Recently, the topic of drought tolerance/resistance among turfgrasses has become a major issue of debate in the sod industry. This is in part due to the national and regional attention that has been given to water conservation. It is estimated that 50 percent of the water used in urban areas involves maintaining landscaped areas which include turf. The high visibility of turf irrigation as opposed to other types of water usage has put grass on the "hit list" in legislative and political circles throughout the country. Researchers and marketers have made many claims promoting selective cultivars of Zoysias, Hybrid Bermudas, Buffalo and Fescues as being "drought tolerant", sometimes insinuating that St. Augustine cultivars are heavy water users. Many St. Augustine grass cultivars are native to the United States and have survived for centuries without cultivation or irrigation. The fact that St. Augustine grass thrives in relatively wet areas of Florida, and coastal Texas may be part of the reason for the misunderstanding, but the ability to withstand frequent and substantial rainfall typical of these areas should not be translated to mean that St. Augustine REQUIRES more water.
Misunderstandings pertaining to terminology of drought and water usage issues have been evident in newspaper, magazine articles,and television news sections. These definitions must be clarified before substantive discussion and research can be properly dissected.
Evapotranspiration (ET) is defined as the total amount of water a plant transpires (loses from evaporation or transpiration through leafy tissue or other means) and the water that evaporates from related soil areas. This sum is measured in experiment evaluations.
Water use rate is defined as the total water needed for a given turfgrass to grow plus the (ET) rate. The water use rate is logically slightly higher than the (ET) rate. These terms are used interchangeably in many non-scientific articles. A significant point to understand about (ET) rates is that they vary depending on available water and environmental conditions, i.e. temperature, humidity, and wind velocity. (ET) rates between cultivars and species are relative only when similar environmental conditions are shared by tested samples. An example of a varied (ET) rate can be based on available water. An example of this is that some cultivars of St. Augustine, such as Floratam and Palmetto will have relatively high (ET) rates when large amounts of water are available and low (ET) rates when less amounts are available. Cultivars of Buffalo and Zoysia grasses tend to have consistently low (ET) rates which may on the surface seem good, but in fact, a high (ET) rate would be favorable during wet and rainy situations to better handle excess water. Palmetto and Floratam cultivars are versatile because they maintain low (ET) rates during drought conditions or low water availability, yet are able to handle excessive amounts of water with high (ET) rates during rainy or wet conditions. In conclusion, grasses that have low water usage rates or (ET) are good for drought situations when minimal water is available, but grasses that have varying (ET) rates are even better due to the fact that they can withstand drought conditions and also have the ability to handle and excess water during rainy seasons and wet conditions.
Drought resistance and drought tolerance are general terms used to describe a wide range of water usage issues. Because these terms often do not mean the same thing to different people, it seems necessary to specifically define four separate terms that can be used to describe the effects of drought for a meaningful analysis:
1.) Dehydration tolerance - the ability to withstand water deficits in tissue caused by insufficient water quantity which may slightly but not significantly affect turf quality. This definition refers to the characteristic of a grass to thrive during a given timeframe of "low" or minimal water in the leaf tissue. An example would be Bahia, Buffalo, and specific cultivars of St. Augustine grass which can thrive with minimal water content available in their respective blades. The term drought tolerance is often used.
2.) Dehydration avoidance - the ability to remain green and maintain acceptable turf quality during drought or low precipitation in a given timeframe. Bermuda, Zoysia, and specific cultivars of St. Augustine grass fall into this category as these grasses can maintain an aestheticly pleasing turf for extended periods of time due to their root structure, root density and relative low water usage. The term drought resistance is most often used.
3.) Dehydration recovery - the ability to go dormant and survive while water is not available and recover when water returns. Bahia and Buffalo grasses are examples of this category. During periods of drought they go into a state of dormancy ( turn brown) and recover when water is applied. This is a desired trait for highway usage or areas where the primary concern is soil stabilization. Both drought tolerance and drought resistance are used, which can be confusing.
4.) Dehydration quantity - the minimal amount and frequency of water required to maintain acceptable turf quality. Both drought resistance and drought tolerance are used in reference to turfgrass that requires a low dehydration quantity, which is again confusing. Articles and promotions sometimes use the term drought tolerance or resistance to describe turfgrasses that exhibit good dehydration recovery. The consumer is subsequently confused and angered when the drought tolerant lawn purchased turns brown and goes dormant during a drought. The consumer will often water these dormant lawns as frequently as a less expensive grass requiring less maintenance in an effort to "green up" his damaged lawn. The performance of the drought resistant or tolerant lawn does not seem to meet the consumers expectation because the expectation was a turfgrass exhibiting dehydration tolerance or avoidance as opposed to dehydration recovery. Most people seem to equate dehydration avoidance and a minimal dehydration quantity with drought tolerance. That is, they want a grass that uses as little supplemental water as possible but can still retain an acceptable turf quality. The concept of brown or dormant grass in the summer during a drought is not acceptable to most homeowners and would not be considered drought tolerant or resistant by their standards. Exceptions to this concept would be roadside areas or places where turf is used primarily for soil stabilization or erosion control. In conclusion, consumers and conservationists generally want a grass that can attain acceptable turf quality while using minimal supplemental water during periods of drought or low rainfall.
