We all know that one of the potential benefits of using cover crops is weed suppression, but do you know how cover crops actually provide weed suppression? There are a couple of ways: through blocking sunlight from reaching weed seeds, and also allelopathy, as the cover crop plants produce a chemical that reduces weed seed germination.

I was very intrigued to learn more about allelopathy at the Southern Cover Crops Conference last month in Auburn, Ala. According to information from USDA NRCS, allelopathic chemicals are released in three different ways — exuded by living plant roots, washed into the soil from leaves and shoots, and released by decaying residues.

Research has shown allelopathic attributes in cereal rye and other winter cereal grains, sorghums, sorghum-sudangrass hybrids, rapeseed, mustards, radishes, buckwheat and subterranean clover were strong enough to significantly contribute to weed control in field conditions.

You can even utilize specific covers if you have problems with one specific type of weed. For example, cereal rye is active against pigweeds, lambsquarters, purslane and crabgrass. Sorghums can suppress bermudagrass and nutsedge. Sunflower and subterreanean clover suppress morning glories.

The effects of allelopathy can last even a few weeks after covers are terminated, depending on weather conditions. There can be some impact on cash crops, particularly those grown from smaller seeds.

So how do cover crop growers maximize weed suppression? Growers should consider species selection, choose effective seeding and tillage practices and having a termination plan, suggests information published by NRCS.

When selecting species of covers, think about the weed you’re trying to combat, how quickly the cover crop establishes and how much biomass it produces. Your ultimate goal is to create direct competition between the cover crop and weeds so the covers take all the soil resources instead of weeds. Cover crop mixes are usually more effective than a monocrop, especially one that provides continuous living vegetation, says information from NRCS.

Seeding the covers immediately after harvest or even before is usually the best bet to get fast cover crop establishment, and it also reduces or eliminates a fallow period.

Termination timing is critical with some cover crop species to prevent allelopathic toxicity to the cash crop. If allelopathy is a concern, termination should occur 10-14 days prior to spring planting to allow time for the allelopathic residues to disperse, depending on weather conditions. Complete termination of covers also allows for more precise seed placement during cash crop planting, according to information from NRCS.

Although some experts express doubt about the ability of cover crops to fight weeds, I think this shows it’s definitely possibly if you manage covers properly. Have you used allelopathic properties in certain cover species with success to fight weeds? Email me and let me know how you did it!