Cover crops are common, and most growers use grass.
However, a few soil researchers suggest growers consider covering their alleys with legumes instead, as some organic growers already do.
Several research projects in Washington and Utah have recently shown legumes — nitrogen-fixing plants such as alfalfa, clover and soybean — provide more carbon and nitrogen for soil. They also lead to faster crop growth, compared to grass, when planted in alleyways, cut and blown into tree rows.
What’s more, Utah trials indicate the root systems of legumes are more compatible with trees than are those of grass and do not necessarily lead to more pest damage.
“So, that was encouraging,” Jennifer Reeve, a professor of sustainable agriculture and a soil scientist at Utah State University, told growers at the Washington State Tree Fruit Association Annual Meeting in December in Wenatchee.
This is not a new idea. Decades ago, farmers planted alfalfa as alley crops to eke value from their land while their new fruit trees matured, Reeve said. Some still do that.
Later, university extension educators advised against legumes, worried the high-nitrogen content would attract pests. However, most of that advice came from the Eastern U.S. growing areas where pest pressure is higher.
In the arid hills of the West, several orchardists use legumes for soil and weed benefits without negative pest impacts, Reeve said.
At Utah State, Reeve worked with trefoil because it grows low and tolerates drought, traffic, shade and shallow soils. At the Kaysville, Utah, research orchards, her team compared the growth of new peach trees with trefoil versus grass in the alley in a trial that ran from 2009 to 2014. They mowed four to five times a year, blowing the clippings into the tree row to capitalize on nitrogen and organic matter. The experiment also involved comparison of tree row treatments and different ways of managing weeds directly underneath the canopies. Portions of her research were funded by a U.S. Department of Agriculture Organic Agriculture Research and Extension Initiative grant.
With grass in the alleys and no in-row weed control, tree growth slowed. “What we found is when you had trefoil in the alleyway, that didn’t happen,” she said.
In fact, trees with legumes in alleys grew just as well as trees with grass alleys and in-row weed control. The nutrient boost from legume clippings made up for what the trees lacked in weed control directly underneath them.
Meanwhile, trees with trefoil in the alleys outgrew their grass counterparts regardless of novel weed treatments (mowed weeds and straw) in the tree rows. That means those novel weed treatments, meant as alternatives to traditional tillage and expensive weed fabric, worked better with legumes in the alleys than with grass. Soil samples showed higher levels of nitrogen and carbon, too.
Legumes also did not seem to boost pest damage. Reeve said her biggest concern was lygus bugs, also called tarnished plant bugs, which leave unmarketable marks on fruit. The researchers found higher populations of lygus bugs in the legumes, compared to grass, but not an increase in fruit damage.
What’s more, soil cores showed the trefoil-flanked trees had a larger root biomass, giving them more access to nutrients. In the comparison rows, tree roots typically grew only in the rows and stopped when they reached the grass alleys. Reeve suspects that legumes don’t compete with tree roots the way grass does. Legumes have taproots that reach deeper for water, while grass sends out roots more horizontally.
David Granatstein, a retired sustainable agriculture professor at Washington State University, thinks the root compatibility of legumes and trees warrants extra study.
“That’s wide open,” he said.
Under his belt, Granatstein has several research projects regarding legumes, as do some of his WSU colleagues. Generally speaking, legumes can provide 50 percent or more of the nitrogen needs of orchard trees at a cost competitive with fertilizer.
One of his larger studies compared 25 different varieties. Alfalfa appeared able to provide the most nitrogen, but there were differences among varieties and types, such as hay, grazing and dryland. Timing was an issue, too. Compared to compost or grass, legumes break down to release nitrogen relatively quickly, which means apple growers could time their final mow to start fertilizing immediately after harvest.
As for pests, meadow voles were Granatstein’s biggest problem. In Central Washington, he recommends avoiding white clover because that’s a favorite food of voles.
He advocates for more research but encouraged growers to try out legumes on small patches. “I think there’s value here,” he said.
On the farm
Count Steve Ela, the fourth-generation owner of Ela Family Farms in Western Colorado, among legume believers.
Ela has about 80 acres of tree fruit — peaches, apples, pears, sweet cherries and plums, all organic since 2004. He plants alfalfa and Dutch white clover in alleys and directly under tree rows. Now, after Reeve’s research, he plans to try trefoil.
The legumes provide nitrogen when mowed and dropped in place, he said. They keep quack grass, a native undesirable that competes with trees, at bay. The taproots penetrate his heavy clay subsoil to improve his overall soil profile.
He doesn’t have voles, but he definitely gets lygus bugs. However, just as Reeve found, in his orchard they prefer the ground cover to the trees, Ela said. He alternates his mowing every other row, leaving them some preferred habitat, until right before harvest, when he’s going to pick the fruit anyway.
He started with legumes about 20 years ago and plans to continue every time he plants anew.
“We use them in almost all of our blocks,” he said.•
—by Ross Courtney