Crape myrtles (Lagerstroemia spp.) demonstrate a high tolerance to drought conditions, although an evenly moist, well-drained soil that is occasionally irrigated promotes the fastest and lushest growth. Crape myrtles, like most other trees, do not have a large, carrotlike taproot. Neither do trees have a root structure that mirrors the size or shape of the branching canopy. Root systems of crape myrtles extend well beyond the reach of the branches.
Hundreds of different varieties or cultivars of crape myrtles exist. They range in size and habit from dwarf shrubs only 18 inches tall and 3 feet wide to medium-sized trees 30 feet tall and 35 feet wide. Therefore, depending on cultivar, the plant matures to any array of sizes. Larger-sized plants develop larger, more extensive root systems compared to the smaller shrub crape myrtles. Regardless of crape myrtle cultivar, plants develop numerous lateral, shallow roots that radiate outward from the plant.
Root Zone Depth
A healthy growing crape myrtle tree creates an expansive root zone in the top 6 to 24 inches of soil. In denser clay soils, the roots may not penetrate as deeply as compared to the more porous, faster draining sandy soil types. Crape myrtle roots often dwell near the soil surface and give rise to suckering shoots, called water sprouts. An occasional root does angle downward to act as more of a stabilizing or anchoring root, mainly in taller crape myrtles that experience wind resistance.
Root Zone Size
Expect roots from a crape myrtle to grow well beyond the farther reach of the branch tips. Actively growing roots do not remain under the canopy of the plant since the canopy blocks rainfall to the soil underneath. The roots grow outward, seeking new soil that is richer in moisture and nutrients. Crape myrtle roots grow two to four times the width of the plant's canopy. For example, a crape myrtle with leafy canopy 10 feet wide can have a root system that measures 20 to 40 feet across, with the tree trunk in the middle.
Root System Insight
Dwarf shrub crape myrtles may not develop wide root systems, as compared to tree forms, since there is less leaf and twig mass to support with water and nutrients. However, still expect roots to grow several feet beyond the branch tips even on miniature shrub cultivars. All crape myrtles are well-adapted to average fertility soils, but fertilizing young and newly planted crape myrtles does encourage better root establishment and initial growth of leaves and new branches. Since roots are generally wide spreading, special fertilization applications aren't always warranted for older crape myrtles, as the roots invade into nearby lawns and other garden beds where other fertilizers or organic materials such as compost and mulch may exist.