Outdoor Classroom – pruning wisteria

Wisterias are vigorous vines which lose all their leaves in winter. You can not plant them and leave them. If you are not going to prune them at least once a year, you may be wiser to take out the whole plant. After several years of less than thorough pruning, this particular plant had multiple runners which had escaped and run along both the base and the top of the block wall for at least fifteen metres. Planted against a house they will split the spouting if left unchecked.

Wisterias flower on old growth so you can’t cut them off at the base and get flowers this season. Look at the plant and decide the shape you want. Check the old stems for borer and rot. Wisterias are vulnerable to the borer larvae. Cut out any bad damage. Choose which stems and canes you wish to keep. You probably don’t need to keep them all.

Take out all thin or surplus canes and growths, starting from the base of the plant. Some wisterias are grafted. If you can see where the graft is, you must cut off any growths below that because they will be from the vigorous root stock which will be a stronger grower. This is a cutting grown plant so it does not apply. Don’t put all your trust in one trunk only. It always pays to train a replacement alongside it. The older and more gnarled the trunk, the more chance of borer and rot taking hold.

Further up the plant, decide your central framework (the shape, or skeleton of the plant) and shorten all minor growths to two to four spurs (leaf buds). This is the same principle to pruning an apple tree. You can use the prunings to weave supports for other plants in the garden because they are flexible and they won’t take root easily.

Look for tell tale borer holes in remaining stems and treat these. Spraying kitchen oil or fly spray down the hole can work.

There are two main groups of wisterias, the Chinese ones (“sinensis” which just means from China) and the Japanese ones (floribunda). The Chinese ones usually have finer leaves and they flower on bare wood before the spring foliage appears. Japanese ones tend to have longer flower racemes to compensate for the fact they flower with their new growth. As a relatively random piece of information, the Chinese ones twine anti clockwise whereas the Japanese ones twine clockwise.