This last weekend I noticed a couple forsythia bushes starting to flower which reminded me that it's time to control the crabgrass in my lawn. Many may consider it strange that the blooming of forsythia is related to the presence of crabgrass, but there is a correlation. In general, crabgrass seed germination coincides with the time when forsythia blossoms start to fall. Thus, by observing the forsythia, one can determine early or late warming of the soil temperature. This in return, allows us to apply our preemergence product just before germination which insures us that there will be enough herbicide in the seed germination zone of the soil to control the crabgrass. Of course, we will need to make sure that the herbicide has moved into this zone and is not on the soil surface. Most often Mother Nature does this for us, but if the forecast is for dry weather, one will need to irrigate within a couple days after the application.
Now that we know when to apply our crabgrass preventer, the question of what product to use often comes up. There is no single product that is superior to all other products available. Most of the products available to homeowners for crabgrass control on established lawns will contain either: benefin, bensulide or pendimethalin. All provide excellent crabgrass control when applied properly. Benefin may have shorter residual control, but that will vary with the rate of application and environmental conditions. Therefore, one needs to compare the cost of these products, other weeds controlled that may be a problem, as well as the fertilization cost if the two are pre-mixed in order to choose the most economical/effective product.
Once this decision has been made, an emphasis needs to be placed on the actual application. Uniformly applying the proper amount is one of the most important aspects related to chemical control of crabgrass. If not enough herbicide is used, crabgrass control will be unsatisfactory; and if too much herbicide is used, turfgrass injury is likely.
For more information, refer to: FG-401, Home Lawn Care: Crabgrass Control.
Remember: Always read and follow the label of a specific product. The label provides mixing instructions and any precautions that may be important and are the law. If any questions arise, contact the manufacturer or your local county extension office before making any application. If your crabgrass infestation is severe, consider a total renovation of that area in the fall. Also, don't forget that establishing and maintaining a dense, vigorous growing turfgrass is the first step and most important method to reduce crabgrass infestations. Crabgrass seed needs light to germinate. By using proper cultural practices (mowing, irrigation and fertilization) one reduces the amount of light penetrating to the soil surface, which decreases crabgrass germination. Cultural practices generally won't eliminate crabgrass. But in conjunction with a herbicide treatment, one can be sure that lawn spots which were once crabgrass will quickly become turfgrass.
This article originally appeared in the April 8, 1992 issue, p. 53.