Herbicide strips within rows of commercial tree fruit or small fruits orchards, and in vineyards, have become standard practice. Runoff and erosion, depletion of soil moisture, reduction in organic matter, decrease in soil microbes and problems with soil fertility can be concerns under this type of orchard floor management. Some growers are turning to cover crops to try to control weeds, eliminate bare soil and enhance tree and soil health.
“There are so many things to consider when it comes to the cover crops,” Amaya Atucha, University of Wisconsin Extension specialist, said during a recent Advanced Apple Production and Management webinar. “It’s just not only thinking about the development of the trees and the yields. It changes the whole ecosystem down there.”
While perennial cover crops are typically grown in orchard aisles in the Midwest and in some other regions of the country, growing cover crops, annual or perennial, within the rows could be a viable option to leaving bare soil, said Annie Klodd, University of Minnesota Extension educator, during the presentation.
Going Under Cover
“There are regions in the United States where they do utilize annual cover crops in orchards and vineyards,” Klodd said. She provided examples of annuals which can be used as in-row cover crops: red clover, cereal rye mixes or annual mustard; Austrian pea and rye cover crop mixes have been recently researched. Perennial cover crops can include a variety of grasses, perennial white clovers, native vegetation or perhaps creeping thyme or wild strawberry.
Using soil pits and soil coring techniques, researchers can study the root zone of trees and compare the impact of in-row cover crops, Klodd said. Studying the system long-term is important in order to determine how cover crops impact tree fruit or vineyard crops over time, as the immediate effect may differ from the long-term outcome.
Cover crops will have an impact on the trees or vines under which they are growing. They can compete with trees for water and nutrients, leading to decreased fruit yield or quality, attract pests or rodents and possibly decrease tree vigor.
In a study where peach tree seedlings were planted into containers either with a living fescue sod or with a killed sod, whether or not the containers were irrigated, trees in the “living soil” system were shorter than in those without the cover crop. Without irrigation, the above-ground tree growth was most stunted. Establishing newly planted trees or vines without cover crops, and planting cover crops after a year or two of growth, might prevent this initial growth loss in the crop, Klodd said.
In another study, perennial creeping red fescue was planted under grapevines; other rows were treated with herbicide strips. While the cover crop vines showed a lesser amount of root growth in the topsoil, those vines pushed their roots farther into the soil layers, so as not to compete with the fescue roots. This cover crop was very successful in eliminating weeds.
However, the vines in the rows with cover crops showed decreased vigor over the first few years of planting. But after seven years there was no significant difference in fruit quality or vine health. Although vines have been shown to establish more slowly when in-row cover crops are present, in the long term, those vines adapt and have the same yields as vines planted into herbicide strips, Klodd said.
If fruit crops are newly planted, their growth can be stunted in the first few seasons when compared to those planted into bare soil systems. Cover crops will have different impacts depending on the age of the trees or vines, as well as the availability of soil nutrients and water. If nutrition or water is limited, cover crops will be competing with trees.
In one New York apple orchard study, trees established more slowly when a grass cover crop was planted, but over time were able to adjust and yields were comparable to those grown on herbicide strips.
According to Christina Curell, cover crop and soil health educator at Michigan State University, some cover crops can help direct water into the deeper soil layers, where it becomes available to the tree roots.
Various studies have shown that cover crops can decrease nitrogen availability. Grasses take up and reuse nitrogen efficiently, so even though soil nitrogen might be retained with cover crops and nitrogen runoff decreased, it does not mean that the added nitrogen is available to the trees, Klodd said.
Atucha was involved in a New York study where wood chips, grass cover crops or herbicide strips were used in an apple orchard. After 16 years, the organic matter in the wood chip system was found to be 7%, followed by the herbicide strip at about 4%, then the grass cover crop soils at 3%. Despite the organic matter being lower in the grass cover crop system, the cover crops and wood chips both resulted in healthier trees and yields in the long term.
“From year number 10 on, the treatment with the mulch and with the grasses had better growth and better yields,” Atucha said. “They outperform the bare soil, because soil health overall was so much better.”
The problems being seen in high-density orchards today, said Atucha, seem to be related to excessive use of herbicides in those management systems and the resulting degradation of soil structure.
Choosing Cover Crops
When opting to utilize cover crops in-row, Klodd recommended establishing them under mature trees, not new plantings. Irrigation, when needed, will decrease any competition between the fruit crop and the cover crop for water, and regularly assessment of fertility – she recommended foliar sampling of the crop – can prevent any competition for scarce nutrients.
Depending on cover crop density, the impact on weeds can vary. Dense cover crop canopies can be quite effective against weeds. Selecting a cover crop with a height below 12 inches can reduce the need for maintenance. Drought tolerance might be a beneficial attribute in some locations.
Cover crops could provide some rodent or insect pests with habitat and increase disease pressures, but can also be used to attract beneficial insects. Keeping cover crops removed from the tree base and leaving a bit of bare soil around the trunk will decrease concern of rodents or disease issues.
While flowering cover crops could attract pollinators to the orchard, orchard chemicals (even OMRI-listed ones) might be detrimental to bees, so flowering crops may not be a good selection for in-row cover crops, Klodd said.
Cover crop selection is dependent upon soil needs, crop needs and location. Some cover crops are actually weeds in some regions and can quickly become a problem. They might cover soil but not provide any other benefits to the ecosystem. Selecting cover crops that enhance the primary crop and the overall ecosystem is the goal.
Klodd isn’t advocating for the use of in-row cover crops, just seeking to provide growers with information to make informed decisions. The use of in-row cover crops, which cover crops to select and how to establish and maintain them is complex. “This is a topic where research is really important; however, there hasn’t been a lot of research.”