How do I fertilize crepe myrtles?

before the leaf buds break open use a bloom booster with the higher center number.... and again, in midsummer if you trim off old blooms and ask it to bloom again..... if you allow the old blooms to go to seed, then wait until the seeds are set and then fertilize again with a fertilizer without huge first number.... you're looking for plant and root strength to go thru winter, not more leaves, like you'd get with a high first number and thus, a lot of Nitrogen.... 5-10-10 or thereabouts.....