Why is my lawn turning brown after I mow it?

When a lawn browns right after mowing, look at the mower before the lawn. A dull blade or a deck set too low accounts for most cases, and both are easy to fix. If the mower checks out, the usual suspects are summer heat and drought, clumps of wet clippings and, in hot humid weather, a fungal disease called brown patch.

1. A dull blade

This is the most common cause, and it shows up within a day or two of mowing as a pale, tan or whitish cast over the whole lawn. Dull blades rip and tear the grass, which promotes disease, leaves the shredded tips tan and ragged, and weakens the plants.^[1] Pull a blade of grass and look at the end. A clean, straight cut means the blade is fine. A frayed, split tip means it is time for sharpening. Blades dull faster when they hit sticks, stones and sandy soil, so a mower that mows a lot of rough ground needs attention more often.

2. Cutting too short

The second cause is taking off too much at once. The guideline is to never remove more than one-third of the leaf blade at any one mowing.^[4] Cut lower than that and the lawn is often left with pale, stemmy stubble that looks brown for days. Low cutting also weakens the lawn over time, since grass mowed too short is more prone to disease.^[2]

For tall fescue, Henrico County's Extension office recommends 3 to 4 inches for fescue lawns.^[2] For summer, Virginia Tech's drought guidance sets a minimum of 2.5 to 3 inches for tall fescue, Kentucky bluegrass, perennial ryegrass and fine fescues,^[3] and says to raise the cutting height on cool-season lawns ahead of summer stress.^[3] See what height to mow fescue for the details.

Scalping can also be local. If the brown is in stripes or on bumps and the edges of beds, the deck is dipping over high spots. Raise the deck a notch, or smooth out the worst bumps in the fall.

3. Heat and drought

Sometimes mowing simply reveals stress the lawn was already under. Lawns that get little or no water from irrigation or rain during the summer go dormant, their color lightens, and most recover when water returns.^[3] Kentucky bluegrass, for example, survives extended drought by slowing its growth, turning straw colored and going dormant.^[3] A mower pass over grass in that state, especially in the heat of the day, makes the browning more obvious.

As a rule of thumb, most turfgrasses need roughly 1 inch of water a week, from rain or irrigation, to keep growing in warm weather.^[3] If you water, aim for watering deeply and infrequently, soaking the root zone to a 4-inch depth,^[3] rather than light daily sprinkles. If you do not water, let the lawn go dormant and stay off it: the advice is to keep mowers and other traffic off the lawn during extreme drought.^[3]

4. Clumps of wet clippings

Mowing wet or overgrown grass leaves rows and piles of clippings. Wet grass sticks to the mower and falls in clumps on the lawn,^[1] and a thick layer of wet clippings can smother turf.^[1] The result is yellow or brown strips where the clumps sat. Scatter clumps with a rake or blower the same day, and mow when the grass is dry and before it gets too tall.

5. Brown patch and other diseases

If the browning comes as roughly round patches in hot, humid weather, think disease. Henrico County's Extension office says brown patch is the most common lawn disease in its area,^[2] caused by a fungus that thrives in warm, humid summers; the leaf blades turn brown and die in patches, but the grass plant itself usually survives and recovers when the weather turns cooler and less humid.^[2] Mowing does not cause it, but short mowing and too much fertilizer at the wrong time both leave grass more open to disease.^[2]

To confirm it, your local Virginia Cooperative Extension office can help; Henrico's office asks for a six-inch round sample of turf with the roots intact.^[2] It also recommends that you wash mowing equipment after each use, because disease pathogens can live on plant debris and be transported to other locations.^[2]

6. It is the season, not the mower

Warm-season lawns, such as bermudagrass and zoysiagrass, go dormant after repeated frosts and lose their green color for 4 to 5 months from late fall to early spring.^[6] If your lawn browned in October and the mower happens to be the last thing that touched it, the mower is not to blame.

When the lawn does not come back

After a dry summer, wait for rain before deciding what to do. Fall rains quickly show which cool-season turf is alive and which is dead, with living grass greening up within a day or so of a significant rain.^[5] Most damaged lawns end up with problem spots of 50 to 150 square feet surrounded by healthy grass,^[5] which are worth repairing on their own rather than renovating the whole yard, and fall is the best season to establish cool-season grasses by seed or sod.^[5]

Stop and call a pro if

• Brown patches keep spreading through the lawn even after you have fixed the blade, height and watering.
• Large areas are dead after fall rains and you are considering a full renovation.
• You want a fungicide or insect treatment applied; that is work for a business holding a Virginia pesticide license.
• The mower vibrates, cuts unevenly from side to side, or the blade is bent. A shop that does lawn mower repair can fix it.

For more, read how often to mow in Virginia, browse the lawn and landscaping answers, or see lawn mowing services. If you would rather have someone else keep the lawn cut right, describe the job once and local businesses quote it.