Extremely tolerant of hot summer temperatures, baking direct sun and dry soil, crape myrtles (Lagerstroemia spp.) still manage to bloom profusely in summer and make low maintenance plants in the southern United States. Crape myrtles -- in both shrub and tree forms -- include dozens of cultivars that are best grown in U.S. Department of Agriculture plant hardiness zones 7 through 10. Never plant them in wet, soggy soils. They need a well-drained soil to prosper, although more watering is needed after first planted.
Crape myrtles need irrigation really only under two circumstances. The first is maintaining an evenly moist soil for the first two years after transplanting the tree into the landscape. The second circumstance is during periods of excessive drought. Once a newly planted crape myrtle develops and establishes a good root system two years after planting, it survives perfectly on natural rainfall alone. Only when soil becomes bone dry in a seasonal drought and lower leaves show signs of withering or yellowing does a deep irrigation become necessary.
The amount of water needed to appropriately water a crape myrtle varies based on the size of the tree and the type of soil. A general rule in applying enough water is to moisten the top 12 to 24 inches of soil in the root zone. In newly planted trees, the root zone includes the original root ball and an area about 2 to 4 feet outward in all directions from the trunk. Larger, established trees extend their roots 10 to 30 feet beyond the reach of the branch tips. Sandy soils drain faster and may need more water to wet the top 2 feet of soil; clay and loamy soils hold more water.
Crape myrtle trees need the most moisture in the soil profile when they are actively growing -- when the plant displays leaves. Once the leaves naturally drop away in fall, they don't require irrigation when dormant unless they're newly planted or the soil becomes unusually dry in winter. You may water anytime the soil is not frozen in winter, if necessary. Overwatering crape myrtles in spring and summer can diminish flowering, since overly moist soil promotes leafy growth, not flower production.
Placing a 2- to 4-inch layer of organic mulch over the root zone of a crape myrtle tree provides numerous benefits. Foremost, the mulch shades the soil and helps conserve soil moisture, diminishing the need to water the tree. Often, natural rainfall maintains an appropriate soil moisture level for all established trees, and the mulch layer keeps the soil barely moist -- perfect for crape myrtles. The mulch also reduces evaporation of water during hot weather and slows the rate of soil drying during seasonal droughts.