Gadenin日本語Get the appThe support is the real decision, not the planting hole
A wisteria in its third year looks like a climber. At twenty years it is a tree that happens to lean on something. The main stems thicken to the diameter of your forearm and beyond, they twist as they thicken, and the twisting exerts a slow crushing force on whatever they are wrapped around. Trellis panels split. Thin wire cuts into the stem and then snaps. Downpipes are pulled off the wall. None of this happens quickly enough to alarm you in any single season, which is exactly why it happens at all.
Build for the mature plant. A freestanding pergola for wisteria wants posts at least six inches (15 cm) square, set in concrete two feet (60 cm) deep, with cross beams of similar substance. Against a wall, use galvanised wire of at least 4 mm gauge or, better, 12 mm steel rod, held on vine eyes that stand the wire three to four inches (8 to 10 cm) clear of the masonry so a thickening stem has somewhere to go. Space horizontal wires eighteen inches (45 cm) apart. Put all of this up before the plant goes in the ground; retrofitting a support under an established wisteria means cutting it back hard and losing years of flower.
The one structure that is never adequate is the house itself, unaided. Wisteria will find its way under weatherboarding, behind fascia boards, into the gap between a gutter bracket and the wall, and under roof tiles at the eaves. Once a stem is in a gap it thickens and levers the two surfaces apart. Grow it on wires set clear of the wall, keep the top of the plant at least three feet (90 cm) below the guttering, and cut back anything heading for the roofline the moment you see it. This is not a caution about a rare failure. It is the ordinary behavior of the plant.
Never let wisteria climb a wall by attaching itself. It has no adhesive pads, so it climbs by wrapping, and the only things available to wrap around on a house are pipes, gutter brackets, cables and board edges, all of which it will eventually distort or pull free.
Grafted or seedling: the difference is years of waiting
Wisteria raised from seed usually takes somewhere between seven and fifteen years to produce its first flower, and some never produce a good one. A grafted plant is a piece of an adult, already-flowering plant joined onto a rootstock, and it carries the adult behavior with it. Grafted wisteria commonly flowers within two to four years of planting, and often has a few trusses on it in the nursery pot.
Look at the base of the stem, an inch or two (3 to 5 cm) above the compost. A graft shows as a distinct swollen kink or a change in bark texture where two pieces were joined, sometimes with a visible diagonal scar. No such join anywhere on the lower stem means the plant is on its own roots, which usually means it was grown from seed or from a cutting. A cutting taken from a flowering adult also flowers young, but you cannot tell a cutting from a seedling by looking, so the graft union is the only reliable evidence you can gather yourself.
This matters after planting too. A grafted plant may throw shoots from below the graft union, out of the rootstock. Those shoots belong to a different, usually seedling, plant and they will not flower. They tend to be more vigorous than the top, with slightly different leaf size, and they arise from the stem below the join or straight out of the ground beside it. Pull or cut them off flush at their point of origin as soon as you notice them, at any time of year. Left alone for two seasons they can outgrow and eventually replace the flowering variety entirely, which is the usual explanation behind a mature wisteria that once flowered and now does not.
| Clue | Grafted plant | Seedling plant |
|---|---|---|
| Base of stem | Swollen kink or diagonal scar low down | Smooth, continuous stem to soil level |
| First flowers | Two to four years after planting | Seven to fifteen years, sometimes never |
| Flowers in the pot | Often, or flower buds visible | Almost never |
| Shoots from below the join | Rootstock; remove flush | Not applicable |
| Worth planting | Yes, for reliable flower | Only if you accept a long wait |
Where to dig, and what the soil needs
Wisteria flowers on wood that has been ripened by sun. A plant in half shade will grow enormously and flower thinly, and no amount of pruning corrects that. Choose a position that gets a minimum of six hours of direct sun through the growing season; a south or west aspect is ideal, an east aspect acceptable, a north wall a waste of a good plant.
The soil requirement is modest and easily overdone. Wisteria is a legume: it fixes its own nitrogen through root nodules and needs no help with nitrogen at all. Deep, moisture-retentive soil that does not lie wet in winter is all it asks. Do not enrich the planting hole with manure or high-nitrogen fertilizer. Rich feeding produces exactly the wrong plant, one that spends everything on shoot growth and flowers reluctantly. A bucket of leaf mold or garden compost forked into the base of the hole for structure is plenty.
Set the plant so the top of the rootball is level with the surrounding soil, or with the graft union at least two inches (5 cm) clear of the surface so buried scion wood cannot root and bypass the rootstock. Plant at least eighteen inches (45 cm) out from a wall, where rain actually reaches the ground, and lead the stems back to the wires on a cane. Water in with two full cans and mulch three inches (8 cm) deep over the root area, keeping the mulch off the stem itself.
- Put the support up first Posts, beams or wall wires all installed and tested before the plant arrives. Wires eighteen inches (45 cm) apart, standing three to four inches (8 to 10 cm) proud of the masonry on vine eyes.
- Check the plant for a graft union Find the join low on the stem before you plant, and remember roughly how high it sits. Everything that later emerges below that point is rootstock and must be removed.
- Dig wide and shallow A hole two feet (60 cm) across and no deeper than the rootball. Fork the sides and base so roots can leave the hole. Do not add manure or fertilizer.
- Set at the same depth it grew Rootball surface level with the surrounding soil, graft union well clear of it. Backfill with the soil you removed, firm with your hands, and water to settle rather than treading it down.
- Tie in the leader and choose the framework Select one strong stem as the leader and tie it loosely to a cane running up to the support. Use soft ties with slack in them and plan to check every tie annually for the life of the plant.
- Mulch, then leave the feeding alone Three inches (8 cm) of compost or bark over the root zone, clear of the stem. Water through the first two summers in dry spells. Apply no nitrogen in the first year or any year after.
The first three years are framework, not flowers
In the establishment years the job is to build a permanent skeleton in the shape you want to keep for decades. On a pergola that usually means a single clear trunk to the top of the post, then two or three main arms trained along the beams. On a wall it means a trunk with pairs of horizontal arms tied along each wire. Everything else that grows is temporary and gets shortened.
Train the leader up the support and let it run. Each summer, shorten the side shoots coming off it to about five leaves, and each winter cut those shortened shoots back to two or three buds. That is the same twice-a-year cycle you will use forever, started early. It builds short flowering spurs along the permanent arms while the framework is still extending, so the plant is ready to flower the moment it is old enough.
Expect a young grafted plant to produce a scattering of trusses in its second or third year and a proper display a year or two after that. Resist the urge to feed it into flowering faster. What speeds flowering is sun, restraint with nitrogen, and consistent summer pruning; what delays it is rich soil and an untouched tangle of growth.