Ask Mr. Smarty Plants

Please read this previous answer, also on septic fields in Austin. Your requirements do not totally duplicate those on the previous answer, but it will give you a good place to start. Excerpted from another previous answer:

"So far as we are able to determine, there is no list or database on root lengths of any plants, including natives. The root length of any plant is variable, depending on the plant size, genetics and age as well as environmental conditions. Keep in mind that when your system was designed, it was a well-known fact that something would grow over it-if not planted by you, then weedy volunteeers. If those volunteers are not eliminated, some of them are bound to be woody plants, and that's what you're trying to avoid. 

All the research we consulted agrees that the best plant material for the earth covering septic systems is grass. They have fibrous roots and will help hold the soil in place, help prevent erosion on your slope, conduct some of the moisture to the surface of the soil, and are easily maintained. Wildflowers will fall into the same category, particularly the annuals. The one thing you want to avoid is woody plants, especially trees, as they develop long roots that stretch out beyond the canopy of the tree and would go for the moisture in the septic lines. Those roots can certainly clog the lines, so keep the trees and shrubs away."

Something that would be really nice for the area is a Meadow Garden. See our How-To Article on Meadow Gardening. Because you already have plants you do not want, particularly bermudagrass, we also suggest you go to our How-To Article A Guide to Native Plant Gardening, paying special attention to the "Soil Preparation" portion.

Most of what you need to know has already been covered in the material from previous answers and the How-To Articles. But, just so we will earn our (non) pay, we'll list a few wildflowers and grasses that we think would be good for your site. Please realize that sowing the wildflower seeds should not be done until Fall in the Austin area, and that just throwing out seeds is not going to control the vegetation you already have there. In fact, it works the other way around, you have to first control the non-wanted plants before you seed, or most of those wildflower seeds will never come up.  We will go to our Recommended Species section, click on Central Texas on the map, and then select first on "herbs" (herbaceous blooming plants) under General Appearance and then, on a separate search on "grasses and grass-like plants." Another option is the Lady Bird Johnson Legacy Wildflower Mix, if you want to try just throwing out some seeds and see what comes up. There are also a number of plant suggestions in the Meadow Gardening How-To Article. Follow the link of any plant you are interested in and learn whether it is annual or perennial, size, light requirement, etc.

Native Grasses for a Septic Field in Austin: 

Muhlenbergia lindheimeri (Lindheimer's muhly)

Muhlenbergia reverchonii (seep muhly)

Nolina texana (Texas sacahuista)

Schizachyrium scoparium (little bluestem)

Herbaceous Blooming Plants for a Septic Field in Austin:  

Callirhoe involucrata (purple poppymallow)

Coreopsis lanceolata (lanceleaf tickseed)

Echinacea purpurea (eastern purple coneflower)

Melampodium leucanthum (plains blackfoot)

From our Native Plant Image Gallery: