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The ladino clover cover crop thrived for the first two years but has begun to decline.

The ladino clover cover crop thrived for the first two years but has begun to decline.

Geraldine Warner

Apple growers who are experimenting with growing a nitrogen source in the alleys of their orchards are finding that not all cover crops are equal.

Warren Morgan, a Quincy, Washington, grower has on-farm trials in which he planted different cover crops between orchard rows. The cover crops are cut during the growing season with a special mower that directs the clippings into the tree rows to form a mulch.

The primary goal is have the legumes provide nitrogen for biomass grown in the alley that is then mowed and blown onto the tree row to form a thin mulch layer, orchard manager Amos Kukes reported during a visit to the orchard this spring organized by the Natural Resources Conservation Service.  The mulch can improve tree growth, suppress weeds, add organic matter to the soil, and conserve water.

Morgan planted his first trial in May 2008, in a mature orchard, seeding directly into the grass drive alley with a four-foot-wide no-till drill. He planted alfalfa, jumbo ladino clover, kura clover, and birdsfoot trefoil, all perennial legumes. In some areas, the alleys were sprayed with herbicide before seeding to reduce the competition from weeds. Growth was affected by the shade that the trees cast on the alleys and by tractor traffic, which some plants withstood better than others.

David Granatstein, sustainable agriculture specialist with Washington State University, said it takes a number of years to figure out the benefits and drawbacks of the various types of cover crops, and there’s a lack of research in this area. “What we see in year one or two may be really different than in years three, four, and five,” he said.

Biomass

  • The alfalfa was quick to grow and produced the greatest amount of biomass of all the cover crops. The alfalfa tended to grow tall and spindly (probably due to shade), and  might work well mixed with other shorter-growing crops, such as clover, to provide a two-tier cover crop with even more biomass, Granatstein suggested. Alfalfa can serve as habitat for pests such as thrips and lygus bugs, but if rows are mowed alternately, the pests generally  move from one row to the next, rather than into the trees.
  • The ladino clover produced a good, weed-free stand early on, but started to decline by year three, possibly because rodents were eating it. Although some cover crops provide habitat for beneficial insects, they can also attract pests, and rodents are a big concern, Granatstein said.
  • The kura clover was the slowest to become established, and was barely visible the first year. However, by year three, it had dramatically increased its growth and started to move out towards the tree rows. “Can we wait that long?” Granatstein wondered. “Or can we plant it with something else as a nurse crop?”
  • A drawback of kura is that the seed is not that easy to find. Granatstein said he got the idea of planting it from experiments done in the Midwest in field crops and obtained the seed from Wisconsin. Kura also seemed attractive to rodents.
  • The trefoil takes a little longer to get growing in the spring, but it equaled alfalfa in biomass by year three.  The seed is easy to obtain.  There are a number of varieties, but good information on which ones are better suited to an orchard setting is lacking.

Nitrogen

The mulch under the trees should be light and actively decomposing, Granatstein said. He sampled the cover crops and soil in 2009 and 2010 to assess the nitrogen contribution of the plant material. The alfalfa leaf tissue contained 4.1 percent nitrogen, compared with 3.9 for the ladino, 3.4 for the trefoil, 3.1 for the kura, and 2.3 for grass. The alfalfa also had a relatively low carbon-to-nitrogen ratio of 10.6, making it quicker to decompose. Ladino clover had a ratio of 11.2, trefoil 13.0, kura clover 14.9, and grass 18.8. Granatstein estimates that the alfalfa, which produced close to 3,000 pounds of biomass per acre over the first three years of the trial, was providing about 33 pounds per acre of available nitrogen to the trees each year.

“It’s not going to meet your whole nitrogen need, but it’s going to be a nice contribution,” he said.

Morgan calculates the cost of applying the herbicide and planting the crop at $84 per acre. Based on those costs, Granatstein figures that a four-foot-wide swath of alfalfa supplies available nitrogen at a cost of around 65 cents per pound, versus 70 cents for a commercial ­fertilizer.

A wide swath of cover crop that is mowed and blown onto the tree row in midseason might provide the trees with too much nitrogen, affecting fruit quality and delaying maturity, Granatstein warned. To avoid that, the clippings can be dropped into the alley at certain times in the season.

Although there was no statistical difference between the amount of biomass in the areas that were sprayed or unsprayed before planting, the nitrogen level in the sprayed areas is greater. This is because the unsprayed cover crop contains a greater proportion of grass and other species, he reported.

Young orchard

In 2010, Morgan planted another cover crop trial in a young, grafted Fuji block that had greater light penetration. Alfalfa and white clover were seeded in May into existing vegetation, doing a double pass with the same drill to create a seven-foot-wide swath.

Kukes said that as well as blowing the cover crop clippings under the trees, he rakes prunings into the tree row after flailing. He has tried growing white clover in mature orchards, and, although it grew well at first, it was gradually outcompeted by the grass. “In older orchards, there’s so much shade from the trees, it’s hard to establish the cover crops without taking out the grass,” he said. “From now on, when we have a new orchard, we’ll establish cover crops when the ground’s bare and the trees are in,” he said.

Red clover

Neighboring orchardist Jack Toevs has planted red clover as a cover crop in a block of organic Piñata apples. The clover was planted in the fall of 2009, along with grass, into a bare alley using a grass drill with an attachment for small seeds. He got the seed from Montana.

Red clover is a biennial plant that can be perpetuated by letting it go to seed, Granatstein said. It can be mowed close after harvest to remove hiding places for voles. Toevs said he does not control voles, but he’s seen a barn owl, hawks, and even an osprey around his orchard that he thinks are preying on mice.

Junell Wentz, a cherry grower at Wenatchee Heights, said she uses a mixture of oats and plaster of Paris to kill voles. She attaches short sections of pipe to a PVC T-joint, and places a mixture made of three parts oats and one part dry plaster of Paris in the middle pipe with a cap on. Voles enter the open pipes and eat the mixture. The plaster of Paris moistens and hardens in their gut, killing them. Wentz said birds will not enter the pipe to eat the mixture, and birds of prey that eat the dead voles are unaffected because the mixture is nontoxic.