Landscape Management and Culture
Lagerstroemia plants grow well in most soils, tolerating a relatively wide range of physical and chemical characteristics. A main requirement is full sun, as insufficient light exposure and temperatures below 70oF impair or limit flowering. Crape myrtle plants require irrigation for a period following transplant, but once established they can thrive with minimum irrigation, and are known to tolerate moderate lengthy drought conditions (very common in Texas).
Although high and frequent fertilizer applications are often employed during nursery production, landscape established Lagerstroemia plants are known to adjust their growth and internal nutrient reserves to most existing soil fertility conditions in such a way that their foliage and flowering remains attractive. Minimal fertilization of landscape Lagerstroemia is often recommended, and in particular the avoidance of excessive nitrogen levels, often present in mixed landscapes with intensively managed turfgrass areas. Our research indicates that excessive nitrogen fertilization could delay flowering and cause undesirable nutrient imbalances in the plant.
Nutrient deficiencies or disorders, like the chlorosis shown in this Lagerstroemia indica plant, are rare, as these plants are fairly adaptable to most soil physical and chemical characteristics.
In our research studies and trials with crape myrtles in Dallas, Texas we have observed this leaf disorder, and to this date we do not know if it is caused by a nutrient deficiency or imbalance, or if it is caused by an environmental stress like excessive heat. This disorder is only observed in container-grown plants, and it only affects cultivars of Lagerstroemia fauriei and the interspecific hybrids of L. indica x L. fauriei. We have not yet observed these symptoms in cultivars of L. indica.
Some ornamental Lagerstroemia cultivars are reported to be cold hardy to USDA Zone 6b (-20oC = -5oF) and perhaps even some slightly colder conditions, but without protective measures this often represents the death of the aerial parts every winter, with subsequent re-sprouting the following spring. Reliable above-ground hardiness, without winter protection measures, is guaranteed to climates no colder than Zone 7b (-15o C = 5oF). While many of the L. fauriei hybrids are considered to be hardier than L. indica cultivars, comparative cold hardiness studies have shown inconsistent patterns, likely the result of changing environmental factors, cultural conditions and pruning practices.
Some crape myrtle cultivars are not above-ground cold hardy in some areas, but as long as the root system is not damaged, they will grow (sprout) back every spring, and will be able to flower quite well within that season, thus behaving like an herbaceous perennial plant.