I have three 1 1/2" caliper, 7-8', multi-trunk crepe myrtles that need to be moved by next summer. I've been told not to prune top growth till February. I should have root pruned this past growing season. Is it too late to root prune now?
In general, root pruning should be done when the trees are dormant. Since you need to move them by next summer, you should plan to transplant these trees in March (a good time to transplant crape myrtles) and do your root pruning this fall AFTER the leaves have dropped (which is probably now). The soil is still warm now and feeder roots will begin to grow and continue to grow through the winter.
If you were planning to transplant them in the fall, the time to root prune would be in the previous spring before growth begins.
It is not recommended to root prune during the growing season because the tree becomes prone to water stress since you are removing most of the feeder roots when you root prune.
It is a good idea to water the trees well a day or two before you do the root pruning. This will ensure that the soil is softer and the roots are turgid. Be sure to use a sharp shovel so the roots are cut cleanly and not mashed during the process.
Fertilize with Plant-tone, Holly-tone, or Tree-tone after root pruning.