Why is my wisteria not flowering?

Working through the five causes — age, origin, pruning, nitrogen and frost — in the order that identifies most cases quickly.

Difficulty: beginner
Time: 20 min
Read: 3 min

Short answer

Ask two questions first: how old is it, and was it grafted or grown from seed. A seedling wisteria can take fifteen years and may never flower well. After that, the usual cause is no summer pruning, or too much nitrogen from a nearby lawn feed.

Wisteria is a plant where the reason for failure is usually decided before you bought it. A grafted named variety on a suitable wall, pruned twice a year, flowers reliably. A cheap unnamed seedling may not.

Step by step

Find the graft union.Look at the base of the main stem for a knuckle or change in bark texture within about 300mm of the ground. That is a graft, and grafted plants flower within a few years. No graft usually means a seedling.
Count the years.Grafted plants generally flower within three to five years of planting. Seedlings can take ten to twenty, and some never flower acceptably.
Check whether it is pruned twice.Summer shortening of side shoots to five or six leaves, then winter shortening to two or three buds. One cut a year, or none, gives leaf rather than flower.
Look at what else gets fed nearby.Wisteria on a wall beside a lawn receives whatever the lawn does. Nitrogen produces growth and suppresses flowering.
Apply sulphate of potash in spring.A potash feed supports flower initiation. Applied in early spring around the root area, watered in.
Consider frost damage to the buds.Fat flower buds killed by a hard April frost give a year with leaves and no flowers, particularly on an east-facing wall where morning sun thaws them too fast.
Check the light.Wisteria needs a lot of sun to flower. A north-facing wall or a position that has become shaded by a growing tree will not produce a good display.
Check water in the previous summer.Flower buds for next year form in mid to late summer. A wisteria that was drought-stressed in July often flowers poorly the following May.
Consider root pruning as a last resort.On a mature, well-sited, well-pruned plant that still will not flower, severing some roots with a spade in a circle about a metre out in autumn can check vegetative growth and trigger flowering.

Tips

Look for a graft union at the base — it tells you most of what you need to know.
Buy wisteria in flower, so you know it is a flowering-age grafted plant.
A wisteria beside a fed lawn is being fed nitrogen whether you intended it or not.

Common mistakes

Feeding a non-flowering wisteria with a general fertiliser — It usually makes it worse. High nitrogen drives leaf and shoot growth at the expense of flower initiation. Potash is the useful nutrient here.
Buying an unnamed cheap wisteria — Unnamed plants are often seed-raised, and seedlings are notoriously slow and unreliable to flower. A named grafted variety costs more once and flowers within a few years.

If it doesn't work

Young plant, vigorous, no flowers

Cause: Too young, or seed-raised — Fix: Check for a graft. If seed-raised, be patient or replace with a grafted named variety.

Old plant, huge, flowers only at the top

Cause: Never pruned into spurs — Fix: Begin the summer and winter pruning cycle and cut the top growth back to a framework.

Buds formed then browned

Cause: Late frost, worsened by early morning sun — Fix: No remedy after the event. On an east wall it is a recurring risk.

Flowered well, then stopped

Cause: Nitrogen feeding nearby, or increasing shade — Fix: Stop nitrogen, apply potash, and cut back whatever is shading it.

Questions people ask

How long before a wisteria flowers?

A grafted named variety, three to five years from planting. A seed-raised plant, ten to twenty, and sometimes never satisfactorily.

Does root pruning really work?

Sometimes, on a mature, correctly pruned plant growing too vigorously. Cutting a spade's depth in a circle about a metre from the trunk in autumn checks growth and can trigger flowering. It is a last resort, not a first.

What should I feed it?

Sulphate of potash in early spring. Avoid nitrogen-rich general and lawn fertilisers, which produce growth instead of flowers.