Crabgrass is an annual weed that can become a rather pesky problem in a lawn. I think it is impossible to not have a little crabgrass in a lawn, but there are both cultural and chemical steps that can be taken to reduce the growth of crabgrass patches in lawns.
The best and environmentally-friendly way to control it is to create a dense, healthy crop of turf grass. Mowing at the correct height for the turf species will help. This height is normally 2½ to 3 inches. Mowing any lower than this will thin blade presence, which can allow sunlight to reach the crabgrass seed. When needed, irrigate deeply. Light watering promotes shallow rooting of turf grass which will not tolerate drought conditions well. Hand-pull any crabgrass plants. It is a fast growing weed that can take over an area quickly. Lastly, follow the recommended fertilization schedule, which is to apply nitrogen in two applications in the fall: one in September and one in October or November after the final mowing. I use Labor Day and Halloween as my reminders.
There may be conditions in which a pre-emergence herbicide will need to be applied to help control the spread of crabgrass. This annual weed germinates when the soil temperature reaches 55 to 60 degrees F for 3-5 days at the ¼-inch level. Most likely this will not happen until sometime in April or May, depending on the weather. Pre-emergence herbicides need to be applied at least two weeks prior to germination for them to be effective. Very often you will find these herbicides combined with fertilizers. It is recommended that fertilization in the spring be kept minimal, so look for a product that contains a slow release nitrogen in the form of methylene urea, sulfur or polymer-coated urea. Avoid the quick release products. To get the most bang for your buck these products will need to be watered in after application. Do not use these herbicides on newly seeded areas.
It is very important when working with any chemical to read, understand fully and follow all label directions. In trying to be environmentally-friendly, use herbicidal control only if necessary.
As always, Happy Gardening!
The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange County.