Gadenin日本語Get the appWork through the four causes in order
Wisteria that refuses to flower is the most common complaint about the plant, and it has a small number of causes that can be checked in a few minutes. Diagnose before you act, because the remedies are quite different and two of them take years.
First, age and origin. A plant grown from seed spends seven to fifteen years in a juvenile state during which it cannot flower no matter what you do to it. Look for a graft union low on the stem: a swollen kink or diagonal scar an inch or two (3 to 5 cm) above soil level. If there is none, and the plant is under about seven years old, patience is the only treatment. If there is a graft union and the plant is over three or four years old, the cause is something else.
Second, pruning. Wisteria flowers on short spurs, and spurs only form if the long summer growth is repeatedly shortened. A plant that is either never pruned or pruned only once a year will not build a spur system. This is the cause in most cases where a mature grafted plant grows vigorously and produces a handful of trusses at the very top.
Third, nitrogen. Wisteria fixes its own nitrogen and needs none from you. Feeding it, or growing it in the middle of a lawn that gets a spring lawn feed, or planting it over a border you dress with a general fertilizer, pushes it hard into leaf and shoot at the expense of flower buds. Fourth, light. Six hours of direct summer sun is the minimum for the wood to ripen properly. A wisteria that has been slowly overshadowed by a growing tree will taper off in flowering over several years for no other reason.
| Symptom | Likely cause | What to do | Time to flower |
|---|---|---|---|
| Vigorous, no graft union, young plant | Grown from seed, still juvenile | Prune correctly and wait, or replace with a grafted plant | Up to fifteen years, or two to four if replaced |
| Huge tangle, a few trusses at the top only | Never pruned, or winter only | Start the twice-yearly cycle and renovate over two winters | One to three years |
| Dense dark leaves, strong shoots, no buds | Nitrogen from feed or nearby lawn treatment | Stop all feeding; keep lawn feed off the root zone | One to two years |
| Flowering has declined year by year | Increasing shade from a nearby tree | Thin or reduce the overhanging tree | One to two years |
| Once flowered well, now only leaves | Rootstock shoot has replaced the grafted top | Trace stems to origin; remove all growth from below the graft | Two years or more, if any grafted top survives |
| Buds formed, then dropped before opening | Spring frost on swelling buds, or drought last summer | Nothing this year; water through dry spells next summer | Next season |
Where the flower buds come from and when they are made
Next spring's flowers are decided during the second half of this summer. When you shorten a long shoot to five leaves, the buds remaining at its base receive the growth energy that would have gone into extending it, and over the following weeks some of them differentiate into flower buds rather than leaf buds. Anything that disrupts the plant during that window, particularly a severe drought or a late hard pruning that provokes fresh soft growth, reduces the number of flower buds set.
By the dormant season you can count next spring's display. Flower buds are noticeably fat and rounded, roughly twice the girth of a leaf bud, and often carry a fine silvery down. Leaf buds are narrow, flat and pressed against the stem. Walk the plant in winter, look along the spurs, and you will know before spring whether the previous summer's pruning worked.
Flowers open on the bare or barely-leafing plant in spring, before or with the emerging foliage, which is why the display looks so dense. The trusses hang and open progressively from the top of the truss downward, so a single truss stays in color for one to two weeks and the whole plant for two to three weeks depending on temperature. Cool weather during flowering prolongs it; a sudden hot spell shortens it markedly.
Two interventions worth trying on a stubborn mature plant
If a grafted plant is well over five years old, correctly pruned, unfed and in full sun, and still will not flower, there are two traditional checks that shift the balance from growth to reproduction. Both work by mild, deliberate stress. Neither is a substitute for correct pruning, and there is no point attempting either on a plant that is still being fed or is still a seedling.
Root pruning is the more effective. In the dormant season, mark a circle around the trunk at a radius of about three feet (90 cm) for a mature plant, then drive a sharp spade vertically to its full depth along roughly half of that circle, severing the roots it meets. Do half the circle one winter and, if needed, the other half two winters later. Do not sever roots all the way round in a single operation on a plant that is also carrying a heavy structure, and water through the following summer while it recovers.
A dressing of sulfate of potash in late winter, at about half an ounce per square yard (roughly 20 g per square meter) over the root area, is the other. Potassium supports flowering where nitrogen suppresses it. Apply it once a year at most, and never reach for a general purpose fertilizer as a substitute, because the nitrogen in it will undo the point of the exercise.
All parts of wisteria are poisonous if eaten, and the seed pods and seeds are the most dangerous part. After a good flowering year a mature plant sets long velvety pods that split and throw seed. Remove pods as they form if children or pets use the garden, and clear fallen pods from beneath the plant.
After the flowers, and what to write down
As the trusses brown, cut them off. Deadheading is not essential to next year's flowering, but the spent trusses are unsightly for weeks and, more usefully, removing them stops the plant putting resources into seed pods. On a plant you are trying to persuade into better flowering, taking off every truss as it fades is worth doing. Cut back to the first pair of buds behind the truss and leave the rest of the shoot for the summer cut.
Water matters in the six weeks after flowering more than at any other time. The plant is refoliating and starting to build next year's buds. A prolonged dry spell then, particularly on a plant close to a wall where the soil is naturally dry, shows up as a poor display the following spring.
Keep a simple flowering record year on year: the week the first truss opened, the week the last faded, a rough count or a note of whether the trusses covered the whole framework or only the top, and whether a spring frost caught the buds. Note also the week you made the previous summer's cut. Four seasons of that will tell you more about your particular plant in your particular position than any general advice, because the connection you need to see is between a cut made in summer and a display that arrives ten months later.