Ask Mr. Smarty Plants

If only. The grasses you would like to use are all excellent choices. Native American Seed in Junction, TX has a Native Sun Turfgrass seed mix consisting of 66% Bouteloua dactyloides (buffalograss) and  34% Bouteloua gracilis (blue grama).  Hilaria belangeri (curly-mesquite) is not usually sold as a turf grass, as it is coarser in texture than the other two. 

One of the problems with intermixing these native grasses with the non-native St. Augustine  is that St. Augustine is often used because it is shade tolerant, which the two native grasses are not. They require full sun, which we consider to be 6 or more hours of sun a day. So, if you have trees in your lawn, or portions of your house shade some of your lawn, the two native grasses are not going to compete.  If the St. Augustine is left in place, it will shade out the seedlings or plugs of the native grasses you put in, and defeat your purpose. Further, the St. Augustine, as you no doubt know, needs a LOT of water, and the buffalograss and blue grama do not. They will need some watering when they first emerge until they get established, but one of the virtues of the natives is that you will have to water less, as well as fertilize less or none. 

You may have already read it, but we would like to refer you to our How-To Article Native Lawns; Buffalograss for more information on preparing the area and planting. 

From our Native Plant Image Gallery:

Bouteloua dactyloides

Bouteloua gracilis

Hilaria belangeri