Ask Mr. Smarty Plants

Sorry, but there is no definitive answer to your question. 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 raised system, 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 away. 

We are going to go to Recommended Species for Kansas, select first on herbs (herbaceous flowering plants) in Habit and annual in Duration. We will then do the same thing in grasses, but not indicate a duration. Even perennial grasses are going to maintain a root system that is compatible with the septic system. If your area has a lot of shade, or you are interested in other species, you can do the same thing, putting in the different characteristics (like bloom time and color, for instance) and make your own selections. We included those on your list, all of which are native to Kansas and excellent choices.

Grasses for Kansas

Sorghastrum nutans (Indiangrass)

Andropogon ternarius (splitbeard bluestem)

Aristida purpurea (purple threeawn)

Bouteloua curtipendula (sideoats grama)

Bothriochloa laguroides ssp. torreyana (silver beardgrass)

Carex blanda (eastern woodland sedge)

Calamagrostis canadensis (bluejoint)

Chasmanthium latifolium (Inland sea oats)

Wildflowers for Kansas

Ratibida columnifera (upright prairie coneflower)

Rudbeckia hirta var. pulcherrima (blackeyed Susan)

Dalea purpurea (purple prairie clover)

Amphiachyris dracunculoides (prairie broomweed)

Aphanostephus skirrhobasis (Arkansas dozedaisy)

Chamaecrista fasciculata var. fasciculata (partridge pea)

Coreopsis tinctoria (golden tickseed)

Gaillardia pulchella (firewheel)

Sorghastrum nutans

Andropogon ternarius

Aristida purpurea

Bouteloua curtipendula

Bothriochloa laguroides ssp. torreyana

Carex blanda

Calamagrostis canadensis

Chasmanthium latifolium

Ratibida columnifera

Rudbeckia hirta var. pulcherrima

Dalea purpurea

Amphiachyris dracunculoides

Aphanostephus skirrhobasis

Chamaecrista fasciculata var. fasciculata

Coreopsis tinctoria

Gaillardia pulchella