It seems like you can’t read a story these days without hearing about cover crops.

They are great tools for keeping soil covered, to build organic matter and to keeping vital nutrients in the soil. Farmers have largely bought into their use in corn and soybean rotations.

But their use in vegetables is much less talked about, even though vegetable growers have found some interesting ways to include cover crops in their rotations, too.

“So the key is organic matter. That’s the driver for soil health, and certainly one of the things we want to do is use cover crops to provide that organic matter,” Gordon Johnson, Extension fruit and vegetable specialist with University of Delaware Cooperative Extension, told a group of vegetable growers at the recent Mid-Atlantic Fruit and Vegetable Convention in Hershey, Pa.

The sandy soils on Delmarva are already low in organic matter, he said. The region also suffers from many soilborne diseases, limited land for rotation and widespread compaction from machinery operations on the same soils.

Cover crops can be used for different purposes in vegetables, Johnson said, so it is crucial for growers to figure out what they want out of their covers and how it will fit into their rotations. It is also important to know how to manage them.

Mustards, for example, can be used as biofumigants to reduce soilborne diseases. The best way to manage them, Johnson said, is to let it grow, flail-mow it and then immediately incorporate it into the soil. This enables the plant to release its biofumigant into the soil.

Rape seed, which can overwinter, is another cover crop Delmarva growers are using as a biofumigant, he said, along with newer Caliente mustard varieties.

Sorghum is another crop with biofumigant properties, but it also has trace amounts of cyanide compounds, which can kill the next crop. So, Johnson recommends waiting three weeks after flail-mowing to incorporate sorghum into the soil.

Sunnhemp, a tropical legume, grows well in the hot Delmarva sun, and it can provide good biomass and nitrogen, but it is also hard to incorporate. Johnson recommends flail-mowing this crop early before it gets too woody.

Many Delmarva vegetable growers also use cover crops as a mulch in no-till. For example, pumpkin growers regular use rye or rye vetch as a mulch before planting pumpkins right into them. “They produce clean pumpkins with few problems with rot,” he said.

Cover crops are also being used in what Johnson describes as biological strip-till systems. One example is tillage radishes planted in rows into mixed cover crop fields using an 8-foot drill — legumes in the small seed box, rye in the grain box and tillage radishes in the middle. The rows are 7 inches apart. 

The tillage radishes winter-kill to help break up compaction, but they also create a good spot for transplants to be planted the following spring. The result are cash crops growing in rows with already established cover crops beside them.

Perspectives from the West

In Utah, the second-driest state in the country, cover crops have become an important tool to build organic matter in the desert soils, said Daniel Drost, Extension vegetable specialist at Utah State University and a Michigan native, who also spoke at the convention.

Desert soils don’t have much organic matter, he said, and historically manure has been brought in to provide it. But animal sources aren’t close enough, and it has become too expensive to move manure to farms.

Growers are interested in fall cover crops that can overwinter, grow in early spring and can be incorporated into the soil. Some growers are incorporating cover crops in a summer fallow period where they can stabilize and add organic matter after a vegetable crop. Millet has become a popular option for summer as it grows fast, although it doesn’t produce as much dry matter as rye.

Broadleaf cover crops, including brassicas such as buckwheat, mustards, radishes and rapes have become popular as they can overwinter and be killed the following spring. Kale is a popular one, too, Drost said, because it can also overwinter and be harvested as a crop the following spring.

VIEW FROM UTAH: Daniel Drost, Extension vegetable specialist at Utah State University and a Michigan native, spoke at the recent Mid-Atlantic Fruit and Vegetable Convention about work he’s doing on cover crop uses in the dry desert soils of Utah.

While not a good plant for nutrients or soil building, Drost said many growers, especially organic growers, like buckwheat because of its ability to control weeds.

Of course, mixes are always an option, too. Drost said he’s been doing trials with growers on combinations of summer cover crops such as black beans, buckwheat and millet with fall cover crops like vetch, wheat, and either kale or rape. The crops are planted in plots in opposite directions.

The purpose of the trials was to look at the amount of nutrients produced by the combos, number of weeds and each combination’s later effect on cash crops such as broccoli and sweet corn.

Each combination produced different biomass accumulation and weeds, but the goal is to give growers options, whether it is building up organic matter, better weed control or a combination of both.

"What the growers are saying, we've got to figure out ways to put these things together and then look at the problems," Drost said. “The important thing is there are these beautiful combinations out there that can do the job for you and make your life a whole lot easier, and still get those levels of productivity that are going to make your farm successful.”

Johnson thinks cover crop mixes should be limited to no more than three different cover crops from different plant species.

And if increased organic matter is the goal, there is another option you could consider, but it is more of a long-term solution.

“Don’t forget one of the best ways to increase organic matter is to put in hay or pasture. If you have animals, three years in hay or pasture will increase organic matter greatly, though this will come with pest issues,” Johnson said. “If you have this kind of rotation with animals, that’s the best way to deliver organic matter.”