In 2011 we built a pergola in our backyard and purposely planted a wisteria beside it so it would weave its way up one of the posts and across the latticework up top. Over a decade later, the wisteria’s well-established foundational limbs do indeed encircle one of the posts in several spots and provide much shade. This year, however, it became apparent that several posts are leaning and the pergola needs to be rebuilt. Is there any way we can safely save the wisteria — and how? The pergola is 8-feet high; its limbs begin to use their post for support at about a foot off the ground, twisting around it about ⅓ and ⅔ of the way up.
You have a challenge before you. But, the good news is that wisteria grows very rapidly and will come back from the root quickly even if killed off above ground! I had an ancient one that finally rotted away and within 2 years, it has rejuvenated to climb up the metal posts and cover the trellis … and it bloomed this year!
So don’t be afraid. We had a similar inquiry from another gardener and this is what we advised, “Time the replacement work for early September. Just before the contractors are due to begin, cut back the wisteria to about 36” and place boards over the area where the roots are growing so the contractors will not compact the soil. Water well when the work is completed.”
Another person with a similar challenge suggested using wood braces to prop up the Wisteria, cutting out pieces of the arbor bit by bit and building a new one exactly where the old one was. I think that would be a bit tricky to dig new holes and pour concrete footings in exactly the same spot.
I would recommend that the new pergola posts be built outside the existing footprint, and that you should tie the vines to the posts to start them off, but remove the ties as soon as new growth appears.
In any case, the wisteria is unlikely to die.
More importantly, as you have learned, you must ensure that the new structure is strong enough to support the wisteria, since they are notorious for destroying arbors. Mature Wisteria vines can grow up to 75-100 feet long, becoming exceptionally heavy!
Proper pruning is essential. Here is the guidance we have provided to others:
“Wisteria can indeed grow out of control if not pruned on a regular basis. Two good prunings per year should keep the plant healthy and flowering happily. These are best done, once in mid-summer just after flowering and again in mid-September when vigorous growth has stopped.
For both of these prunings, you should prune the entire plant back following the smaller branches back to where they join a larger branch. On the smaller branches, close to the joining, you will find nodes or buds. In mid summer, you want to cut the smaller branch leaving one or two of these nodes/buds as these will be where the plant will continue to grow. In the mid-September pruning you will leave four or five nodes/buds.
Overall, you need to keep in mind the shape of the plant, keep vigorous strong shoots and cut away branches that hang down. Dead, dangerous & diseased branches need to be removed as do overcrowded stems. When making your cuts, try and find a down facing bud/node and make your cut one half – one inch beside it, leaving the bud/node on the branch to form new growth.
Every third year, give the plant a more severe pruning as this will open up the plant, help keep it in the desired shape, allow light to penetrate the plant, encourage continuous flowering and take some weight off the support structure. Renewal pruning is usually done right after flowering and cutting back to within 7.5 centimetres of the main stem is advisable. Any suckers that grow from the root of the plant must also be taken off as many varieties of Wisteria are grafted onto rootstock and these suckers can crowd out the good plant. Also, remove any seed pods as you want the plant to put its energy into growth and flower production.
Wisterias bloom on the lower 30 centimetres of last years growth so by pruning back a couple of times a season, you can create a multi-branched stem with lots of flowers. Make sure to keep your cutting tools sharp and make clean cuts. Sterilizing tools when pruning will also cut down on disease spread in the garden.”
Good luck with the construction and the rejuventaion of your wisteria vine!