Mar 15, 2024

Summer cover crops: A matter of timing, seed choice

It’s easy for Ajay Nair, professor in Iowa State University’s Department of Agriculture, to point out the reasons growers should include cover crops in their planning process. He’s seen the results of increased microbial diversity add to the richness of the soil.

“Why is that important to vegetable growers?” Nair asked during a session on integrating summer cover crops in organic vegetable cropping systems at the 2023 Great Lakes Fruit, Vegetable and Farm Market Expo in early December in Grand Rapids, Michigan.

“It’s important because we are operating under an annual cropping system,” he said, explaining that after spring or early summer planting, everything from root development to fruit development has to happen in a relatively short time, as opposed to perennial crops.

“We need those nutrients when the plants require it,” Nair said. “So we need that active biology in the soil to release those nutrients. We need that organic matter to be broken down.”

Results aren’t immediate, however; cover crops can build the soil’s water-holding capacity, but that usually takes three or four years of continued cover cropping on the same land. Nair recalls a visit to a Bondurant, Iowa, pumpkin grower’s field in August 2018, a dry year for that area.

Side-by-side plots of the same cultivar that were planted on the same date looked very different, with one plot showing wilting and other signs of water stress. The difference was the healthy plants were on land with an eight-year history of cover cropping.

For organic growers, a key benefit of cover cropping is weed suppression without using chemicals.

Summer crop considerations

But does the typical Midwest vegetable planting, growing and harvesting cycle lend itself to a summer cover crop between spring and fall crops? Yes, if they are planted in mid- to late-June and given 50-60 days to generate enough biomass and weed suppression capabilities to be of use for the late-summer/fall cash crop.

Nair’s research focused on eight cover crops, including buckwheat, sunn hemp, sorghum sudangrass, teff grass and golden flax.

Sorghum sudangrass and teff produced the most biomass in a 60-day growing cycle in the summer, and a higher biomass means fewer weeds. Cowpeas, as a cover crop, however, is not a weed suppressor, Nair said, but being a legume, it had the highest nitrogen content in the biomass. Mung bean and sunn hemp, also legumes, had high nitrogen content.

After terminating the cover crops, Nair and his research team planted cabbage and beets on the various plots. The legumes produced more nitrogen, leading to larger beets and heads of cabbage. Nair said that properly inoculated, the legume cover crops will fix nitrogen in the soil, which leads to larger vegetables.

On the other hand, the grasses led to “excellent” weed suppression, Nair said.

There are caveats, however, such as sorghum sudangrass’ allelopathic properties — the chemical inhibition of one plant by another — due to the release of substances that inhibit growth or germination. Nair said lettuce that was transplanted immediately after termination of sorghum sudangrass turned yellow and refused to grow. Had they waited 10-14 days, the result would have been different, he said.

There are other considerations, such as equipment needed to terminate a crop. Teff is very “wispy” and light and requires a flail mower.

Benefits to the soil, crop

Seeding rates necessary to produce a favorable result must be taken into account when considering which cover crop to plant. Nair said some seeds cost more but may require fewer pounds of seed per acre, resulting in a lower overall cover crop expense.

The timing of a summer cover crop between cash crop seasons is important: When is the cover crop planted, how fast does it develop, when must it be terminated to avoid reseeding and will the process be completed in time to meet the needs of the fall cash crop?

“In all of this, for you as growers, this system has to work for you,” Nair said. “The cover crop has to be manageable for you. And you need to get the seeds for that. And you need to see what benefits it provides to the soil and also to the vegetable crops.”

Nair referred growers to the Midwest Cover Crops Council, midwestcovercrops.org, for more information on specific crops.