Fertilize a new crape myrtle in early spring, beginning in March or early April. Add 1 teaspoon of a general-purpose fertilize once a month until August, spreading the fertilizer evenly over the soil around the plant. Keep the fertilizer localized to the planting hole to avoid getting the fertilizer too close to the base of the plant.
Fertilize mature crape myrtles in early spring when the plant begins to actively grow. Fertilize again in early summer to encourage blooming. Well-established crape myrtles that are large and several years old need only need the first round of fertilizer in spring, but you can fertilize again in summer if desired.
The amount of fertilizer depends on the type of concentration of the fertilizer product used. In general, a crape myrtle needs 1 pound of fertilizer for every inch of trunk, or 1/2 pound of fertilizer for every 100 square feet of canopy spread. Measure the diameter of each trunk that emerges from the ground in inches and add these measurements together to obtain the total trunk diameter. Measure from the center of the plant to one outside edge of canopy in feet to the other to obtain the radius of the canopy. Multiply the radius by itself and then multiply by 3.14 to determine the number of square feet of the canopy. You can round the numbers to simplify the calculations, but err on the low side to avoid adding too much fertilizer.
Fertilize when you expect rain the same day or the following day when possible. If you do not expect rain, water thoroughly after adding fertilizer. Soak the ground where you spread the fertilizer to allow the fertilizer to penetrate to the roots of the plant. Use an irrigation system or garden hose to water the crape myrtle after each fertilization.