Feed your crape myrtle with high nitrogen fertilizer in the spring a soon as the leaves appear. Make another application two months later and then don't feed for the rest of the year.
Overfeeding can cause the plant to put its energy into growth at the expense of blooms. Use slow release fertilizer to prevent the growth of suckers that are susceptible to disease and insects. If suckers do appear, prune them off.
just concern yourself with ensuring that the trees get plenty of water. Theyll drop those leaves when winter hits. Cut em back right around mardi gras and theyll explode late spring/early summer with new growth.