Ask Mr. Smarty Plants

A centralized listing of nitrogen-fixing plants native to the Texas Hill Country does not appear to exist at this point, but there is a relatively simple way to construct one. Since almost all members of the legume family, Fabaceae, are known to fix nitrogen, finding a list of native Central Texas plants in that family would cover most nitrogen-fixing plants in your region. Since you're in Williamson County, this flora of neighboring Travis County would be a good start. Simply go down to the Fabaceae section to get to the leguminous plants. There are many, including some commonly used in landscaping, such as Texas Mountain Laurel (Sophora secundiflora), redbud (Cercis canadensis), Honey Mesquite (Prosopis glandulosa), and bluebonnets (Lupinus spp.).

Consulting books that cover a broader region, such as Shinners and Mahler's Flora of North Central Texas (which includes most of Williamson County) and the Manual of the Vascular Plants of Texas (which lists leguminous plants under the older name, Leguminosae), paying attention to which species occur in the Hill Country, can help produce a more complete list. Both books are commonly available in Texas libraries.

In addition to the legumes, there are several other plant genera that fix nitrogen. Central Texas examples of these include Mountain Mahogany (Cercocarpus montanus), native to the western edge of the Hill Country, and Redroot (Ceanothus herbaceous).