By Denise Ellsworth, Akron Beacon Journal
Have you found yourself (or your spouse) getting a bit crabby over the past few weeks? One look at the lawn, and your normally pleasant disposition morphs into a decidedly less amicable state? It’s more than the brutal summer temperatures we’ve endured, it’s what the weather has done to the lawn. Instead of lush, dark grass, the lawn is uneven, full of fast-growing light green patches. In a word: crabgrass.
To the uninitiated, crabgrass is just another weed, but it’s much more than that. This is a supercharged, heat-loving pest that takes advantage of any sparse area in the lawn to choke out desirable turf. Give it just an inch, and it takes a foot in every direction, then goes on to produce about 150,000 seeds per plant. These seeds either sprout next spring, or lie in the soil for a dozen years or more, waiting until conditions are right.
Conditions are right when spring is cold and wet. When soil temperatures reach about 55 degrees, crabgrass seeds begin to sprout. Desirable plants grown from seed may all sprout within a day or two of each other, while weeds are successful in part because they continue to germinate over a much longer time span. For crabgrass, this span may begin in April and not end until the later days of June.
A lawn with a heavy crabgrass infestation may appear normal in June, but look again once hot temperatures arrive in July. Crabgrass belongs to a special group of plants called C-4 plants, a group that includes desirable grasses such as sorghum and corn. These plants are all adapted to grow especially well in drought and high heat. They also grow well when nutrient levels are low.
According to Dr. John Cardina, a weed scientist with Ohio State University based in Wooster, Ohio, spring-applied fertilizer is, “candy for crabgrass” , giving the weeds more fuel to become established. Bring on summer temperatures, when our desirable, cool-season turf grasses begin to slow down, and crabgrass is primed to explode.
Cardina recommends fertilizing the lawn in October, to benefit the turf instead of the crabgrass. “Put nitrogen on in autumn to help build the root system of the lawn grasses, not to fertilize crabgrass,” he said.
Technically, this discussion should be about “crabgrasses,” not simply crabgrass, since there are two common species of crabgrasses in our lawns and landscapes, each with a different preference for growing conditions and a slightly different appearance. Large crabgrass has stiff hairs on the leaf blade, and is more of a problem in ornamental beds. Smooth crabgrass is generally smaller, has few hairs on the leaf blade, and is a serious pest of lawns due to its tolerance of mowing.
Cardina studies the germination of smooth and large crabgrass to help pinpoint the timing of herbicide applications and ultimately reduce pesticide use. His research has demonstrated just how long the germination period can stretch, making management of the weed with pre-emergence herbicides a challenge. These products must be applied before seeds begin to germinate, and they must remain effective throughout the germination period.
Proper mowing height is another key to favor turfgrass over crabgrass. Shading reduces weed seed germination and reduces early seedling growth. Dr. Cardina recommends that homeowners, “raise the height of their mower and resist the urge to cut the grass before May 1 or so, to keep the soil cool and keep light from reaching the surface where crabgrass germinates.”
Keeping turf thick and healthy is also important in the fight over lawn weeds. According to Dr. Cardina, “crabgrass is an opportunist. It takes advantage of weak spots or damage in the lawn. Sites where light is able to reach the soil are sites where crabgrass gets started, because a light gap means low competition from neighbors and availability of resources. And before you know it, crabgrass has got a foothold.”
What to do now? Besides trying to keep up with mowing, nothing can be done at this point to rid the lawn of crabgrass. While some herbicides can selectively kill crabgrass in the lawn, these products are best applied when plants are small, not when they are large and vigorous.
Cool fall temperatures will kill crabgrass plants, giving desirable grasses a chance to grow before winter. Next year, applying a pre-emergent herbicide in early spring, around the time forsythia begins to bloom, will prevent a repeat performance. That, and a little luck.
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Denise Ellsworth is a horticultural educator with Ohio State University Extension.
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