Crape myrtles produce their flower buds on active new growth. Therefore, you’ll want to apply a high-nitrogen fertilizer that will stimulate that new growth. Many different analyses are available, many of them categorized as “lawn foods.” Curious, isn’t it, that the same fertilizer you apply to your lawn to make it green will also result in the maximum number of flowers in your crape myrtles.
Follow the product’s instructions on amount of fertilizer to use. However, for most inorganic products you’ll be somewhere in the range of one pound of fertilizer per accumulated inch of trunk diameter at ground level. For dwarf types you’ll want 1 pound per 100 square feet of bed space. Follow each feeding with a thorough watering.
First application should be as the buds are breaking and new growth is beginning, probably late March into early April. The second application should be 6 to 8 weeks later, most likely in late May or early June. A third and final feeding can be made in late summer (mid- to late August) to promote a flush of fall growth.