Ask Mr. Smarty Plants

I recently had an experience much like yours.  I replaced the St. Augustilne grass in part of my lawn with Habiturf, the mixture developed at the Wildflower Center for water conservation.  Habiturf does not thrive in shade, which I have in one part of the lawn.  So I planted in that part a number of mowable forbs and sedges that grow in my neighborhood, and, unlike you, looked forward to a groundcover war to discover the fittest species.  I wanted a lawn of mixed grass and various forbs.

Here are some of the species I tried, and I indicate after each whether they grew well (G) among the sparse grass or struggled (S).  Carex blanda (Eastern woodland sedge)(G), Carex texensis (Texas sedge)(G), Carex planostachys (Cedar sedge)(G, but did not spread), Calyptocarpus vialis (Straggler daisy) (GG, but in late Spring), Ruellia humilis (Fringeleaf wild petunia)(G), Phyla nodiflora (Texas frogfruit)(G, but straggly), Salvia lyrata (Lyreleaf sage)(G), Dichondra recurvata (Oakwoods ponysfoot) (GGG), Viola pedata (Birdfoot violet) (S), Salvia roemeriana (Cedar sage)(G, but did  not spread), Oxalis stricta (Common yellow oxalis) (GGG) and Oenothera speciosa (Pink evening primrose)(S). I expect that those species that are successful will coexist fairly peacefully.

Seeds from Lupinus texensis (Texas bluebonnet) and Gaillardia pulchella (Firewheel) somehow found their way in and germinated freely in the sunny spots the second season.  Echinacea purpurea (Eastern purple coneflower) (S) encroached from a nearby bed.  I am keeping some non-natives, e.g., ajuga, which quickly invaded the shady lawn without my permission but I am pulling out chickweed and henbit, which also do well.  I mow every several weeks in Summer, skirting around taller bloom stalks.

Now for the question of whether some or all of these would do well in St. Augustine.  I notice Wild petunia, Straggler daisy and Lyreleaf sage occasionally growing in my other (St. Augustine) lawn without any encouragement from me. But I think most of the species I mentioned would have trouble competing with St. Augustine, especially in full sun.  You would have to give them some help.  I have read that these plants grow better in Habiturf if it is cut short, given lots of water, and/or fertilized heavily.  That might help with St. Augustine too.  Otherwise, you will have to prune away the grass from around the forbs until they get a good start.  This would be much more effective in shady or even partly shady spots.

I have enjoyed both the successes and the failures, and I think you will too.