ASHS 2025 Conference

Planting a cover crop in orchard middles can have many benefits for the health of the trees. In California, improved water infiltration, especially during the winter rainy season, is a common motivation for cover cropping, and the benefit to soil health through increased biological activity resulting from biomass incorporation is also gaining renewed attention. Cover crops can also play a part in nutrient management by “mining” and holding existing nutrients, especially nitrogen, at the end of the season and, when legumes are included, by direct sequestration of nitrogen from the atmosphere through the symbiosis of their roots with nitrogen-fixing bacteria. Different cover crop types can contribute in different ways to these outcomes, but benefits depend on successful establishment and a significant amount of biomass being produced. We evaluated three common cover crop types over three years in a walnut orchard in northwestern San Joaquin valley with a heavy clay-loam soil. Treatments included a pure stand of winter triticale, a ‘pollinator’ mustard mix (multiple Brassica spp. plus Raphanus sativus [Daikon]), and a multi-species mix including grain rye and triticale, mustards (Sinapis alba and Raphanus sativus), and legumes (Vicia faba, Vicia sativa, Lathyrus oleraceus). Treatments were planted with a seed drill in two adjacent orchard middles at two parts of the orchard in early Nov. 2022, 2023, and 2024 and depended on rain for germination and growth. In 2023 and 2024, unplanted controls were maintained in rows adjacent to the trial rows. Sampling plots were defined at five representative points in the planted rows. Just before cover crop termination early in the April following each planting, above ground vegetation was sampled using a one-meter quadrat. While triticale and the mustard blend produced above ground dry biomass at 0.13 to 0.28 and 0.04 to 0.27 kg/sq. m, respectively, the multi-species mix consistently produced three to five times that amount, ranging from 0.53 to 0.68 kg/sq. m. Analysis of soil sampled one week before cover crop termination in 2025 did not show significant differences in organic matter, N, P, and K content, but samples of chopped cover crop taken at termination showed plant N content twice as high in the multi-mix treatment than in the others. This trial has highlighted the resilience of planting a diverse plant mix in comparison to more homogeneous cover crop types, and the advantage of including legumes when N may be a limiting factor to cover crop biomass growth.