When to Prune
Winter (when the plant is dormant), with some summer trimming to help keep the winter pruning more manageable.
Flowers/Fruits on New or Old Wood
Wisteria flowers on the previous year's growth, i.e., old wood.
How & What to Prune
Winter: At the outset, cut out the dead, diseased and damaged vines. After their removal, the spacing of the main vines is our priority, so that the side shoots have room to produce buds that flower. There is no magic number as to how far apart the main vines should be, but approximately 12” works well. (See diagram A). If the vines are well established, they will not be easy to move apart, but do your best to separate them. Here is where the pruning comes in.
If you have a group of vines that are close together, remove enough of the vines at their point of origin until you have a well-spaced structure. If the wisteria hasn’t been pruned in a while, you will be removing a great deal of material. Wisteria is a resilient plant, so don’t be bashful in your pruning. To maintain the spacing, you will need to tie the main vines to the structure supporting them.
Once you have addressed the spacing of the main vines, we will now turn to the side shoots on which the buds will form. Often the side shoots will develop into a slingshot shape, with the shoot growing away from the main vine and into the shape of a “Y,” with the vines extending many feet until they are pruned back. (See diagram B) Space the side shoots approximately 12 inches apart, again to provide room for the flowers. You will notice buds forming along the base of the “Y” shaped spurs. To encourage development of the buds, prune the side shoot vines back to 2 to 3 buds. (See diagram C) This will concentrate the wisteria’s nutrients into the formation of buds along the spurs. Again, you will be removing a lot of material if your wisteria hasn’t been pruned recently.
The result of the pruning will be an orderly structure that results in cascading flowers in the spring. You can make your winter pruning much easier if you do a quick pruning in the summer. At that time prune the side shoots extending from the main vines to approximately 12 inches. (See diagram D) This will prevent the vines from finding their way into places they should not be, and it will concentrate the nutrients in the areas where you want buds to form.