Bruce Peterson has been farming near Northfield for more than 30 years, but this year he did something brand new. In mid-September, he shared the cost of hiring a plane with about a dozen other farmers and dropped a mix of cereal rye grass and radish seeds on about 16 acres of standing corn.

The farmers are experimenting with cover crops, which can improve soil and reduce erosion during the many months between fall ­harvest and spring planting. It's a time when millions of acres of Minnesota cropland lie brown and exposed for weeks before and after winter's snow, vulnerable to heavy rains and gusty winds.

"Our topsoil is pretty much like gold out there," said Jill Sackett Eberhart, University of Minnesota Extension educator in Mankato. "Decrease in erosion is one of the main benefits of cover crops, hands down."

At a time when crop farmers are often blamed for polluting water with chemicals, sediment and fertilizer, the crops also can curb runoff and absorb residual nitrogen that might otherwise be washed out of the soil and into ditches and streams, both farmers and environmentalists say.

However, growing them in northern states like Minnesota can be tricky, as Peterson and others are learning.

Cover crops are plants grown mainly for soil and conservation purposes rather than for cash, said Sackett Eberhart. Some of the more common ones in Minnesota are oats, cereal rye and forage turnips. The 2012 federal farm census estimated that cover crops are grown on about 1.5 percent of Minnesota farmland, but there are no more recent figures.

Southern farmers have grown cover crops for decades after their row crops are ­harvested, Sackett Eberhart said, mainly because those states have longer growing seasons. Minnesota farmers who cultivate peas and sweet corn with midsummer harvest dates also have grown cover crops, she said, but the 15 million acres devoted to corn and soybean rotations represent a bigger challenge.

Most of those cash crops aren't normally harvested until October or even into November, and by then soil temperatures are too cool to germinate most seeds and warm weather and sunlight are waning. In order to get established and grow, cover crops in Minnesota need to be planted earlier than that — while corn and soybeans are still standing in the fields — to have much chance of success.

Paul Meints, research director for the Minnesota Corn Research and Promotion Council, said establishing cover crops is the biggest challenge to large-scale planting, as well as the costs to buy and apply the seed.

"There's growing interest in cover crops, but they're very difficult to get growing because of our colder climate," he said. "It probably won't work everywhere, but we need to find where it can be really be an option."

Cover crops are a high priority for research projects that the council helps to fund, Meints said, and there's no doubt that providing cover can offer multiple benefits, such as improving soil over time.

"There's a number of interactions where keeping active root systems growing is good for the biotic part of the soil," he said.

Some cover crops can also take up any residual nitrogen and tie it up in plant material during the winter months and early spring, Meints said. Then the nutrient is available for corn, soybeans or other cash crops as the material decomposes or is tilled into the soil for the next growing season.

That's one of the things Peterson will test on his cover crop acreage. Standing last week amid the 4-inch rye and radish greens growing through corn stubble, he said the radishes will root deeply and help break up soil compaction, and the rye grass will store residual nitrogen left in the soil. Next spring, Peterson will plant the area with soybeans and test whether he gets more bushels per acre on the land where cover crops grew. He'll be comparing that extra yield to the cost to buy the rye-radish seed blend and have it sown by air, which came to about $44 an acre.

"The question that remains to be seen is what kind of benefits we will see," Peterson said. "There's not a lot of extra dollars to throw at experimentation" because corn and soybean prices are low, he said, "so if farmers are going to try this, they may start small scale at first."

Bryan Biegler, a corn and soybean farmer near Lake Wilson in southwestern Minnesota's Murray County, has been experimenting with cover crops for the past three seasons with a mix of successes and failures.

The first year he seeded cover crops by plane in September, but there was no rain for weeks so nothing grew. Last year, he had good results when he planted cereal rye early and mixed results when he planted a five-species mix by plane in September.

This year, Biegler purchased a high clearance sprayer and converted it to apply a cover crop seed mix on corn and soybeans in mid-August.

The rains came and the cover crops grew pretty well between the corn rows, but not on the soybean field. "I think the canopy [of leaves] was too tight, and not enough sunlight got down to the ground, so those crops all died off," Biegler said. Seeds planted on a different bean field one week later did much better, he said.

In total, Biegler said this year he planted cover crops on about 300 acres, about one-seventh of the total land that he farms, at a cost of $25 to $35 an acre, depending on the amount and type of seed and how it was applied.

"It's just hard to figure out that best end-of-season time to plant," he said.

Planting too early can also be a problem, experts say, because if cover crops start growing before corn and soybeans mature, they may compete enough to reduce yields, and some crop insurance policies will be invalidated.

Biegler said that where he has successfully grown the cover crops, erosion has been reduced dramatically. Other farmers are also showing interest, he said, especially about the potential to use cover crops to graze cattle.

The University of Minnesota is also researching ways to better use farmland in its Forever Green Initiative. Agronomists and other scientists are looking at various plant species not just as cover crops but as adjacent crops to the row crop system that can be harvested for income.

Sackett Eberhart is optimistic that while cover crops are still considered fairly new for many in the state, farmers are curious and willing to experiment, whether it be using planes or helicopters or retrofitting equipment to get them planted early enough. "What we're seeing around the Midwest is that farmers and researchers are thinking outside the box," she said. "It's that ingenuity that has renewed the interest in cover crops."