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Wisteria is a legume with nitrogen-fixing nodules on its roots, so it manufactures the nutrient that most garden plants need supplied. Adding more produces a plant that grows too fast to flower well. This is the most common care mistake made with wisteria, and it is usually made with good intentions, either as a spring feed of general fertilizer or indirectly by a lawn feed applied over the root zone.
The one supplement worth giving is potassium. A light dressing of sulfate of potash in late winter, around half an ounce per square yard (about 20 g per square meter) spread over the root area and lightly scratched in, supports flowering. Once a year is enough. Skip it entirely rather than substituting a general purpose feed.
Mulch is more useful than fertilizer. Two to three inches (5 to 8 cm) of garden compost or bark over the root zone in late winter conserves moisture, suppresses weeds and slowly improves the soil. Keep it off the trunk itself, and do not use fresh manure, which is a nitrogen source in disguise. If the wisteria is set in a lawn, keep a clear mulched circle at least three feet (90 cm) across around the trunk and take care that lawn feed does not reach it.
| Input | Established plant | Reason |
|---|---|---|
| Nitrogen fertilizer | None, ever | The plant fixes its own; extra suppresses flowering |
| Sulfate of potash | Once in late winter, a light dressing | Supports flower bud formation |
| Compost or bark mulch | 2 to 3 in (5 to 8 cm) in late winter | Moisture and soil structure, no feeding effect |
| Fresh manure | Never | A concentrated nitrogen source |
| Lawn feed near the trunk | Keep off the root zone | The commonest hidden cause of a leafy, flowerless plant |
| Water | Deeply in dry spells for six weeks after flowering | That is when next year's buds are being set |
Water rarely but properly
An established wisteria in open ground is deep-rooted and drought-tolerant, and in most seasons needs no watering at all. Two situations change that. The first is the establishment period, the first two or three summers after planting, when the roots have not yet reached beyond the original hole; water thoroughly every week through dry weather in that period, two full cans poured slowly rather than a daily sprinkle.
The second is the six weeks after flowering on any plant, of any age, growing in the dry strip along a house wall. That soil is sheltered from rain by the eaves and dried by the masonry, and a wisteria there can be genuinely short of water while the rest of the garden looks fine. Push a finger or a trowel four inches (10 cm) down; if it is dry at that depth in the post-flowering period, water heavily. Drought during that window costs you flowers ten months later.
Wisteria in a large container is a different plant altogether in this respect. Container plants need watering through the whole growing season, often daily in hot weather, and they need a low-nitrogen liquid feed occasionally because the compost has nothing to give. A container wisteria also needs harder and more frequent pruning, since the pot cannot support the growth the plant attempts.
Ties and supports are an annual job, not a one-off
The main stems thicken every year for the whole life of the plant. Any tie, wire or clip that fitted last year will be tighter this year, and one that fitted five years ago may now be embedded in the wood. A stem that has been ring-barked by an old tie dies above the constriction, sometimes suddenly, taking a whole arm of a mature plant with it. This is the most common avoidable loss on a wisteria.
Check every tie once a year, during the summer prune when the framework is visible and you are already at the plant. Use soft, wide, flexible ties with a deliberate loop of slack in them rather than tight wire or plastic clips. Where a stem crosses a wire, look at whether the wire is starting to indent the bark and, if so, retie or move the stem to a different position on the wire.
Look at the structure itself at the same time. Sight along pergola beams for sag, check post bases for rot at ground level, and test wall wires for tension and vine eyes for movement in the mortar. A mature wisteria in leaf and in full flower is extremely heavy, and heavier still when wet, so a support failure tends to happen during rain in the flowering season, which is the worst possible moment.
- Walk the plant once a year with pruners and ties Do this at the summer prune. Work along each main arm from the trunk outward, checking every attachment point rather than only the ones you can see easily from the ground.
- Cut off any tie that has no slack Replace with a soft tie in a loose figure of eight, so the stem can thicken without pressing directly on the material. Do not simply add a new tie beside an old tight one.
- Free stems that have wrapped around the support Wisteria twines. A stem that has wound around a post or a pipe will thicken and crush it. Unwind it while it is still flexible and tie it in against the support instead.
- Check the structure for movement and rot Push firmly on posts and beams. Probe post bases at ground level. Test wall fixings by pulling on the wire. Repair anything doubtful in the dormant season when you can cut the plant back for access.
- Keep a three-foot clearance from the roofline Cut back anything growing towards gutters, fascias, weatherboarding or roof tiles, whatever the season. Do not wait for the summer prune to deal with a shoot heading under the eaves.
Suckers, seedlings and the plant's habit of spreading
Wisteria produces shoots from the base and sometimes from roots at a distance from the trunk. On a grafted plant these arise from the rootstock and will never flower like the top. On any plant they steal vigor and quickly become a thicket at the base. Remove them flush at their point of origin, pulling rather than cutting where you can, since a pulled sucker takes the dormant buds with it and is less likely to regrow. Do this whenever you see one rather than saving it for the pruning rounds.
Long, low shoots that run along the ground will root where they touch soil, particularly through a mulch, and start a new plant several feet from the parent. Any shoot heading horizontally at ground level should be removed. Where wisteria grows near a hedge or a shrub border, check under the neighboring plants once a year for stems that have traveled in and started climbing something you did not intend.
Seed pods are the other route to unwanted plants, and they are the part of the plant that is genuinely hazardous. The seeds are poisonous if eaten and the pods split and scatter them. Deadheading after flowering prevents pods forming, which deals with both problems at once.
Wisteria has no aerial roots or adhesive pads, so it cannot support itself on a flat surface. Everything it climbs, it climbs by twining. That means every point of contact is a point of pressure, and the pressure only increases with time.
The record that pays for itself
Because wisteria responds slowly and lives a long time, memory is a poor guide to what is happening to it. A few lines a year turn the plant into something you can actually manage. Record the week of the summer prune and the winter prune, the week the first truss opened, and how heavily it flowered.
Add a short structural note: which ties you replaced and where, whether any post or wire needed attention, and where you removed suckers or renewed a spur cluster. On a large plant on a pergola it helps to number the posts or the arms in your notes so that next year you know which arm carried a heavy truss load and which one is thinning out.
The third useful entry is anything you did that was out of the ordinary: a root prune, a hard renovation cut, a potash dressing, a dry summer where you did or did not water. Those are the events whose effects arrive one to three years later, far too long to hold reliably in your head.