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
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
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
"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
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
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,
"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
Writer: Paul Schattenberg