WINTER-KILL  Our north Florida sod customers had serious winterkill to their Floratam lawns. How can they help their lawns recover? If they have to replant, how can they avoid such severe problems in the future? Gadsen County

ANSWER: Lawns with scattered surviving runners (i.e., every couple of feet) usually grow back in one season, so homeowners who can tolerate temporary brown may wish to wait and watch. Unfortunately, winterkill is often irregular. Typically, strands of green turf surround dead expanses, the latter on their way to becoming weed gardens. In my opinion, bare patches more than five feet across should be replanted, with plugs or sod. This is a judgment call, which your customers can make once it is warm enough to see the green returning.

While Floratam St. Augustinegrass remains the best general-purpose lawn grass for most of Florida, it has the worst cold tolerance. Substantial winterkill can result whenever air temperatures fall below 20F (-7C). Temperatures below 16F (-9C) are associated with total loss of Floratam. If your customers have encountered repeated winter kill to Floratam, they might consider replacing it with a different grass, which incidentally presents other new characteristics that may or may not be desirable.

Three possible replacements, Delmar, Mercedes, and Raleigh St. Augustinegrasses, showed good winter survival based on 1992 data from Louisiana State University at Calhoun, and 1991 through 1993 data from Mississippi State University. Floratam had poor survival at both locations. The observed differences in winter survival in the field are backed up by laboratory data. The lethal temperature for Raleigh is 2-3 degrees F (1-1.5 C) lower than Floratam, based on laboratory tests by Dr. Jack Fry and others at Louisiana State University, and independent work using another method, by Mr. Wayne Philley and others at Mississippi State University. While there is not a one-to-one correspondence between the lab and the field, the relative rankings are very similar. Because of the erratic effects of microenvironment--e.g., tree shade, soil moisture, thatch thickness, and the distinction between frost damage and killing injury--one must be cautious in predicting winter survival.

Palmetto St. Augustinegrass over-wintered as good or better than Raleigh, according to Dr. Jimmy Golden of Clemson University. Experimental plots at the Sandhill Research and Education Center, Columbia, experienced 18F temperatures during the last cold snap of 1995-96. While only time will tell, Golden believes that Palmetto has a niche in colder areas, and that it is comparable to centipedegrass in cold tolerance.

A more cold tolerant cultivar, carefully managed, might give your customers the edge to pull through an occasional bad winter. At Mississippi State University, Mr. Wayne Philley and others showed that late fall fertilization with a high rate of nitrogen adds about 2 F (1 C) to the lethal temperature of St. Augustinegrass, potentially reducing survival. However, that research, plus older research from Texas, showed that fall fertilization with a high rate of nitrogen led to earlier resumption of spring growth, provided that the winter was not severe.