The Wisteria Bonsai: A Cascade of Lilac Flowers

Wisteria (Wisteria floribunda or sinensis) is the spring bonsai par excellence. When it blooms, the coloured racemes (lilac, white or pink) hanging from the bare branches are a breathtaking sight. It is an extremely vigorous climber, almost invasive in the wild, which makes it easy to grow but demanding to keep in check as a bonsai. It is not grown for fine ramification (which is coarse), but for the elegance of the flowers and the charm of the twisted trunk.

Origin & species

Native to China and Japan (W. Sinensis and W. Floribunda respectively), it belongs to the Fabaceae family (legumes). The Japanese variety has longer, more elegant racemes and is often used for medium/large size bonsai. The trunk ages beautifully, becoming fluted and muscular.

Position and microclimate

Wisteria is a sun worshipper.
Full sun at all times. Sun is the fuel needed to set the flower buds for the following year. If you keep it in the shade, you will get plenty of green leaves and no flowers.
It does not mind heat, but in summer it drinks like a sponge.

Watering (the drip tray trick)

In summer, Wisteria is one of the very few exceptions to the "no standing water" rule.
During flowering and in the hot months (June-August) it drinks so much that keeping up with a watering can alone is almost impossible.
Tip: keep the pot standing in a tray with 2-3 cm of water during the scorching summer days. The roots will drink it all within the day. Remove the tray as soon as temperatures drop, and in winter.

Water quality

It is not fussy, but it appreciates abundant water. Hard water is not ideal in the long run, but it tolerates it better than a maple does.

Soil mix

A standard mix, but one that holds water.

• Recommended mix: 60% Akadama (or fine expanded clay) and 40% universal soil mix.
• Do not use pumice or pure akadama alone unless you can water three times a day in summer.

Repotting

Repot often (every 1-2 years for young plants, 2-3 for old ones) because the roots fill the pot in no time.
When: in autumn (after leaf drop) or in very early spring.
Be drastic with the thick roots, Wisteria flushes out new growth regardless.

Fertilizing

Strategy is essential for flowers.
Spring (after flowering): balanced fertilizer to recover energy.
Summer/autumn: fertilizer rich in phosphorus (P) and potassium (K) and low in nitrogen. Excess nitrogen favours leaves at the expense of flowers.

Flowering and fruiting

It flowers in April-May. The flowers appear before the leaves.
After flowering, long velvety pods form: remove them at once! Producing seed costs the tree a great deal of energy and may compromise the following year's flowering.

Pruning

Wisteria throws out shoots metres long in a few weeks.
Summer pruning: keep shortening the long shoots back to 2-3 buds, to maintain the shape.
Winter pruning (essential): in January-February, shorten the current year's branches leaving only 2-3 buds. The flower buds are the large round ones at the base of the branches; the pointed ones are leaf buds. Do not cut the round ones!

Wiring & bending

Wire is used little because the branches thicken quickly and the wire bites in immediately. It is better to shape by pruning ("clip and grow"). If you do use wire, apply it to young shoots in summer and check it weekly.

Diseases and pests

Very tough, but it attracts:

• Aphids: on the buds and tender shoots.
• Rust or powdery mildew: on the leaves late in the season.

Winter & protection

It is quite cold hardy (down to -10°C in the ground), but in a pot it is better to protect the roots from hard frosts, which could damage the already formed future flowers.

Yearly calendar

• January-February: pruning and selection of flower buds.
• March-April: flowering (water plenty!).
• May: removal of spent flowers and pods, start fertilizing.
• June-August: tray with water, continuous pinching of the long shoots.
• September: remove the tray, PK fertilizing.
• November-December: dormancy.

Useful products for this bonsai

• Rectangular drip tray (for summer hydration)
• Flowering fertilizer (high phosphorus content)
• Long scissors (to trim the shoots quickly)
• Copper-coloured aluminium wire (for small adjustments)
• Enriched universal soil mix (to hold water)
• Insecticide for aphids (to protect the buds)

Frequently asked questions about Wisteria

Why does my wisteria never flower?
Common causes: the plant is too young (from seed it takes 10-15 years, from cutting/graft it is quick), too much nitrogen in the fertilizer, or incorrect winter pruning that removed the flower buds.

Can I keep it indoors?
No, Wisteria is an outdoor plant. Indoors it would die from lack of light and winter dormancy.

Do I have to repot every year?
Yes, if it is young. The roots are so powerful that they often crack the pot if they have no room. Check in spring whether the root ball is too compact.