David Granatstein

While pest management and tree horticulture dominate the research agenda for temperate tree fruits, organic production requires more attention to weed control and nutrient management because of the limited options growers have. Weed control and tree nutrition are interlinked and can have impacts on other parts of the system such as rodent pests, beneficial insect habitat, water use, and soil quality. Trade-offs commonly occur when trying to solve weed control and nutrient management in organic orchards. This article reviews numerous orchard-floor management studies relevant to finding solutions for organic systems and suggests areas for further research. Alternatives to soil tillage are needed, as it can degrade soil and tree performance. Conversely, mulching frequently leads to improved tree performance and increased soil quality but can be a costly practice relative to other options. Currently effective organic analogues to the herbicide strip-grass alley system are lacking. A plant-based solution would be ideal from a sustainability standpoint, but no satisfactory solution has been developed for widespread use.

Living mulch cover crops can improve soil health and build organic matter, yet their use in fruit orchards comes with a risk of encouraging meadow vole ( Microtus pennsylvanicus ), a rodent that can be destructive to fruit trees. Several living mulch cover crop species were assessed in an apple ( Malus × domestica ) orchard understory along with wood chip mulch and bare ground. Desired species characteristics were weed competitiveness, low growth habit, nitrogen fixation, and potential rodent repellency. Legume species included birdsfoot trefoil ( Lotus corniculatus ), medic ( Medicago spp.), and subterranean clover ( Trifolium subterraneum ), which were planted in solid stands as well as mixtures. Nonlegume species included sweet woodruff ( Galium odoratum ), sweet alyssum ( Lobularia maritima ), creeping thyme ( Thymus serpyllum ), and colonial bentgrass ( Agrostis tenuis ). Meadow vole presence was evaluated in fall and spring with point-intersect and run-length measurements. A legume mix (medic, birdsfoot trefoil, subterranean clover, and colonial bentgrass) had the highest meadow vole presence, with no reduction under the “sandwich” system of tilling either side of the tree trunks while leaving a cover crop in a narrow strip with the trunks. The nonlegume mix [colonial bentgrass, sweet alyssum, creeping thyme, and fivespot ( Neomophila maculata )] had similar results. However, the sweet woodruff (planted in the “sandwich” system) had significantly lower presence of meadow voles than the other living mulches. Wood chip mulch, cultivation, and bare ground control were all similar, with very low presence, indicating low risk of meadow vole damage. The results from the sweet woodruff suggest that we need more research on the potential to select living mulches that are nonattractive or repellent to meadow voles for use in orchards.

Living mulch cover crops can improve soil health and build organic matter, yet their use in fruit orchards comes with a risk of encouraging meadow vole (Microtus pennsylvanicus), a rodent that can be destructive to fruit trees. Several living mulch cover crop species were assessed in an apple (Malus ×domestica) orchard understory along with wood chip mulch and bare ground. Desired species characteristics were weed competitiveness, low growth habit, nitrogen fixation, and potential rodent repellency. Legume species included birdsfoot trefoil (Lotus corniculatus), medic (Medicago spp.), and subterranean clover (Trifolium subterraneum), which were planted in solid stands as well as mixtures. Nonlegume species included sweet woodruff (Galium odoratum), sweet alyssum (Lobularia maritima), creeping thyme (Thymus serpyllum), and colonial bentgrass (Agrostis tenuis). Meadow vole presence was evaluated in fall and spring with point-intersect and run-length measurements. A legume mix (medic, birdsfoot trefoil, subterranean clover, and colonial bentgrass) had the highest meadow vole presence, with no reduction under the “sandwich” system of tilling either side of the tree trunks while leaving a cover crop in a narrow strip with the trunks. The nonlegume mix [colonial bentgrass, sweet alyssum, creeping thyme, and fivespot (Neomophila maculata)] had similar results. However, the sweet woodruff (planted in the “sandwich” system) had significantly lower presence of meadow voles than the other living mulches. Wood chip mulch, cultivation, and bare ground control were all similar, with very low presence, indicating low risk of meadow vole damage. The results from the sweet woodruff suggest that we need more research on the potential to select living mulches that are nonattractive or repellent to meadow voles for use in orchards.

The semiarid climate of the Pacific Northwest allows for the production of organic, temperate tree fruit relatively free of disease and with fewer key insect pests compared with other regions of the United States. Weed control and soil fertility are two of the higher cost areas for organic tree fruit where alternatives are being sought through research and on-farm innovation. Mulches, both living [e.g., white clover [( Trifolium repens )] and inert (e.g., wood chips) show promise for controlling weeds, conserving water, providing nitrogen (N), or improving tree growth, but potentially have system trade-offs such as increased rodent pests and unwanted late-season N. Growers need orchard floor management practices that help them maintain or improve soil quality per the requirements of the National Organic Standards.