Answer: Robinia pseudoacacia 'Twisty Baby' is a black locust, popular because of its twisted branches and trunk. Unfortunately, locust trees do not take well to pruning. Established trees are best left unpruned; large cuts rarely heal well and are prone to rot. After pruning, the tree may respond with a profusion of vigorous, but structurally weak vertical "water shoots".
If you'd still like to try pruning your 'Twisty Baby', wait until mid-winter when sap flow is at its lowest; cut the branches back by no more than one-third; then remove any dead, diseased, or misplaced branches.
If your timing is right and the cuts are small, your Robina should recover. If water sprouts (upright slender growth) appears, cut them off as soon as possible.
Best wishes with your 'Twisty Baby'.
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