Mow high to keep weeds under control
- Published: Monday, June 15, 2020
COLUMBIA, Mo. – Use your mower’s highest setting for fewer weeds and lusher grass.
University of Missouri Extension turf pathologist Lee Miller says mowing too low is a common mistake.
“We want your grass to be the trees of your lawn,” he says. “Your feet should sink into the grass.”
Miller says more than 10 separate studies since 1958 have found a direct correlation between mowing heights below 3.5 inches and substantial increases of weeds such as dandelion, white clover and crabgrass for tall fescue and Kentucky bluegrass lawns.
Mowing grasses to their optimal height—3 1/2 to 4 inches for tall fescue and Kentucky bluegrass, including mixes; and 1-2 inches for zoysiagrass—gives them a competitive advantage over weeds. Cutting high keeps sunlight and heat from weeds. As grasses grow, there is less room for weeds to grow. In short, a weed in need is a good thing.
“The current bank of weed seeds in your lawn needs sunlight to heat the soil surface for germination and aid young seedlings in growth,” says Miller. “Growing Kentucky bluegrass or tall fescue tall robs them of sunlight and prevents weeds from establishing and competing for turf.”
When weeds cannot grow, turfgrass roots get more room to spread and become more dense. The more leaf a plant has now, the more sunlight it can harvest and the more food it can make for itself. When temperatures rise in summer, grasses face intense stress from heat, drought and disease. Giving grasses ample leaf tissue helps them withstand that stress.
For a healthier and more attractive lawn, never mow more than a third of the leaf and avoid clippings that clump, says Miller.
Mow often to avoid taking off too much at one time, but make sure the grass is actively growing when you do so. “We often see a lack of growth and turfgrass dormancy during drought and high temperatures,” he says. “If it isn’t growing, you shouldn’t be mowing.”
Conversely, take care when mowing after big rainstorms. Under the weight of a mower, saturated soils can rut and are more easily compacted.
Also, avoid “ring around the collar” trimming – cutting short in areas around roads, sidewalks and lawn margins. Scalped areas become a hot spot for weeds, particularly foxtails and crabgrass, which will soak up the extra heat from the asphalt and quickly outgrow scalped turfgrass.
For more lawn care information, visit MU’s Integrated Pest Management website at ipm.missouri.edu.
Writer: Linda Geist
Lee Miller Jr
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