Extension Educator, Horticulture
Some like it hot: some like it not. Plants have their own optimal growing temperatures. For example most lawns are cool season grasses, Kentucky bluegrass and perennial ryegrass. The designation as a "cool season grass" or a quick look out your window is a good indication that this summer weather has been hard on our lawns.
On the other end of the heat spectrum crabgrass is a warm-season annual weed. Predictably crabgrass has emerged the victor in the grass war this year. The light green carpet of wide bladed crabgrass is a common sight.
So what is causing this perfect storm of crabgrass invasion and lawn grass demise? Tom Voigt UI turf specialist states our desirable lawn grasses have had it pretty good over the last couple years. Remember how nice August was last year? This year is a reminder of how cool season grasses naturally brown and go dormant in hot dry weather.
Voigt also shared how this spring our lawn grass didn't develop a good root system; either because the soil was so waterlogged it couldn't grow or there was plenty of available water so it didn't need to develop an extensive root system.
Enter hot dry weather. Lawn grass suffers and the warm loving weeds flourish. Weather is a factor but also lawn management techniques can determine the extent of weed proliferation.
Crabgrass and weeds in general prefer full sun and thin lawns that allow sunlight to penetrate to the soil surface. A dense, vigorously growing lawn that shades the soil will prevent crabgrass and most other weed seeds from germinating.
Proper lawn management can help prevent crabgrass. Mowing no shorter than 2-1/2 to 3 inches high will typically result in fewer crabgrass plants appearing in lawns. Crabgrass also likes frequent light watering. If you decide to water your lawn, water thoroughly at about one inch of water a week.
So what do you do now that you have a lawn of crabgrass? The good news is we are entering a great time to renovate and thicken the lawn. Generally mid-August through September provides the best time to reseed, overseed or resod lawns.
If lawn areas are all weeds and no desirable grass remains, a complete renovation may be needed. Nonselective herbicides such as glyphosate may be used to kill existing plants. Be sure to read and follow all label directions. After weeds are dead the area can be reseeded or resodded.
However many lawns just need thickening. Planting seed into live or dead plants often referred to as "overseeding" may be adequate. For successful seeding, the seed must be in good contact with the soil either by using a rake or with a slit seeder machine.
Before renovation thoroughly examine why the lawn is not growing well and correct problems. Lawns can decline for a combination of reasons beyond hot dry weather such as insects, excessive thatch and unfavorable growing conditions such as shade, poor drainage or poor soil.
Weed killers are not the only answer to weeds. Postemergence [translation-the plant is already growing] crabgrass herbicides need to be applied when crabgrass plants are very small; typically crabgrass is noticed too late for these to be very effective. Lawn care professionals have herbicides that are more effective on large crabgrass plants; however the long term solution is to concentrate on thickening the lawn. Also keep in mind some herbicides can affect the germination of lawn grass seeds so have a plan first.
The suggested strategy is to improve the lawn through cultural practices of mowing at the correct height and watering and fertilizing appropriately, renovate now if needed to create a denser lawn then avoid crabgrass next season by continuing good management practices and consider a preemergence [translation-applied before the plants emerge] crabgrass herbicide in the spring.
Check out http://urbanext.illinois.edu/lawntalk/