Chinese wisteria

Identification and origin

Wisteria sinensis belongs to the Fabaceae, the pea and bean family. It is native to China, where it grows wild in the north-central, south-central and southeastern parts of the country.

It is a fast-growing, deciduous woody vine. Its stems twine around their support counterclockwise, which is a good way to tell it apart from Japanese wisteria, which twines the other way. The alternate leaves are made up of seven to thirteen oval to elliptic leaflets. With age, the stems become thick and twisted.

In spring, hanging clusters 6–12 in long open almost all at once, before the leaves have fully emerged. The pea-like flowers are mauve to blue-violet and mildly fragrant. They are followed by brown, velvety pods 4–6 in long that hang on into fall. In cultivation the vine commonly reaches 10–25 ft tall; trained against a wall, it can spread more than 33 ft.

Common cultivars include ‘Amethyst’, with dense violet-blue clusters flushed with red, ‘Prolific’, which blooms at a younger age than others, as well as ‘Blue Sapphire’, ‘Jako’ and ‘White Diamond’, the last two with white flowers. In the eastern United States it has become invasive.

Light

Full sun is the key to flowering. Choose a south- or west-facing wall or pergola. Wisteria also grows in partial shade, but blooms less there, and hardly at all in deep shade.

Watering

Once established, wisteria tolerates drought, but it needs water in its first years and during dry spells, especially in sandy soil. Check the moisture a few inches down and water deeply when the soil is dry.

Pay particular attention in late summer. That is when the flower buds for the following spring form, and dry soil at that point makes them abort. In a container, the root ball dries out much faster: check it during hot weather.

On the other hand, waterlogged soil rots the roots. Never leave water standing in the saucer.

Potting mix

In the ground, wisteria copes with clay, loam, sandy or chalky soils, acid to alkaline, as long as they are fertile, moist and well drained. It does, however, prefer slightly acidic soil.

In spring, apply a balanced fertilizer or one made for flowering shrubs. In sandy soils, which are often low in potassium, add sulfate of potash: potassium encourages next year’s flowers. In containers, feed with a liquid fertilizer for flowering plants, such as a tomato fertilizer, or mix a slow-release fertilizer into the potting mix.

For container growing, use a pot at least 18 in across filled with a peat-free, loam-based mix.

Temperature

Chinese wisteria is cold-hardy, coping with winters down to 5 °F and even close to -4 °F; it is grown in the ground in USDA zones 5a to 8b. The real danger is a late spring frost, which damages or distorts the developing clusters and makes the buds drop before they open.

Repotting

Plant wisteria between fall and spring. Set it at the same depth as it was in its nursery pot. Choose the site carefully: an established wisteria does not take well to being moved. Put a sturdy support in place from the start.

In a container, start with a pot only slightly larger than the root ball and move it up gradually, as the roots fill each pot, until it reaches its final container.

Pruning

Wisteria flowers more reliably when pruned twice a year. Let a young plant grow freely until it covers its support, then begin routine pruning.

• In summer, after flowering: cut the long, whippy shoots of the current year back to five or six leaves, about 12 in. The wood ripens better and the plant forms flower buds rather than leafy growth.
• In winter, when leafless: cut those same shoots back further to two or three buds, within 1–2 in of the older wood.

Against a wall, training it as an espalier on horizontal wires set 12 in apart works best. Hard pruning in early or midsummer disrupts the next season’s flowering. An old wisteria can be renovated by drastic pruning between leaf fall and the end of winter; it usually flowers again two or three years later. Trained as a small standard tree, it can be grown in a container.

Propagation

Layering in summer is the easiest and most reliable method for home gardeners: bend a long, flexible stem down to the ground, bury a section and keep it moist until it roots, then sever it. You can also take softwood cuttings from spring to midsummer, or hardwood cuttings in winter. Nurseries graft the named cultivars.

Growing from seed is possible but disappointing. Collect the pods after leaf fall, let them ripen in an open tray, then twist them to release the seeds. Sow them fresh, 1 in deep; soak dry seed for a day first. A seed-grown wisteria can take twenty years to bloom, so buy a grafted cultivar or a plant already in flower instead.

Common problems

• No flowers: seed-grown plant, young plant not yet established, badly timed summer pruning, shade, dry soil in late summer, lack of potassium, or a late frost.
• Buds dropping or distorted flowers: spring frost.
• Brown blotches with a yellow margin on the leaves: powdery mildew, which on wisteria rarely shows the usual white coating. Mottling that comes back year after year can point to a virus.
• Shoots suddenly wilting and dying back: honey fungus, Phytophthora root rot encouraged by waterlogged soil, black vine weevil grubs in containers, or graft failure, betrayed by shoots coming up from below the graft. Coral-pink pustules on dead wood indicate coral spot.
• Brown, limpet-like bumps on the bark: scale insects.
• Other pests: aphids and spider mites; birds or mice shredding the flowers.

The same symptom can have several causes: look at the exposure, soil moisture, pruning and roots before drawing conclusions.

Toxicity

All parts of wisteria are toxic, seeds included. It is toxic to dogs, cats and horses, and mildly toxic to people. The compounds involved are a lectin and a glycoside, wisterin. Eating the plant causes vomiting, sometimes with blood, diarrhea and depression. Wisteria does not cause contact dermatitis, but wearing gloves when handling it is advised. If any part is swallowed, contact a veterinarian or a poison control center.