lawn crabgrass

Asked August 26, 2013, 8:02 AM EDT

I have leveled my yard by adding import topsoil, mostly sandy loam, reseeded the entire area ( 150 sq. yds ). I now have large population of crabgrass either from import topsoil or poor seed. I do not want to tear it up but salvage it, Other seed is bluegrass, fescules, rye grass. The lawn is on sand soil, and I cannot overwater. What and when can I start killing the crabgrass. It is very hot now

Crow Wing County Minnesota lawns and turf weed issues horticulture

1 Response

Thank you for the question. Controlling the crabgrass this season will be difficult but Mother Nature will help you. Crabgrass is a warm season grass and the first frost will kill the plants and you will probably have frost by the end of the month. It's the seeds that will create new crabgrass next summer. I don't know how you prepared your space for new seed, but crab grass seed could have come in with your topsoil or you could have poor quality seed that contained a large number of weed seeds. Since the crabgrass is already up, you can't use a pre emergent herbicide. A post emergent product for weeds that are already growing can't be used until your seeded area has grown enough to require 3 mowings. The new grass seed is unlikely to grow enough to need this amount of mowing this fall and if you use chemicals now, you risk injuring the new grass plants. Your best option is to water the new grass frequently and if it does require mowing this year, remove no more than 1/3 of the grass blade length. It is best to mow no shorter than 2 1/2-3 inches tall. This way the new seedlings can put down deep roots to withstand the winter. Attack the crabgrass in the spring by applying a pre emergent herbicide that will keep the weed from sprouting. Then work to create a vigorous lawn that will outcompete the weeds. Please read this publication to learn more: http://www.ext.colostate.edu/pubs/garden/03101.html