I could go on and on about how beautiful Crape Myrtle are and how no landscape where they will grow is complete without at least one. But this article is about feeding these trees. So here goes...
Evaluate Soil Conditions...
Crape Myrtle thrive in a slightly acid to acid soil with a pH between 5.0 and 6.5. With this somewhat narrow range, it might be a good idea to test the soil and make adjustments if necessary. You can purchase a test kit at your local nursery and garden center or buy a soil test kit online here. Or check with your local extension service to see if they provide soil testing services.
Feeding A Newly Planted Crape Myrtle
Before planting a new crape myrtle, it's helpful to amend the soil in the planting area with an organic material such as compost or a good planting mix or soil conditioner/amendment. Doing so will help boost the soils fertility and get the roots off to a good start. Do not add peat moss to clay soils as it retains too much water, which clay itself already does. How much amendment you add to the native soil removed from the planting hole will depend the type of soil and its fertility. When planting in heavy, compacted clay soils, I recommend thoroughly mixing in an amendment at a 50/50 ratio with the existing soil. Make sure to dig a much wider planting hole in clay soils - at least 2 to 3 times the width of the container the crape myrtle was growing in.
After planting a young crape myrtle I suggest watering it in with a solution of Root Stimulator. Follow instruictions on the product label for mixing and application. I also broadcast a dose of a slow-release, granular, "shrub and tree" fertilizer that contains micronutrients. Avoid using fertilizer that is too high in nitrogen (the first number in fertilizer) as this can cause accelerated growth that can weaken branches and also diminish flowering.
Maintenance Feeding Of Crape Myrtle
Fertilizing an established crape myrtle each spring helps the plant produce more blooms, keep the foliage nice and green, and maintain overall good health. Wait to feed until after new leaves have emerged and any chance of a late frost has passed. Again, I suggest using a slow-release shrub and tree type fertilizer containing micronutrients, avoiding the use of quick-release fertilizers that are high in nitrogen (the first number in fertilizer). Not only will too much nitrogen cause accelerated growth, you'll get more foliage and less flowers. Follow instructions on product label for application rates. Broadcast the fertilizer around the outside perimeter of the tree, where you would find most of the feeder roots.
Other Cultural Needs
Hardiness Zones: Most crape myrtles grow well in USDA Plant Hardiness Zones 7 through 9, though some will tolerate the colder temperatures in Zone 6.
Sun Needs: Crape myrtle perform and flower best in full sun.
Water Needs: During the first summer, newly planted crape myrtle should be watered enough to keep soil moist but not constantly soggy or wet. Though they tolerate drought fairly well when established, flowering will be best when plants are provided sufficient supplemental water during prolonged periods of dry weather.