Ask Mr. Smarty Plants

While Lagerstroemia indica (crape myrtle) is not native to North America and therefore out of our range of expertise, aphids do not discriminate. The aphids are a nuisance, and you don't want to park a car or even stand very long under a crape myrtle infested with them, but they really don't do any damage to the tree. You're right, a hard spray of water would probably knock off more blossoms than aphids. Depending on how much it is bothering you and how big the tree is, you could try spraying it with a weak solution of Safer insecticide, or even soapy water. 

Stenotaphrum secundatum (St. Augustine grass) is also non-native to North America and although widely used, is not really suitable for Austin, especially in a time of extreme heat and drought. It needs water and shade. It is fairly drought-tolerant when it has become well-established. If your watering is restricted and you get some brown areas in the lawn, they will still probably come back when (and if) we get some rain. Even if the grass in a particular area does not green up next Spring, the stolons around it will spread back into the area.