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Weather causing brown, weedy lawns
Central Kentuckians are probably noticing brown spots and lots of weeds in their lawns. While the area has had above-average rainfall for the year, no significant rainfall has occurred in the past couple of weeks. Mix that with temperatures in the 90s, and that has put stress on grass.
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.
No selective herbicides are labeled for lawns to kill perennial weeds like nimblewill, bermudagrass and dallisgrass. As soon as cooler, wetter weather arrives in late September or early October, homeowners should add nitrogen fertilizer to lawns to get as much bluegrass and tall fescue recovery as possible. If possible, repeat fertilization in November or December. Homeowners should continue to mow regularly during the fall until the grass stops growing. A crabgrass pre-emergence herbicide should be applied before April 15. If crabgrass is extremely bad now, repeat that application again in late May or early June.
Homeowners and lawn care professionals wanting additional information on heat, drought and serious lawn weeds, can view several extension publications about those topics on the UK turfgrass website at http://www.uky.edu/Ag/ukturf/Publications.
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