Timing of crabgrass control makes difference

GREENFIELD — It seems the $64,000 question right now is “When should I apply my pre-emergence herbicide for crabgrass control?”

Crabgrass is a summer annual weed, a grass, that germinates in lawns each spring and seems to cause great distress in homeowners who hold up a well-manicured lawn as a sign of accomplishment and pride.

Pre-emergence herbicides prevent emergence of crabgrass plants from the soil. Such products must be applied prior to crabgrass germination, which could occur as early as April 1 in southern Indiana and three or more weeks later in northern Indiana in an average year (there don’t seem to be many average years these days).

Purdue research has shown that modern forms of these herbicides are very stable in the soil and can be applied in late winter (February or later) and still be effective all season. It is essential to apply these products early in spring prior to crabgrass germination, or they will not be effective. Once the crabgrass has germinated, applying these products will not kill this weed and the application will be for nothing.

Often, pre-emergence herbicides are combined with fertilizers, and since fertilization should be minimized in the spring, consider purchasing products with most of the nitrogen in slow release forms such as methylene ureas or sulfur or polymer-coated ureas.

Products with mostly quick-release nitrogen (more than 70 percent of total nitrogen), such as urea or ammonical nitrogen, should be avoided; they will be more readily lost through leaching and runoff before the grass plant can access and tie them up. Slow-release product may reduce that early flurry of mowing that quick-release products can cause.

The use of most pre-emergence herbicides on newly seeded lawns or before seeding your lawn should be avoided — except with products that contain the active ingredient siduron, which will actually allow for the germination of cool season grasses while preventing crabgrass from sprouting. They’re really quite amazing. Other common products will not be able to distinguish and will kill desirable turf grass seed and perhaps even very young newly emerged grass plants.

To be most effective, pre-emergence herbicides need to be watered-in after application, which usually means rainfall in the spring, since most irrigation systems aren’t yet operational.

The best approach to controlling crabgrass is to use a pre-emergence herbicide such as dithiopyr (Dimension), pendimethalin (Pendulum), prodiamine (Barricade), prodiamine and quinclorac (Cavalcade PQ), sulfentrazone and prodiamine (Echelon), and others.

These herbicides inhibit cell division and prevent crabgrass seeds from properly emerging. Since these herbicides work on germinating seeds, you must apply them prior to germination — with the exception of dithiopyr, which controls crabgrass after germination until it reaches one tiller. This is the product of choice if you wait to apply until after crabgrass germinates. This could be you.

Purdue research has shown that all of these ingredients provide similar and effective crabgrass control in Indiana.

Roy Ballard is an agriculture and natural resources educator with the Hancock County office of Purdue Extension (www.extension.purdue.edu/hancock). Contact him at (317) 462-1113 or rballard@purdue.edu.