Crape myrtle (Lagerstroemia indica) can bloom for two months or longer in the right conditions and with suitable care, but a shady site, excessive amounts of fertilizer, hard pruning and pest infestations often mean fewer flowers. Hardy in U.S. Department of Agriculture plant hardiness zones 6 through 9, crape myrtle grows 6 to 25 feet tall and 6 to 20 feet wide, depending on the cultivar. This deciduous shrub is invasive in some areas of the U.S.
Growing a crape myrtle in a partial shade or full shade reduces flowering. This shrub needs at least eight hours of sun a day during the growing season to promote blooming. Crape myrtle also grows best in well-draining loam or clay soil.
In late spring when the shrub's leaves are fully open, check the hours of direct sun it receives over one day. If the crape myrtle is exposed to less than eight hours of sun, remove sources of shade, such as overhanging branches from other shrubs and trees, if possible. Transplanting the shrub to a full-sun site should also improve flowering. Wait until spring the following year and move the shrub before new growth appears.
Overfertilizing a crape myrtle encourages it to produce leaves, not flowers. Excessive fertilizer also encourages soft, sappy growth that's attractive to pests.
Fertilize crape myrtle in early spring just before new growth appears. Apply a 12-4-8, slow-release, granular fertilizer at a rate of 4 tablespoons for each 4 square feet of ground. Evenly sprinkle the granules over the area beneath the shrub's canopy, and lightly rake them into the soil surface. Don't allow the fertilizer granules to touch the shrub's stem. Manufacturer's instructions vary from product to product. Read the fertilizer label and follow the instructions.
A crape myrtle growing in fertile soil may not need fertilizer. You can test the soil's fertility with a home soil test kit.
Pruning faded flowers and stem tips encourages more blooms in a crape myrtle. Sterilize pruning shear blades by wiping them with a cloth that was dipped in rubbing alcohol, before and after pruning this shrub.
When more than one-half of the crape myrtle's blooms have wilted, prune the final 3 to 4 inches of all the crape myrtle stems, which also removes faded blooms. Prune the stems just above a leaf.
Crape myrtle flowers on new wood, and severe pruning can delay or prevent flowering. Modern crape myrtle cultivars are disease-resistant and don't need pruning to improve air circulation and discourage diseases.
Aphids and other pests can cause severe damage on a crape myrtle, and reduce or prevent flowering. Aphids are small, sap-sucking insects that cluster on stem tips, the undersides of leaves and on flower buds.
Ready-to-use insecticidal soap spray treats a small crape myrtle infested with aphids. On a cloudy day, spray all the leaves and stems. Wear gloves, long sleeves, long pants and eye protection when you use the spray. Spray the shrub every week, if necessary, or according to label instructions. Spraying aphid clusters with high pressure jet of water from a garden hose helps treat a large crape myrtle. You can also discourage aphid infestations by only fertilizing if necessary.