GALLATIN, Mo. – Missouri’s unpredictable spring weather makes it tough to decide when to apply preemergent herbicides on lawns, says Manoj Chhetri, University of Missouri Extension horticulturist.

Preemergent herbicides, a form of chemical weed control, prevent germinated weed seedlings from growing. Preemergents must be applied at the right time of year to be effective, says Chhetri.

Preemergent herbicides are applied mainly to control crabgrass. “Hence, some folks call it a crabgrass preventer,” he says. “But the reality is that it will stop many grassy and broadleaf weeds.”

Germination of weeds depends on soil temperature. Research shows that crabgrass germinates when the mean soil temperature at a depth of 1 inch reaches 60 degrees Fahrenheit or above for several days. The preemergent product is most effective when the mean soil temperature at 1-inch depth reaches 50 to 55 degrees Fahrenheit for five days in a row.

Data at indicates that this is the time to put down preemergent in southern and central Missouri. “The optimum window would last until the end of April, but the earlier the better,” says Chhetri. “For northern Missouri, the window would extend one to two weeks later.”

The split application of the recommended amount at 30-45 days intervals would provide more long-lasting results than a single full dose, he says.

Products containing prodiamine, dithiopyr, pendimethalin and mesotrione effectively control crabgrass. Corn gluten meal derived from organic sources can be an alternative preemergent for organic lawns. Follow label directions for mixing and application.

Two-way granular products (fertilizer plus preemergent) are common, but Chhetri does not recommend them since the timing for fertilizer and herbicide do not coincide.

Cool-season lawns in Missouri need spring fertilizer in May; early fertilizer would increase the chance of nutrient leaching and runoff, creating environmental pollutants. Slow-release nitrogen-containing fertilizers would remediate the potential pollutant problems, as these are less likely to leach.

“Herbicides are just one of many practices to manage crabgrass and other weeds,” says Chhetri. “Growing dense, healthy turf with proper fertilizer, irrigation and proper mowing practices are equally essential to compete against weeds.”

More information: “It’s time to start thinking about that pre-emergence,” Missouri Environment & Garden newsletter,