Chinese wisteria bonsai

Placement

Wisteria sinensis is a deciduous woody vine from China. Along with Japanese wisteria, Wisteria floribunda, it is one of the two species grown as bonsai. One way to tell them apart is the direction in which their stems twine: counterclockwise for Chinese wisteria and clockwise for Japanese wisteria. The leaves are compound, up to 14 in long, and the very fragrant flower clusters, blue, purple or white depending on the variety, can reach 12 in.

It is an outdoor bonsai. Throughout the growing season, keep it in full sun: light is essential for flower formation, even though the tree tolerates partial shade. With its long clusters and heavy canopy, the tree easily becomes top-heavy: secure the pot or shelter it from strong winds that could knock it over.

In the ground, wisteria is hardy. In a pot, its water-filled roots are more fragile: protect them once temperatures fall below 23 °F. A cold greenhouse, a garage or a frost-free shed works well for overwintering.

Soil mix

Use a standard bonsai mix that is slightly acidic, with a pH of about 5.5 to 6. It should hold enough water for this very thirsty species while draining well: a root ball that stays soaked during dormancy, or a poor-quality soil, encourages root rot.

Watering

Wisteria drinks far more than most bonsai. During the growing season, water heavily until water runs out of the drainage holes, and keep the root ball constantly moist. Check the soil with a finger and lift the pot regularly, especially in hot weather. During hot spells, you can stand the pot in a shallow tray that you refill at each watering. In fall and winter, keep the roots moist but never soggy.

Fertilizer: wisteria belongs to the legume family and fixes nitrogen from the air. A high-nitrogen fertilizer therefore favors leaves at the expense of flowers: choose a low-nitrogen fertilizer. Apply it after flowering and through to fall, as a liquid every week or every two weeks, or as a solid organic fertilizer once a month; the two can be combined.

Pruning

Hard pruning is done in early spring or right after flowering. In summer, let the new twining shoots grow until the middle of the season, then cut them back. As with a garden wisteria, this summer pruning of long green shoots to five or six leaves pushes the plant to form flower buds rather than new stems; in winter, the shoots pruned this way are shortened to two or three buds.

After leaf drop, prune branches and twigs, but never cut the short shoots that carry flower buds. On close inspection, flower buds can be told apart from leaf buds. In summer, if the foliage becomes too dense and the inner twigs lack light, thin the leaves, but never fully defoliate the tree. Also remove seed pods before many develop: they drain the tree’s energy.

Getting flowers takes time. A wisteria will not flower until it is about ten years old, whatever the technique, and a seedling can take ten to fifteen years. A young tree mostly produces leaves and long shoots; a mature tree forms short flowering spurs. So you first develop the trunk and branches by giving the roots room, then, once the trunk is built, let the tree fill its pot with roots, which slows vegetative growth and encourages flowering.

The style serves the flowers rather than the foliage: informal upright, slanting or cascade, in medium to extra-large sizes, so the clusters can hang at their full length.

Wiring

Wire the branch structure after leaf drop, when the tree is bare. Choose aluminum wire about one-third the thickness of the branch, coiled at roughly 45°, and apply force from the inside of the bend to reduce the risk of splitting. Avoid bending the same branch repeatedly. During the growing season, check the wire regularly, since a branch can grow around it within one to four months, and cut it at each turn to remove it. For a branch that is too thick, use guy wires instead.

Repotting

Repot a young tree every two years and an older one every three to five years. To protect the flowers, repot right after flowering; repotting in early spring, before the buds open, is still possible but costs some bloom. Wisteria tolerates root pruning well, but do not prune an old tree’s roots hard. Bear in mind that a tree slightly cramped in its pot flowers better, while frequent repotting restarts leafy growth at the expense of flowers.

Wisteria is propagated from seed, cuttings, layers and grafts. Basal cuttings taken from side shoots in early to midsummer root with bottom heat. Ground layering is done in fall and air layering after flowering. Cuttings, layers and grafts flower sooner than seedlings.

Wisteria is generally vigorous and healthy. Powdery mildew and leaf spot can appear: remove affected leaves early. Also watch for aphids and brown scale, and cut out crown galls and cankers. The most dangerous pest is a boring insect that tunnels through the tree’s transport tissue and can kill it; it is hard to get rid of. Finally, wisteria is toxic to dogs, cats and horses, which can suffer vomiting, sometimes with blood, diarrhea and depression: keep the tree and its pods out of their reach.