Once established, crape myrtles are among our most drought-tolerant shrubs and small trees. However, you will want to hand-water new plants for their first couple of summers. Sprinkler irrigation alone will not be sufficient to maintain them in good vigor. Young plants that are allowed to get too dry will grow and flower lethargically.
Use soaker hoses, bubblers or drip irrigation emitters. You can also take the sprinkler off the end of your hose and water your plants directly. Create a berm of soil as you plant the new crape myrtles, then irrigate within it. Soak the soil deeply, then wait until the plants begin to show signs of drying before watering again.
Keep water off the foliage and flowers as much as you can. Not only will it weight the floral heads down, but moisture can also accelerate development of powdery mildew. To that end it’s a good idea to plant crape myrtles where air circulation is good.