Gadenin日本語Get the appThe shape of the wisteria year
Almost everything that determines how a wisteria performs happens in two short windows. The summer prune, made about two months after flowering, converts extension growth into flower buds. The winter prune, made when the plant is bare, exposes those buds and maintains the framework. Miss either and the following spring is thinner; miss both for a few years and the plant becomes a leafy tangle that flowers only at the top.
Everything else in the calendar is inspection rather than intervention. Ties are checked once a year, the structure is checked once a year, scale is looked for on bare wood, suckers are removed on sight, and water is given only in the specific circumstances that matter. There is no feeding routine, which surprises people, and the absence of one is deliberate.
The seasons below are written so they read the same in either hemisphere. Anchor the calendar to the plant rather than to dates: the summer prune is triggered by long whips waving free of the support about two months after the flowers faded, and the winter prune by the plant standing bare with its buds visible.
| Season | Main job | Also check | Do not do |
|---|---|---|---|
| Late winter, before buds move | Winter prune: stubs back to two or three buds; structural cuts | Scale on bare wood; mulch; optional potash dressing | Do not apply nitrogen or general fertilizer |
| Early spring, buds swelling | Nothing; let the buds run | Note frost nights and bud condition | Do not prune; you would remove the flowers |
| Spring, in flower | Enjoy it and record dates and coverage | Support under full flowering weight, especially in rain | Do not cut anything except a dangerous shoot |
| Late spring, after flowering | Deadhead spent trusses; water in dry spells | Rootstock and ground suckers emerging | Do not let pods form where children reach |
| Mid to late summer | Summer prune: long shoots to five or six leaves | Every tie, every fixing, every post base | Do not shorten shoots you want as new framework |
| Autumn | Tidy; keep growth clear of gutters and roofline | Clear fallen pods; a second light tidy of late whips | Do not cut into the framework; wait for winter |
Winter and early spring
The dormant season is when you can see the plant. Do the winter prune first: take every summer-shortened stub back to two or three buds, a stub of about two to four inches (5 to 10 cm), cutting just above a plump rounded bud rather than leaving a snag. Then deal with structure, removing dead, crossing and misplaced wood and, on a congested old plant, cutting out up to a fifth of the fist-sized spur clusters so new spurs can form.
With the wood bare, inspect for scale insects along the undersides of the arms and in the crotches, and brush off what you find. Mulch two to three inches (5 to 8 cm) over the root area, keeping it clear of the trunk, and if you use potash apply it now, once, at about half an ounce per square yard (20 g per square meter). That is the only nutrient input of the year.
Once the buds begin to swell, stop. Any cut made from that point removes the flowers you spent last summer creating. Note frost nights during the bud-swelling period, because bud drop later in spring is nearly always explained by them.
Spring and the weeks after flowering
During flowering the plant is at its heaviest, especially after rain, and this is the moment when a marginal support fails. Look up at the beams and fixings while the plant is in full flower rather than assuming the structure that held last year will hold this one.
As the trusses brown, cut them off back to the first pair of buds behind the truss, leaving the rest of the shoot for the summer prune. This is not essential to next year's flowering but it prevents seed pods forming, which is worth doing on any plant where children or animals have access, since the seeds are the most poisonous part of a poisonous plant.
The six weeks after flowering are the one watering window that matters on an established plant. New foliage is expanding and next year's buds are beginning to be determined. If the soil is dry four inches (10 cm) down during that period, especially in the sheltered strip along a house wall, water heavily rather than lightly. This is also when rootstock suckers appear most obviously; pull them off flush at their origin as soon as they show.
- Check the structure under load While the plant is in full flower and heavy, look for sag in beams, movement at wall fixings, and any softness at post bases. Anything doubtful is repaired in the following dormant season when the plant can be cut back for access.
- Deadhead as the trusses brown Cut each faded truss back to the first pair of buds behind it. Leave the rest of the shoot alone; it is material for the summer prune.
- Water deeply if the soil is dry Test four inches (10 cm) down. If dry, give two full cans slowly over the root zone, repeating weekly while the dry spell lasts. Wall plantings need this far more often than open-ground ones.
- Remove suckers and rootstock shoots Anything from below the graft union or straight out of the ground comes off flush at the base. Pull rather than cut where the shoot is young enough, so the dormant buds come away too.
Summer and autumn
When the season's long whips are waving free of the support and their bases have started to firm, about two months after flowering, do the summer prune. Every long side shoot off the framework goes back to five or six leaves. Shoots you have chosen as new permanent arms are tied in full length instead, and shoots from below the graft or from the ground are removed entirely.
Do the annual maintenance at the same visit, because the plant is open and you are already working along the arms. Check every tie and replace any without slack, free any stem that has twined around a post or pipe, and look over each fixing point on the wall wires. An old tie embedded in bark is the single most common cause of losing a mature arm, and the summer prune is when you will see it.
Through late summer and autumn the plant will make some further growth. A second, lighter pass over the longest of those late whips does no harm and keeps the plant tidy through the dormant season. The one job that cannot wait for a scheduled visit at any time of year is cutting back growth heading for gutters, fascias, weatherboarding or roof tiles. Keep the top of the plant three feet (90 cm) clear of the roofline and deal with any shoot heading into that zone the day you notice it.
In autumn, clear fallen seed pods from beneath the plant along with the prunings. The pods split and scatter seeds that are poisonous if eaten and that can germinate into unwanted seedlings which would take a decade to flower.
What to write down each year
Wisteria works on a ten-month delay between cause and effect, which is longer than anyone reliably remembers. A short annual record turns guesswork into diagnosis. Note four things: the week of the summer prune, the week of the winter prune, the week the first truss opened, and how heavily and how evenly the plant flowered.
Add the exceptions. Frost nights while the buds were swelling. A dry summer where you did or did not water in the post-flowering weeks. A root prune, a potash dressing, a heavy renovation cut, or a year where you were late with the summer prune. These are the entries that explain a good or bad spring two years later.
Finally, record the structural work by location: which arm lost a tie, where you removed a spur cluster for renewal, which post you repaired, and where a rootstock sucker keeps coming back. On a big plant on a pergola, number the posts or the arms in your notes. Three or four years of that record and you will be able to predict your own plant rather than reacting to it.