In spring, the cool-season lawns common across much of Iowa turn a vibrant green. Occasionally, you will notice large "dead" patches of grass. Often, the brown patch of grass seems to get larger every year. Upon closer inspection of the grass, one can see that there are no spots on the blades (from fungal fruiting bodies) or darkening of the roots or crowns (the part of the plant at the soil line). These brown spots are typically caused by several environmental factors.
Warm-Season Grassy Weeds | Winter Drying | Dog Spots & Salt Damage | More Information
Warm-Season Perennial Grassy Weeds
A closer inspection of the grass may reveal that the "dead" plants aren't typical cool-season lawn grasses like Kentucky bluegrass, ryegrass, fine fescue, or tall fescue.
Certain grasses, such as zoysiagrass and nimblewill, can be invasive in Iowa lawns. Both are perennial warm-season grasses that don't begin to green up until mid- to late-May. They perform well during the heat of summer when cool-season grasses, such as Kentucky bluegrass and ryegrass, struggle, since cool-season grasses grow best in the cooler temperatures early and late in the season. In early fall, when temperatures begin to drop and the health of cool-season grasses begins to improve, warm-season grasses start their way into dormancy.
Control of Perennial Warm-Season Grassy Weeds in Cool-Season Lawns
Perennial warm-season grasses, like nimblewill and zoysiagrass, are difficult to control in bluegrass lawns as control options are limited. One way to control infested areas is to completely destroy the warm-season grass with a non-selective herbicide. Herbicide applications should be made when the grass is green. At least two, often three, applications are needed to completely eradicate the weed. Before applying a second or third application, be sure regrowth is observed. Starting in early to mid-summer can provide enough time to completely kill the weed before reseeding the area at the ideal time of year in late summer/early fall. With this approach, bare or dead patches of grass will be present most of the summer.
Another option is to dig up and destroy the grassy weed. Be sure to remove all stolons, which can be difficult. Bluegrass or other cool-season grasses have to be reestablished by seeding or sodding after the warm-season grass is completely destroyed.
An application of a selective herbicide is another control option. Tenacity (mesotrione) is a systemic herbicide that selectively controls nimblewill in bluegrass lawns. It is also safe to use on lawns with perennial ryegrass, tall fescue, and the fine-leaf fescues. Tenacity is not readily available to home gardeners. It is best applied by lawn care professionals as it can negatively affect desired turfgrasses, especially when it's not applied appropriately. When treated with Tenacity, the warm-season grass stops growing, turns white, and eventually dies. Three applications (at 2- to 3-week intervals) are usually necessary to control nimblewill. Occasionally, white tips can also be seen on Kentucky bluegrass and other cool-season grasses following treatment. This is a temporary symptom, as the white grass tips will be removed during subsequent mowings.
Winter Drying
If the grass does not appear to be a different species, such as a warm-season nimblewill or zoysiagrass, the large brown patches could have been killed by winter drying. If the winter was mild, grass plants continue to actively transport water to leaves and into the surrounding air. If conditions are also dry, grass plants cannot replace the water they lose and die as a result. Grasses growing in typically wet areas, such as near ditches or surface drains, are more likely to be damaged when conditions are dry because they have limited root systems. Lawns that are over-irrigated are also more likely to have limited root systems and be more prone to winter drying.
Management of Dead Patches from Winter Drying
These killed areas likely need to be reseeded. This can be done in April when the dead patches are noticed in spring, before weeds can take root.
Using a rake, remove some of the dead, dry grass so the soil is visible. Obtain seed that is a similar mix of species to what is already growing in your lawn. For most Iowa lawns, this is a mixture of Kentucky bluegrass and perennial ryegrass. Shade mixes will include fine fescues.
Find a handful or two of soil from a garden area or from bagged topsoil and mix it with a couple of handfuls of grass seed. Mixing a small amount of garden soil with the seed helps ensure good seed-to-soil contact. Sprinkle this seed/soil mixture over the spot.
Water regularly until it germinates. Daily watering may be necessary if Mother Nature doesn't provide rain. The area should green up in two to four weeks; over time, the color and texture will fully blend with the surrounding grass. Overseeding in late summer/early fall may be necessary, especially if the newly seeded grass is thin or sparse.
More information can be found in this article: Overseeding a Lawn.
Dog Spots and Salt Damage
Brown patches can also form in areas where dogs have repeatedly urinated over the winter. The high nitrogen concentration in the urine causes grass plants to die. Similar brown patches can form along heavily salted sidewalks or driveways. The high salt concentration causes the grass to die.
Management of Dog Spots and Salt Damage
Spring rains frequently leech out excess salts, but if conditions are dry or salt levels are particularly high, water the areas deeply to remove excess salts before repairing them. Reseed areas killed by excessive dog urine or salt in the same way you would reseed areas killed by winter drying.
More Information
- Overseeding a Lawn
- How to Manage Perennial Grassy Weeds in the Garden and Lawn
- Lawn and Turfgrass Resources for Iowa
Photo credits: 1: Iowa State University Plant and Insect Diagnostic Clinic; 2: Bill Davison; 3: Iowa State University Plant and Insect Diagnostic Clinic; 4: Aaron Steil