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Autumn /Early Winter Pruning of Wisterias
October is the optimal time to prune wisterias. It will bring the rapid summer growth into some manner of control and allow for easier pruning in the early spring.
Prune your wisteria in the autumn by pruning all of the current side shoot growths back to within 12-16 inches of the main lateral (side shoot) branches. This should leave 4-6 leaves or leaf buds on each side shoot. Any side shoot required to extend or train the framework should be left unpruned and trained as desired.
Late Winter Pruning of Wisterias
In January/February, but no later than early March, cut back the wisteria’s side shoots back even further between 2 - 4 inches long, leaving only 2 or 3 buds on the side shoot. These shoots will be the flowering spurs. The only shoots to be left untouched are those which are required to extend the size or direction of your wisteria. Both of these pruning regimens should be done each year.
Note, if you did not do any lateral training of the new lateral framework in autumn it needs to be done at this time.
How to prune wisteria side shoots to encourage flowering instead of new stem shoots.
When the lateral shoots are ready to be cut back hard in the late winter pruning (January/February), the pruning cut is done immediately above a bud. This will allow the flower buds (which are on the older wood nearer to the main stem) to receive the food reserves that are just beginning to travel up through the vine at this time.
Note, if these shoots are left unpruned not only will they become an unruly mess, all the nourishment would be allowed to go directly up through the shoot to make more growth and produce more leaves. This would also hide the flowers and deprive the flower buds of the food reserves.