Drive through any suburban neighborhood, especially on the weekends, and you'll see people tending their turf. "Mowing, blowing, watering and fertilizing ... it seems as if we have almost an obsessive attachment to our lawns," says master gardener Paul James. "But rest assured, nothing makes a lawn nut go nuts faster than the first sign of crabgrass."
Of all the weeds that inevitably invade the lawn, crabgrass is arguably the most unsightly and the most dreaded. The two species that are most common, both of which are members of the genus Digitaria and native to Eurasia, arrived here by accident several decades ago. But as nearly every homeowner knows, crabgrass now invades virtually every lawn in the country. Gardeners dread crabgrass because it reproduces by seed, and one mature clump can reproduce literally thousands of seeds. The germination rate of the seeds is quite high, which means that one mature clump of crabgrass can quickly become hundreds (perhaps even thousands) of clumps of crabgrass.
The most popular and effective way to control crabgrass is to apply a pre-emergent herbicide, and there are both synthetic and organic products available. However, the timing of the applications is critical. And in most cases, James says two applications are required to achieve good control. Apply one application in the spring and another in the fall.
"The spring application is typically done when the forsythia begin to bloom, which may be late winter in your area." The fall application is ordinarily done within two weeks either side of the first day of fall.
"But you need to think twice before applying a pre-emergent herbicide, whether in the spring or fall, because those are also the times when most people sow their grass seeds," warns James. A pre-emergent herbicide will, in addition to preventing crabgrass seed from germinating, also prevent your turfgrass from germinating. It's best to wait six weeks after applying a pre-emergent before you sow your grass seed. And that's why some people prefer to use a post-emergent herbicide on their crabgrass. Post-emergent herbicides can be applied practically any time during the growing season. And they work by disrupting cellular processes within the plant itself. In other words, they destroy the existing plants, not the plants yet to come. As a result, you can safely sow grass seed after you've applied a post-emergent herbicide.
James also points out that most herbicides sold for the control of crabgrass, both pre- and post-emergent also control a number of different lawn weeds. "And if you're like most homeowners, you want to get rid of every single weed in your lawn," Paul says. "However, as you probably know, I like a fairly small percentage of weeds in my lawn, so I rarely rely on herbicides but I do use an organic pre-emergent now and then, especially when the crabgrass gets out of control."