While scores of crape myrtle cultivars exist, originally only the species Lagerstroemia indica and L. fauriei were available to gardeners. Europeans first brought back these newly encountered trees from Asia beginning in the 18th century. Through decades of selection and breeding by horticulturists, crape myrtles that mature anywhere from miniature shrubs only 18 inches tall to trees 30 feet tall are grown in the United States. Some cultivars of trees tend to grow much more quickly than shrubbier types, the result of an serendipitous genetic trait. However, healthy trees and those pampered after planting establish and grow faster in subsequent years.
Overall, crape myrtles grow at a moderate to fast growth rate, according to Clemson University horticulturists. This ambiguous classification does not help a gardener determine which cultivars grow more vigorously compared to others. Crape myrtle plants grow fastest and lushest when in full sun settings -- more than eight hours of uninterrupted sunlight each day -- and where the summers are hot. A fertile, moist but well-drained soil that is slightly acidic promotes better growth, but crape myrtles are remarkably tolerant of soil pH ranges. Cold or wet soils are not conducive for fast or lush growth. Dwarf shrubs do not produce new growth branches very long as compared to larger trees.
Some cultivars of crape myrtles grow faster than others, as based on the length of new branch growth or tree height measured each year in autumn. The U.S. National Arboretum's Lagerstroemia Checklist comments on numerous cultivars, with some demonstrating more vigorous growth rates than others. Consistently faster growing cultivars include Byer's Hardy Lavender, McLeod Pink, Natchez, Osage, Purpurea, Sarah's Favorite, Townhouse, Tuscaloosa Red and Regina. Numerous crape myrtle cultivars with trademark names also grow quickly, such as Whit II (Dynamite), Whit III (Pink Velour), Whit IV (Red Rocket) and Desand 081 (Souvenir d'Andre Desmartis).
Newly planted crape myrtles begin growing to their fullest, fastest growth rates once their root systems establish well in the surrounding soil environs. Although they are drought tolerant, irrigating young crape myrtles to keep the soil evenly moist during the heat of spring to fall promotes a larger root system. Healthier roots supply the water and nutrients to increase growth. Supplement natural rainfall with irrigation around newly planted crape myrtles for two years, especially during dry spells. Lightly fertilize in late winter and late spring, with a balanced, slow-release granular fertilizer -- 5-10-5 -- for the first two years as well. In the third year after planting, fertilization and irrigation isn't necessary, although irrigation in summer drought ensures that the trees will remain robust.
Severely cutting back crapemyrtle trees and shrubs in late winter results in voracious, rapid regrowth in spring. Regardless of cultivar, pruning into mature wood on lower branches or the trunks causes an injury response that forms watersprouts or suckers. Watersprouts grow extremely fast as a survival mechanism to begin photosynthesizing light for vital carbohydrate production. Crape myrtles, especially larger growing trees and shrubs, which are topped in winter produce much faster growth compared to trees not severely pruned back. Watersprout regrowth is weak-wooded, thick and broomlike and ugly compared to the natural branching silhouette of crape myrtles.