Preemergent herbicides (aka “crabgrass preventer”) are most effective at controlling annual grassy weeds like crabgrass, foxtail, goosegrass, and barnyardgrass. Preemergent herbicides kill very young weed seedlings but not seeds, so they must be “in place” prior to weed seed germination to be effective. One half to one inch of water from precipitation or irrigation is needed following application to move the herbicide off foliage and onto the soil/thatch zone where weed seeds can germinate. Apply preemergent products by mid-March in Pueblo and Grand Junction, by early April in the Denver metro area, and by mid-April in the northern Front Range. Sites that warm earlier in the spring like southern exposures and turf near sidewalks should be treated earlier than these dates. Acceptable weed control and prevention of turf injury require uniform application at correct rates. The most commonly available preemergent products contain one of the following: pendimethalin, prodiamine, dithiopyr, benefin/trifluralin, or oxadiazon. Some pre-emergent herbicides can harm the roots of lawn grasses if excessive rates are applied, so follow all instructions carefully when applying them. A healthy, vigorous turf will enhance the effectiveness of any pre-emergent herbicide application. Since all pre-emergent herbicides, except siduron sold as Tupersan, will also kill germinating grass seed, lawn overseeding must be delayed until late summer/early fall when a spring pre-emergent application is made.
For more information, see the following Colorado State University Extension fact sheet(s).
For more information, see the following Planttalk Colorado™ script(s).