You can see that in the drawing above. Typically, the root system of Little Blue Stem (sixth from right) is a little over 6 feet deep and Purple Coneflower is roughly 5 feet deep. By comparison, the roots of Bermuda grass go less than a foot deep, and St.Augustine roots are about 2 inches deep - so they demand a lot of expensive water to stay green.
As you know, a Texas summer bakes the ground and kills shallow-rooted plants. But native plants have grown here naturally for centuries, without anyone watering or fertilizing them.