St. Augustine grass in general has good dehydration avoidance and low dehydration quantity. In other words, it has been shown in studies and been observed in field applications that some St. Augustine cultivars have excellent drought resistant qualities and can thrive in low water situations as good as and sometimes better than Zoysia, Bermuda, and Buffalo grasses. The differences in drought response by cultivar types are most likely due to root characteristics, blade width, and nodal distance (distance between the joints of the grass). The soil type also affects water demands as heavy clay soils retain water more efficiently than sandy soils. It may be surprising to know that studies at Texas A&M have shown cultivars such as Floratam and Floralawn to be "very high" in dehydration avoidance as compared to many cultivars of Bermuda, Zoysia, and Buffalo grasses. Cultivar Palmetto has recently been evaluated by the University of Florida as having low relative water content (RWC) as compared to Floratam . During dry periods, Palmetto has also been observed by several turfgrass producers to wilt after Floratam in side by side field evaluations. Similar examples of St. Augustine grasses performing in low water situations are numerous but seldom reported. In Florida, seasonal drought can be devastating to grasses that do not have good dehydration avoidance. The combination of excessive heat and sandy soils further aggravate the situation. Floratam has "naturalized" in some areas and has survived for many weeks without water and thrived. Palmetto St. Augustine has thrived in non-irrigated areas on turf farms for over 90 days with minimal damage. Homeowners have reported Floratam, FX-10 and Palmetto lawns to show no signs of wilt for weeks during periods of drought. In Texas, a grower in Dallas reported that a field of Raleigh St. Augustine survived and thrived in a area that had no significant rainfall for over 92 days and only minimal water over a 6 month period during the summer of 1995-96. A grower near Houston, Texas reported cultivar Palmetto St. Augustine to survive in a production field with no significant rainfall for 3 months during the summer of 1996. Further research is needed to demonstrate the drought tolerant and resistance qualities of St. Augustine grass.
In conclusion, St. Augustine grass is already well known for its superior shade tolerance, low maintenance, beautiful color and overall impressive aesthetic appeal. In addition, contrary to stereotypes and generalizations, specific cultivars of St. Augustine grass are also drought resistant with significant drought avoidance qualities. A bonus feature of St. Augustine grass is its ability to "withstand" large amounts of water as in the case of a rainy season or frequently wet area, but this fact does not mean that St. Augustine NEEDS OR REQUIRES large quantities of water to thrive. The facts are evident both scientifically and in practical application, that particular cultivars of St. Augustine grass including Floratam, FX-10, Raleigh, and Palmetto are drought resistant cultivars exhibiting comparable and sometimes superior drought avoidance properties when compared to Zoysia, Bermudas, and Buffalo grasses. This drought avoidance characteristic defined as minimal water needed over a given time for a turf of acceptable quality is the logical and most practical meaning of drought resistance. Consumers and legislative bodies need to understand these terms to insure adequate and accurate decision making practices and to provide information as we all make an attempt to conserve our water recourses and provide aesthetically functional and pleasing turfgrass areas.
ACKNOWLEDGMENTS
The references of particular events and observations are based on first hand interviews and personal inspection of listed sites. I would like to express my sincere thanks and personal respect to these innovators and leaders in the turfgrass industry; they have contributed enormously through hard work and dedication.: Mr. Elmer Kirkland, Mr. Malcolm "Buzzy" Romine, Mr. Bill Murff, Mr. Doyle Anderton, and Dr. Philip Busey.
Tobey A. Wagner
Sod Solutions, Inc.
REFERENCES
Beard J. B. Genotype Diversity in Evapotranspiration Rates Within Seven Major Turfgrass Species. PR-4750 pg. 40-41
Sifers S.I., Beard J.B. and Hall M. H. Comparative Dehydration Avoidance and Drought Resistance Among Major Warm-Season Turfgrass Species and Cultivars. pg. 37-40
Busey P. and White R. W. South Florida: A Center of Origin for Turfgrass Production. University of Florida, Ft. Lauderdale, FL, and Turf. Manag. Cons., Tavares, FL Chapter 124 pg.-863-869.
Miller G.L. and McCarty L.B. Turfgrass Rooting Characteristics and Water Use of St. Augustine Grass Varieties Used Commercially in Florida. pg. 57-63.
