Evaluating St. Augustinegrass and Bahiagrass Rooting and Drought Tolerance
University of Florida, Gainesville
Institute of Food and Agricultural Sciences
Many urban areas are under varying degrees of water restrictions with tougher ones anticipated for non-commercial use such as lawns. One strategy to reduce turfgrass irrigation needs is to use drought resistant species and cultivars. A major component of drought avoidance is the development and maintenance of a deep, extensive, and viable root system. Envirotron research has been initiated to investigate relative rooting and drought tolerance of new and standard St. Augustinegrass and Pensacola bahiagrass. In this study, maximum root depth ranged from 40 cm for Palmetto to 65 cm for Floratam and Pensacola. Pensacola roots extended quickly, with roots reaching a depth of 60 cm after 90 days of growth. Floratam and Pensacola were the deepest rooting cultivars. After 180 days of growth Floratam and Pensacola had similar rooting depths. Palmetto and FX-10 had similar root depth until August (150 days of growth) after which FX-10 was a deeper rooting cultivar. An increase in FX-10 root depth occurred after November 1995 with roots extending from 40 cm down to 50 cm. Root depth tended to increase to a particular depth for each cultivar and stabilize for a period of time. Roots were observed at the same depth or continued to grow deeper during the early winter months; whereas, during the early spring of 1996 there was a decrease in roots observed at each cultivar's lowest depth. Other researchers have also observed seasonal root growth of St. Augustinegrass and noted root growth continued after winter shoot dormancy occurred. Just after spring shoot greenup, browning of the entire root system was observed followed by a period of rapid root initiation and replacement.
Greatest variability in RLD from month to month and among cultivars occurred from the surface to 10-cm depth. Pensacola had a higher RLD at the 5- and 10-cm depth compared to other cultivars after 150 days of growth. Palmetto generally had a high RLD in the surface 5-cm depth, but Palmetto compared similarly to FX-10 and Floratam at 10 cm. Due to rapid desiccation of surface soils during periods of drought, shallow roots generally are not considered to play a major role in drought tolerance. Evaluations across months indicated there were no absolute RLD separations among cultivars at the 20- and 25-cm depths. At 30- to 35-cm depths, RLD tended to stabilize for each cultivar and were easily separated into two groups -- Pensacola and Floratam with high RLD and Palmetto and FX-10 with lower RLD. Monthly root counts indicated Floratam and Pensacola had a greater number of deep roots compared to Palmetto and FX-10. Extra root growth at lower depths should provide extra drought tolerance for Floratam and Pensacola.
Leaf firing differences were used as an indication of overall drought resistance of grasses, where drought resistance includes avoidance and tolerance aspects. Number of days before wilt for Pensacola, FX-10, Floratam, and Palmetto were 9.0, 6.7, 6.0 and 4.7, respectively. These results indicated that Pensacola had superior drought tolerance compared to other cultivars and that FX-10 and Floratam had similar tolerance. Palmetto had the poorest drought tolerance, but it was similar to that of Floratam. Superior drought tolerance of Pensacola was supported by its ability to develop and maintain an extensive, deep root system. Since rooting depth and density of Floratam was generally superior to FX-10 and Palmetto, other characteristics must have influenced drought tolerance. It has been noted that there may be differences among some turfgrasses in their ability to maintain root systems. The one component of rooting that was not adequately evaluated in this study that might have influenced drought stress was root viability. These data indicate that while rooting depth and density is an important component of drought tolerance, other factors should also be given attention before ranking drought tolerance.