Question
When is the best time to girdle a tree?  especially thorny locust.

Answer
You can girdle the tree at any time if you want to kill it. IF you do this in the spring you can see the results within a week or so by the foliage dying. IF done in the dormant season with no leaves you will not see much results until next spring at bud out. You do under stand that you will still have a standing dead tree to deal with after you girdle it. IF you want to get rid of a tree I would just cut it down and treat the stump with a herbicide called Roundup to kill the root system to stop it from sprouting from the roots.