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First year of two-year turf grass drought study complete; more research needed

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San Antonio, Texas
April 9, 2007

While the first year of a two-year study on the drought tolerance of warm-season South and Central Texas turf grasses is complete, more research is needed, said a Texas Cooperative Extension expert.

"We've gotten some initial results, but we have to repeat the study again later this year to verify these results before we can say anything definitive," said Dr. David Chalmers, Extension turfgrass specialist.

Initial testing was done last year on 25 turf grasses and cultivars using a 5,000-square-foot drought simulator located on San Antonio's south side. The simulator was built by the Texas A&M University System's Irrigation Technology Center with funding from the San Antonio Water System, Turfgrass Producers of Texas and the Rio Grande Basin Initiative.

"Both San Antonio Water System and the Turfgrass Producers of Texas were instrumental in the study," Chalmers said. "They both worked with us to establish the protocol. And the producers provided in-kind services like leveling the test area and transporting, washing and installing the sod."

At the site, 200 grass plots measuring 4x4 feet were subjected to 60-day drought conditions. These conditions were maintained using a rain-out shelter a large, low-profile galvanized metal roof that automatically covered the plots when rain was detected.

After the 60-day simulated drought, the grasses were irrigated over a 60-day recovery period to see how they responded. All grasses were evaluated through the July 23 through Sept. 20, 2006 drought period and subsequent 60-day recovery period.

Grass varieties studied included eight types of bermudagrass, seven of St. Augustine grass and nine of zoysiagrass, Chalmers said. One variety of buffalo grass was included for scientific comparison. Each was planted in a soil depth of 4 inches and in a "native" unrestricted soil depth of 18 inches or more.

The same grasses were planted in the drought simulator last fall in preparation to repeat the same experiment beginning this coming July.

"The 4-inch soil depth was chosen to ‘mimic' soil conditions in the Hill Country and to help evaluate how grasses would perform based on San Antonio's conservation ordinance for new construction," Chalmers said.

The new ordinance requires residential and commercial builders to install only approved turf grasses with "summer dormancy capabilities" in new construction in the city.

"The ordinance was put in place to assist new homeowners," explained Karen Guz, conservation director at the San Antonio Water System. "New homeowners told us their struggles with grass growing on limited soil.

They also wished there were rules to require tough grasses that would not be so difficult to keep alive during dry summers. But there was little evidence about which cultivars had summer dormancy capabilities. This study was an important first step."

Under the conditions of the study, no grasses in the 4-inch soil depth survived the 60-day simulated drought, Chalmers said. But all 25 grasses with a native soil depth of 18 inches or more survived.

Because none of the grasses in the 4-inch soil depth survived, initial study data focused on the grasses which survived in unrestricted native soil, he said. Among those grasses and cultivars were differences in the time to leaf firing as well as the type and amount of firing.

"Firing refers to grass color during a period of drought stress, ranging from a healthy green on the high end showing no moisture stress to a straw-colored brown on the low end showing severe moisture stress,"

Chalmers said "Firing is what happens when the leaf blades lose chlorophyll, and it's a primary indicator of drought stress."

Grasses were evaluated visually for drought performance, and were assigned ratings from one to nine based on how they responded to and recovered from the drought.

"We assigned ratings relative to firing and grass quality, with one being the lowest and nine being the best," Chalmers said. "Our main emphasis was on how well grasses recovered after the imposed 60-day drought."

This data was used by the San Antonio Water System to help develop a list of approved grasses for use in new construction. These include the bermudagrass varieties of Celebration, Common Bermuda, GN1, Grimes EXP, Tex Turf, TifSport and Tifway 419; the zoysiagrass varieties of El Toro, Empire, Jamur and Palisades; the St. Augustinegrass variety Floratam, and all buffalograss varieties (http://www.saws.org/conservation/Ordinance/TurfGrass/index.shtml ).

"Homeowners hope for a quick recovery of their grass after a drought ends," Guz said. "To most of them, this means full coverage and little bare ground showing a few months later. We chose these cultivars with this in mind."

"We encouraged participation by A&M to provide unbiased scientific data to use in making decisions about which grasses would be allowed in new construction," said John Cosper, executive director of the Turfgrass Producers of Texas.

The study was a good one, said Cosper, whose organization represents about 70 turf grass growers across the state, but further explanation is needed to properly interpret the results. Cosper noted that fine-textured zoysiagrasses probably recovered more slowly than other grasses due to the mowing height used during the study. He also cited a 23-day delay in study's original start date leading to grasses being subjected to a "cool environment not favorable to growth" during some of the Sept. 21-Nov. 19 recovery period.

The climate during the drought was extreme with high temperatures and low humidity, he said.

"SAWS accepted some varieties and rejected others even though all 25 varieties (in native soil depth) survived," Cosper said. "We have asked them to look at the plots again this spring and to be open to accepting other varieties that demonstrate the ability to survive a 60-day drought."

Grasses in the native soil did not begin to show much leaf firing until after about three weeks under drought conditions, he said, and the water system has never limited outdoor irrigation to less than once a week.

However, grasses in the 4-inch soil began to show stress after six days and were mostly browned off after 12 days, Chalmers, added.

"One of the most important aspects of the study for conservation was finding that there are turf varieties that can go completely dormant and recover very well," Guz said. "This supports the idea that turf can be part of a sustainable landscape in South Texas, regardless of what future challenges drought may bring."

"It's important to recognize that factors involved in drought resistance should be weighted differently if water conservation objectives are different," Chalmers said. "The 60-day worst-case scenario conservation strategy for the study is very different from the San Antonio Waters System's Stage 1, 2 and 3 restrictions that would allow for different amounts of irrigation to help turf survive."

These initial results should not be interpreted as the last word on drought-tolerant grasses for the region, Chalmers said.

"Drought-resistance related to turf grass is complex and multifaceted,"

he said "And while this study give us some good initial data, it doesn't give us any information about how these grasses might perform at other soil depths, such as 6-, 8- or 12-inch depths."

Grasses benefit the urban landscape and environment in a number of ways, including temperature moderation, oxygen production, runoff and soil erosion reduction, dust stabilization and aesthetic enhancement, Chalmers said.

"The question is how much water is available – and (how much) are we willing to use – to sustain turf grass quality," he said. "Or better yet, the question for research to answer might be: What is the minimum amount of water we can use to achieve this end?"

More information on lawn grasses suited to South and Central Texas can be found at http://aggieturf.tamu.edu.

Writer: Paul Schattenberg

 

 

 

 

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