Weed suppression with cover crops

Cover crops are one way to control weeds after harvest and into early spring prior to planting. The physical barrier of cover crop residue on the soil surface helps suppress weed growth by changing the soil dynamics, growing quicker than weeds, and smothering weed seedlings.

Bob Hartzler is an extension weed specialist at Iowa State University. He says if planting a cover crop to control weeds is your primary goal, then it’s all about the biomass. Cereal rye is a prolific mulch producer.

"Cereal rye has been shown to consistently produce the most biomass. But also, the carbon-nitrogen ratio of the rye is higher, so the microbes break it down less so it’s more persistent," says Hartzler.

Of course, there are factors that affect success such as when the cover crop is planted and terminated, and weather conditions. This also includes the weed species you’re trying to control. The smaller the seed, the more effective the cover crop will be.

"The waterhemp and Palmer amaranth are the big issue," he says. "The only good thing you can say about those is they do have small seed, and so the cover crop can be very effective. If a grower is struggling with things like velvet leaf, or cocklebur, or giant ragweed, those have larger seeds and are more likely to be able to push their way through that mulch on the soil surface."

Hartzler recommends leaving the biomass as undisturbed on the surface as possible. If your planter has row cleaners that sweep the rye out of the row, it opens up a small strip where the weeds are able to poke through.