Winters like this one are tough on warm season grass—even in Florida. The extent of cold temperature injury that turf sustains is not solely predicted by how cold it gets and for how long. These are very important factors, but the turf species and management inputs leading into the winter also play a role. Fertilizer rate, composition, and timing and disease activity can impact how much turf is lost following low temp exposure.
One particular instance where this seemed to be very apparent to me occurred a few years ago in two St. Augustinegrass lawns in Gainesville, FL. Samples were taken throughout the neighborhood (that one of our Dean’s happened to live in) that all had some amount of black hyphae, lobed hyphopodia, and root rot symptoms associated with take all root rot disease and the fungal pathogen Gaeumannomyces graminis var. graminis. What was striking was that some lawns showed typical patchy yellowing and thinning associated with take all root rot in spring, and some were devastated, having almost total turf loss (see Pic1). Both lawns had the pathogen and were subjected to the same temperatures. Where the disease was active in the lawn on the left, the turf appeared to have been more severely affected by the low temperature, but that didn’t hold on the right; the whole lawn suffered regardless of amount of disease. Why such different results?
Temperature fluctuations preceding the photo in 2009 involved 80F highs in late January and a 19F low in early February. Similar conditions occurred more recently in 2012, when temperatures were mild and my lawn was green through most of January. We went from 80F on the 5th of February to 17F on the morning of the 13th of February. Both years resulted in cold temperature injury to many St. Augustinegrass lawns. A sudden swing in temperature tends to result in more damage than a gradual consistent decrease, but that still doesn’t explain the differences between the lawns in the pic.
I suspect the difference between these two lawns was primarily in the nitrogen fertilizer rates and timings. The lawn on the right received fertilizer in late fall and again in winter at rates in excess of the UF IFAS recommendations. The homeowner did this because it was warm, and the turf looked like it needed it. When the temps were mild, the grass continued to flush growth, and looked better than his neighbors, but the day lengths were short and light intensity was low, so the grass utilized and eventually depleted carbohydrate reserves to achieve this growth. That kind of stress makes the grass more sensitive to the cold temperature exposure and to disease development.
Recommendations developed by my turf colleagues that include moderate levels of nitrogen, increases in late summer and fall potassium rates, and using iron to maintain color during periods with poor growth potential can pay dividends in increased low temperature and winter disease tolerance.