Wisteria is a vine that will take over an area if it is not controlled by pruning. Because the wisteria can grow very large, it can become a threat to any building or structure on which it grows. The speed at which wisteria grows means that pruning once a year is not enough.
Step 1 – Wasted Growth
Identify shoots that are simply going to provide growth and expansion for the vine, not flowers. These will grow very quickly and will extend from the vine with no sign of blossom – almost like a sucker. Because these stems tend to be outward growing, they can create quite problem in high winds. Cut all such shoots back to about a foot in length. This will encourage them to produce flowers next year.
Step 2 – Remove Shoots in Restricted Spaces
Wisteria shoots that have grown through the gaps between down pipes and walls will swell and eventually force the down pipe off the wall. Always cut these shoots back. Train the wisteria around the down pipes if you want them in that area. Keep the wisteria out of guttering and off light weight roofs where the weight alone can be enough to do damage. To maintain your light, prune overgrown wisteria away from windows - the dense foliage can cut out the light very effectively.
Step 3 – Cut Back Side Shoots
Cut back all the side shoots to leave four to six leaves except any that you are going to train. If you don’t want the wisteria to keep growing in the direction it has started in, you can either train the main stem back on itself or prune it and remove the growing tip. This will encourage more lateral growth.
Step 4 – January or February
Pruning in January or February while the wisteria is dormant will give you the chance to decide just how big you want the wisteria to grow and to increase the probable flower production in the coming year.
Step 5 – Thin Out Main Shoots
When the leaves are off the wisteria it is possible to see the great weight of shoots that the wisteria has produced. Choose one or two main shoots. Cut out the other shoots, especially if you can see that they have grown a significant length with little side shoot growth. Prune back the side shoots to two or three buds. The side shoots are where the flower bearing stems will originate so although this might seem a drastic amount of pruning, it will give a huge improvement in the number and size of flowers produced
The plant has to be controlled or it will take over. As the plant expands the sheer bulk and weight of the stems spread over a large area can become a hazard.
To produce a greater number of bigger flowers, the plant has to use more of its energy to produce flowers than simple woody growth.