Most Kentucky lawns are established with either Kentucky bluegrass or tall fescue. Both are cool-season grasses that prefer cool, wet summers much like the state experienced in 2009. Kentucky bluegrass tends to show heat or drought stress before tall fescue, because its roots are shallow. But tall fescue planted in shallow or compacted soil will show stress too. In addition, tall fescue lawns have had lots of the disease brown patch, which thinned the turf cover.
Lawns that are sloped to face the south or west are more susceptible to heat, drought stress and weeds, said A.J. Powell, turfgrass extension professor emeritus with the University of Kentucky College of Agriculture.
"On an average summer day, we lose a quarter-inch of water a day in our turf systems," said David Williams, UK turfgrass scientist. "It doesn't take long under this heat for the moisture that is in the ground to dissipate."
In addition to the heat of the day, the grass also needs cooler nights to cool the roots and help the plant recover.
While it continues to be hot, the area could get some rain this week. Williams said that would significantly help area lawns, but it will also make weeds grow.
"As long as we don't have temperatures in the mid-90s for the remainder of August, most lawn grasses will recover," he said.
Homeowners and lawn care professionals can take several measures to prevent further heat-related injuries to their lawns. These include mowing the grass 2.5 to 3 inches high, mowing as infrequently as possible, not mowing during the heat of the day, not allowing foot or pet traffic on dry lawns and irrigate if possible. If severe thinning of the grass does occur, be prepared to renovate the bad areas in September or October with additional turf-type tall fescue.
Thin, cool-season grasses in summer will result in infestations of warm-season weeds like crabgrass, yellow nutsedge, nimblewill, bermudagrass and dallisgrass. None of these weeds are easy to kill at this stage, and for the most part, homeowners should not apply herbicides because they can put additional stress on dry, hot lawns, Powell said.