Author: Dianne DeLongchamps
What causes brown spots in Colorado lawns during summer?
Late-summer brown spots in Colorado usually trace to one of five things: dog urine, a dull mower blade, uneven sprinkler coverage, white grubs feeding on roots, or necrotic ring spot. The shape of the patch is your first clue, and a cut-and-look test is your confirmation. A lift test alone won’t tell you the cause.
Clean Air Lawn Care fields this question all August across Fort Collins, Windsor, and Timnath. Grubs are the pest we actually deal with here; other lawn insects are uncommon. Most lawns we treat are turf-type tall fescue and Kentucky bluegrass. Luke asks every customer for two habits that prevent a lot of this: mow at 3 inches minimum, 3.5 in July and August, and water 1 to 2 inches a week across two to three deep waterings, closer to 2 inches from mid-July to mid-August.
What does the shape of the brown patch tell you?
Round or oval, dinner-plate size. Usually dog urine, concentrated nitrogen burning the grass in the center.
Straight lines or stripes. Mower-related. A dull blade tears grass instead of cutting it, and torn tips brown out in a day or two. Look at a blade tip: clean cut or shredded?
Patches at the edges of sprinkler zones. Coverage gaps. Run a catch-can test before assuming worse. How Often Should I Water My Lawn in Colorado Summer Heat? walks through it.
Irregular patches that feel loose underfoot. Roots are compromised. Grubs or disease, see below.
Circular or doughnut-shaped, a few inches to a few feet across. CSU’s description of necrotic ring spot, which PlantTalk 1528 calls “the most common lawn problem in Colorado.”
Does turf that lifts like sod mean I have grubs?
You’ve probably read that turf lifting like loose sod means grubs. CSU does describe grub-damaged turf as lifting “as if it had been newly laid sod.”
But necrotic ring spot causes “extensive root and crown discoloration and rotting,” per CSU 2.900. That turf lifts too. So a lift test tells you the roots are gone. It doesn’t tell you what took them. Cut three sides of a 12-inch square, pry the flap back, and look at the top 1 to 3 inches of soil. C-shaped white larvae means grubs. No grubs, plus dark rotted roots and crowns, points toward necrotic ring spot.
Why do brown spots show up now if the damage happened earlier?
Late July through early September is when Colorado lawns look worst. It isn’t all happening now.
CSU’s treatment window for young southwestern masked chafer larvae is mid-July to early August. Rocky Mountain billbug larvae do their damage from late June through July. Second-generation sod webworms feed through July and August. By the time you’re staring at a dead patch on Labor Day weekend, the insect that caused it may have finished weeks ago.
Necrotic ring spot works the same way. CSU 2.900 says the fungus colonizes roots at soil temperatures of 65 to 80°F, and “plants with damaged root systems suffer greater injury during high heat or drought stress, which is why visible symptoms often appear in July-August.”
CSU describes billbugs as “relatively uncommon in Colorado lawns” and chinch bugs as “uncommon in Colorado.”
What should I do about brown spots in my lawn?
- Don’t panic-water. Extra water on an unidentified spot can make things worse. CSU on necrotic ring spot: “Do not overwater. This is perhaps the most important prevention practice for NRS.”
- Cut a flap and look. Three sides of a square foot, pried back. Thirty seconds, and it settles the grub question.
- Check your mower blade. PlantTalk 1515 recommends sharpening “after every third mowing, or, at the least, once a month.”
- Raise the deck. CSU warns that mowing under two inches “can reduce drought and heat tolerance, and cause a higher incidence of insect, disease and weed pest problems.” Our standard for customers: never below 3 inches, 3.5 in July and August.
- Get eyes on it if it’s spreading. Several of these look identical from the patio and need different responses.
Dianne, Co-Owner in Northern Colorado, fields most of these calls, and she’ll tell you the same thing every time: send a photo, then go cut a flap and look. It’s amazing how often a patch that’s been worrying somebody for three weeks turns out to be a sprinkler head that got bumped by a mower. If you’re in Fort Collins, Windsor, or Timnath and want a second set of eyes, just ask.
Clean Air Lawn Care Northern Colorado is owned by Dianne, Paul, and Luke DeLongchamps — a family business serving Fort Collins, Windsor, and Timnath with organic lawn treatments and solar-powered electric equipment.
