CHANGING GRASSES: We are planning to convert a lawn from common bermudagrass to 'Floratam' St. Augustinegrass. How do we prevent the bermudagrass from coming back? Bonita Springs

ANSWER: While your question concerns St. Augustinegrass, the answer can be applied to conversion to new bermudagrasses and other species.  Fumigation with methyl bromide eradicates bermudagrass, but may be inappropriate in a lawn*.   A qualified success can be achieved with three applications of glyphosate (Roundup). There are several requirements for success with glyphosate. First, each application of glyphosate must thoroughly cover actively growing, green bermudagrass. The most complete kill comes from letting the bermudagrass grow, or re-grow between applications, to 6 inches height. Prior irrigation can coax the bermudagrass to emerge from the ground. Second, no tillage may be permitted until 7 or more days after the glyphosate application. Because glyphosate acts systemically, any soil disturbance would disrupt its absorption and translocation into the bermudagrass. Finally, one must plan to deal with scattered renegade bermudagrass sprigs, depending on the extent of hardened rhizomes. Dr. B. J. Johnson, University of Georgia (Griffin) has observed 98 to 100% control with three applications to hybrid cultivars of bermudagrass, but at best only 84% control with two applications. You should delay planting the St. Augustinegrass for weeks, and hopefully detect and hand dig most of the escapes.

While glyphosate can be effective, some people cannot provide the multiple applications and time required. If that is your situation, you may have to settle for some return of bermudagrass, and try to keep the St. Augustinegrass healthy and competitive as a partial suppressant.

* Methyl bromide is a RUP (Restricted Use Pesticide) which can be harmful to people and the environment--it kills everything unless used with extreme care, and it requires training and a special license to be applied.