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 a couple of different environmental issues.
Warm-Season Grassy Weeds
Closer inspection of the grass may show that the "dead" plants aren't the typical cool-season lawn grass species like Kentucky bluegrass, ryegrass, fine fescue, or tall fescue.
Certain grasses, such as zoysiagrass and nimblewill, can be invasive in Iowa lawns. Both are 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 Warm-Season Grassy Weeds in Cool-Season Lawns
Warm-season grasses like nimblewill, 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 an application of glyphosate (Roundup). Another option is to dig up and destroy the grassy weed. 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 the two most common warm-season grassy weeds in bluegrass lawns, nimblewill and zoysiagrass. 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 its not applied appropriately. When treated with Tenacity, the warm-season grass stops growing, turns white, and eventually dies. Three applications (at two-to-three-week intervals) are usually necessary to control nimblewill and zoysiagrass. 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 of grass, such as a warm-season nimblewill or zoysiagrass, the large brown patches could have been killed from 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 are not able to replace the lost water and die as a result.
Mangment 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 top soil and mix it with a couple 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 and 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.
Dog Spots and Salt Damage
Brown patches can also form in areas where dogs have repeatedly urinated over the winter. The high concentration of nitrogen in the urine causes grass plants to die. Similar brown patches can form along heavily salted sidewalks or driveways. The high concentration of salt causes the grass to die.
Management of Dog Spots and Salt Damage
Reseed areas killed by excessive dog urine or salt in the same way you would reseed areas killed by winter drying.