I've been receiving a lot of calls lately from gardeners wanting to know how to prevent crabgrass in their lawns and gardens. I'm not surprised, because this is the time to be doing so.
Crabgrass is a warm season annual weed. It germinates every spring, right about the time forsythia shrubs are showing their yellow blooms. The best way to control it is to apply what we call a "pre-emergent herbicide" to the lawn. This would be more of a weed preventer than a weed killer, since the chemical is designed to keep the crabgrass from even getting started.
But before I tell people all of this, I hearken back to the first rule of pest control: identify the pest. If you don't know what the pest is, you will probably not control it properly.
So, when my caller says they have crabgrass, I ask them to describe it to me. True crabgrass grows in a prostrate (low-lying) clump. It develops a seed head in late summer, which is composed of 4 to 6 large spikes at the top of stems standing high over the lawn.
Yet the weed that is being described to me in many cases is Bermudagrass, one of the most troublesome weeds for the gardener to deal with. Bermudagrass also grows low to the ground, but instead of growing in a clump, it grows as a spreading, almost vinelike mass. This plant is very invasive, and will crowd out other grass species in the lawn. It will invade nearby flower and vegetable gardens, spreading with both rhizomes (underground stems) and stolons (or runners, above ground).
Bermudagrass is a perennial weed, which means that it comes back from its roots every year. It does produce some seed, but that's not the main way it spreads. A preemergent crabgrass preventer will do practically no damage to a Bermudagrass infestation. Bermudagrass is usually controlled during the growing season after it has greened up, using a nonselective herbicide like glyphosate (Roundup).
OK, maybe "controlled" is too strong a word. Let's just say that the Roundup will "temporarily annoy it."
Assuming you had a crabgrass infestation last year, you have two main choices to protect your lawn this year. One option is to control the crabgrass by thickening up the lawn. Overseeding thin lawns, fertilizing properly (starting with a slow-release fertilizer in late April), and raising your mower blades to a higher setting will usually squeeze out crabgrass and other weeds from your lawn.
The other option is to use a preemergent herbicide, as I mentioned before. Look for products that contain the ingredients dithiopyr (Dimension), prodiamine (Barricade) or pendimethalin (Pendulum). These do the best job in preventing crabgrass, but their major drawback is that they also will damage newly sprouted turf grass seeds. These products have to be watered in to be effective; be sure to read and follow all label directions.
For more information on controlling crabgrass and other lawn weeds, contact the Purdue Extension Service at (812) 435-5287.
Larry Caplan is an extension educator-horticulture with the Purdue University Cooperative Extension Service, Vanderburgh County/Southwest Indiana. Email him at LCaplan@purdue.edu.
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