Answer: Crape Myrtles bloom on new wood or new growth. Therefore, anything that you can do to make the crape myrtle grow once it leafs out in the spring will result in more flowers in the current year. You pruned in the fall, which encouraged new growth during the time of the year when the tree does not flower. You'd be better off to prune just as the buds begin to swell in the early spring. Many people including some professional landscapers just arbitrarily pick a height(for example, 6') that they want their tree to be and every spring they get up there with a hedge trimmer or chain saw or whatever and "whack off" everything above 6'.
The plant then makes this flush of new growth and covers itself with flowers. Many people prefer this way and think its the way a crape myrtle should be pruned. If you have a group of plants in a line along a driveway this method makes the plants very uniform in height.
While there is nothing wrong with this method, there's another technique called "natural pruning". In natural pruning of crape myrtles you leave about 4-8 of the strongest branches and remove or prune the weaker spindly branches. When you have removed all the inside weak branches and step back and look at the tree it almost looks like a tall bonsai tree. Either pruning method should be used in early spring.
Put away your pruning shears until next spring, then give your tree a haircut and it should bloom its little head off for you. Enjoy!
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