When deciding the best time to prune or trim a Crepe Myrtle tree, you must first decide what exactly you hope to achieve from it. This is important because, depending on the severity of the cuts and in the intensity of the pruning, the time in which it should be done varies.
General maintenance, such as the pruning away of dead and diseased wood and trimming to maintain shape, is generally recommended in late winter or early spring. The tree will be in its dormancy during this period, making it less susceptible to shock. At this time, take measures to trim you plant in a way that will increase its health for the coming spring season. At all costs, avoid this type of pruning before plants go dormant.
Another form of pruning is used to encourage a second round of blooms. This pruning is very superficial and mainly consists of removing dead and wilted flowers to encourage the tree to produce more. It is more beneficial to wait until most of the first round of blooms has died to begin removing them. It is absolutely essential that this practice end at the start of August. If you continue to deadhead your plant after this point, it will encourage the tree to try and create a new round of blooms, potentially delaying its dormancy. At this time, the tree should be shutting down and preparing itself for winter. By deadheading, the tree actually revs up its production rather than slows it down, making it more susceptible to damage come winter's first frost.
Therefore, in general, severe trimming and general maintenance should be done after the plant has gone dormant or before it comes out of dormancy. Simple procedures, such as deadheading, can be done throughout the flowering season but should end as August approaches.