Don’t Mow Your Grass Too Short

Don’t Mow Your Grass Too Short—and Other Secrets from America’s Lawn Geek



April 15, 2012

It is lawn-care season across much of the US. But while most home owners try to take care of their grass, they don’t always do it properly. Lawns don’t come with instruction manuals, and lawn-care folk wisdom often is wrong.

Answers to  important lawn-care questions…

• Does it matter how short I cut my grass when I mow?

It matters a lot. “Scalping” a lawn—mowing off more than one-third of the grass’s height—is the number-one lawn-care mistake that home owners make. Doing this sends grass into physiological shock, which leaves the lawn prone to invasion by weeds and less able to cope with drought and other environmental stresses.

The more often you mow—and the less grass you remove with each mowing—the thicker and healthier your lawn is likely to become. If the grass gets so long that you can’t get it down to proper length in one mowing, wait a day or two and mow again.

For St. Augustine and bahia grasses and for cool-season grasses, such as Kentucky bluegrass and perennial rye, three to four inches is generally best…for most Bermuda and zoysia, one to two inches. Various Web sites, such as www.American-Lawns.com, can help you identify what type of grass you have.

• Should I mulch or bag my lawn clippings?

Definitely mulch. Grass clippings are full of nitrogen and other nutrients. Removing them deprives the lawn of free fertilizer. Modern mulching mowers do an excellent job of grinding up grass into mulch, particularly when you mow off less than one-third of the grass’s height, as described previously.

Helpful: Research has shown that mulching grass does not create excessive thatch and thus does not make lawns more susceptible to disease or drought.

• My lawn feels hard-packed. Should I aerate? And if so, what’s the best way to do that?

It is worth aerating if your soil is hard-packed. It’s difficult for grass roots to grow properly when the soil is compacted.

Rent a core aerator (about $40 to $50 for two to four hours), or pay a lawn-care professional to aerate for you. The aerator used should pull plugs of soil from the ground, not just slash the soil, which is far less effective.

Aerate when grass is actively growing. With the cool-season grasses of the northern US, such as bluegrass, fescue and ryegrass, that generally means April or September (and perhaps the months that precede or follow these, depending on temperatures). With the warm-season grasses of the South, such as bahia, Bermuda, buffalo or zoysia, it typically means May through July or August. Aerating once each year for three years usually solves compaction problems.

Warning: If the distance between the holes created by the aerator is greater than three inches, you almost certainly need to make another pass with the aerator.

• I know that there are dangers to fertilizing too often, but how often is best?

Excessive fertilizer can “burn” grass, causing severe dehydration that could kill the lawn. Or extra fertilizer might cause grass to grow quickly, making it difficult to keep up with the mowing and increasing the odds that you will scalp the grass, as described earlier. Best strategy: Fertilize on the “holiday plan”—on or around Memorial Day, the Fourth of July, Labor Day and Halloween. Four times a year is all a lawn really needs. Do not exceed the dosages recommended on the fertilizer’s packaging.

Warning: Do not fertilize your lawn if it has not rained recently and local water-use restrictions prevent watering. Some home owners faced with this situation think, Well, if I can’t water my lawn, I can at least feed it fertilizer. Unfortunately, this makes a bad situation worse—recently fertilized lawns require even more water than those that have not been fertilized.

• My lawn is discolored where my dog relieves itself. How can I keep these patches of lawn healthy?

Dog urine contains high concentrations of ammonia salts (nitrogen), just like fertilizer. And like fertilizer, it’s good for a lawn in modest amounts but causes problems when too much is supplied. Trouble is, dogs often relieve themselves in roughly the same areas of the lawn every day, causing “burn spots” of dehydrated or dead grass just as if you were overfertilizing these areas.

The best solution is to saturate the affected area thoroughly with a hose every week, particularly when there hasn’t been much rain. Or pour a pail of water on the area right after the dog urinates. An alternative is to train the dog to urinate in a section of the lawn that is landscaped with gravel or wood chips rather than grass.

Warning: Do not alter the dog’s diet in an attempt to solve this problem. Despite folk wisdom, regularly feeding a dog tomato juice or other fruit juices is unlikely to significantly improve the lawn’s problem and it could cause health problems for the dog.

• How much should I water my lawn?

A good rule of thumb is to give your lawn around one inch of water per week, perhaps a bit more during a stretch of very hot days. That guideline includes rainfall. There are new wireless devices on the market that monitor soil moisture and prevent automated irrigation systems from watering when additional water isn’t needed. That’s not just good for your lawn…it’s also good for your water bill.
Examples: Toro Precision Soil Sensor ($140, www.Toro.com)…UgMO ProHome Soil Sensor System ($499 for a two-sensor system, www.UgMo.com).

Water early in the morning so that the grass blades aren’t unnecessarily damp come nightfall—lawns are particularly susceptible to disease when they’re damp on humid summer nights.

Overwatering is much more common than underwatering except when local drought restrictions ban watering. Signs of an overwatered lawn include the growth of mushrooms or nutsedge (grasslike weeds)…significant runoff from the lawn into the street during watering…or a mushy feeling when walking across the lawn hours after watering. Signs of an underwatered lawn include footprints remaining visible in the grass long after you have walked across it.

• Burrowing animals are digging holes in my lawn. Is that bad for the grass? What’s the best way to get rid of those animals?

It won’t significantly harm your grass. The lawn might get upheaved a bit in places, but you usually can push it back down easily with your foot.

Research suggests that traps are the most reliable way to rid a lawn of burrowing animals. No one has ever shown that burrowing animals are significantly deterred by blocking their holes or by folk-wisdom solutions such as placing chewing gum in their holes.

• There are thin patches in my lawn every year after the snow melts, but overseeding never seems to work. What’s the secret to overseeding?

First, always overseed when your grass is actively growing. With the cool-season grasses of the North, that generally means April or September (and perhaps the months that come before and after these, depending on local climate). In the South, it typically means May through July or August.

Second, don’t use a nonselective preemergent herbicide if you’re overseeding. Such herbicides don’t just prevent the growth of weed seeds, they prevent the growth of all seeds, including grass seed. If you’re overseeding and you want to use a preemergent herbicide, choose one with siduron that allows grass seed to grow. Most major herbicide companies offer a siduron-based product.

Third, take a close look at the thinned patches. If the grass seems matted down and you notice a white, gray or pink growth on the grass blades, snow mold might be the source of your problem. Fluff up this matted-down grass with a rake to allow more sunlight and air circulation to reach these blades.

• My kids play on my lawn. Is there any way I can stop using herbicides without winding up with weeds?

Weeds thrive only when there are gaps in the lawn large enough for them to get the sun they need to grow. If you water, mow and fertilize, your lawn should be thick and healthy enough so that weeds won’t be a problem even without herbicides.

Source: John (Trey) Rogers III, PhD, professor of turfgrass management at Michigan State University, East Lansing. He was a turf consultant and project leader for the 2004 and 2008 Summer Olympic Games and 2008 UEFA World Cup, and he is author of Lawn Geek: Tips and Tricks for the Ultimate Turf from the Guru of Grass (New American Library).

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