I would not plant it beside an existing tree. If allowed to wrap around the trunk a Wisteria can tend to "strangle" the tree as the tree grows larger in diameter. With a tree that is already close to mature this concern is not much of a factor. However another reason to avoid allowing a wisteria to grow up into a tree is that it will reach the top of the tree's canopy and spread out shading out much of the tree in time. Even so the blooms will be so high that you wouldn't be able to enjoy them as much.
Wisteria can be trained to a single trunk on a post and then allowed to branch and develop a tree-like canopy at the top of the post. The long gangly vines are trimmed back some during the summer season and then it is also usually pruned a little more in late winter to keep the canopy in bounds. This "tree form" style of training is popular as it keeps this rampant vine in a tidier, manageable form and brings the attractive blooms down to blooming height.
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