or strangle them with thick and heavy growth,” and notes that once they bring a tree down, the collapse can “open the canopy and expose the forest floor to full sun, encouraging the vines’ continued growth.” They spread two ways at once: “short stems form along the vines and will sprout roots if touching the ground,” and “the vines can produce abundant seeds, which can be carried long distances in riparian settings.”

Escaped Chinese wisteria now has populations “as far south as Louisiana and as far west as Wisconsin.” Japanese wisteria infestations are “concentrated in the mid-Atlantic and mid-south states.”

That vigour is the same trait behind your flowering problem. A plant putting fifteen feet of growth a year into runners is not a plant in a hurry to bloom.

The native alternative

American wisteria (Wisteria frutescens) is native to the lower 48 — NC State Extension gives its range as Virginia to Illinois, south to Florida and Texas, hardy in zones 5a–9b.

It is the better garden plant for most people, and specifically for the problem this article is about. NC State notes it “is not as aggressive a spreader as Wisteria sinensis” and is “less damaging to buildings” than either Asian species. It reaches 15–40 feet rather than swallowing a mature oak, and it carries fragrant lilac-purple racemes to about six inches.

Three ways to tell what you already have:

• Bloom timing. American wisteria flowers in April–May after the leaves have emerged. The Asian species flower on bare or barely-leafed wood.
• Seed pods. American wisteria produces narrow, flattened, smooth pods. Asian pods are velvety.
• Twining direction, with a caveat. Japanese wisteria twines clockwise; both American and Chinese twine counterclockwise. This separates Japanese from the other two — it does not tell you native from introduced, which is how it is usually mis-repeated.

If you are in a region where the Asian species have escaped, and yours has never flowered anyway, replacing it with the native is the easy call.

Set your expectations

If the plant is young, the answer is time. If it is mature, well-lit, unfed and correctly pruned and still will not flower, the likeliest explanation is that it was grown from seed — and the only real fix is to replace it with a named cultivar bought in bloom, or with Wisteria frutescens.

One warning if you do remove an Asian wisteria: do not simply cut it down and walk away. Maryland is explicit that “cutting the vines on trees will cause sprouting from the cut edge, so repeated cutting is necessary.” Expect to go back at it, or to treat the cut stump.