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Impacts of two years of autumn cover crops in northwestern Washington on winter annual weed populations

Published online by Cambridge University Press:  21 February 2023

Steven S. Seefeldt*
Associate Research Faculty (Retired), Department of Crop and Soil Sciences, Washington State University, Northwestern Washington Research and Extension Center, Mount Vernon, WA, USA
Toby M. Una
Graduate Student, Department of Crop and Soil Sciences, Washington State University, Northwestern Washington Research and Extension Center, Mount Vernon, WA, USA
Don McMoran
Director, Skagit County Extension, Washington State University, Mount Vernon, WA, USA
Brian Maupin
Technician, Department of Crop and Soil Sciences, Washington State University, Northwestern Washington Research and Extension Center, Mount Vernon, WA, USA
Elizabeth Myhre
Technician, Department of Crop and Soil Sciences, Washington State University, Northwestern Washington Research and Extension Center, Mount Vernon, WA, USA
Deirdre Griffin-LaHue
Assistant Professor, Department of Crop and Soil Sciences, Washington State University, Northwestern Washington Research and Extension Center, Mount Vernon, WA, USA
Author for correspondence: Steven S. Seefeldt, Washington State University–Mount Vernon, Northwestern Washington Research and Extension Center, 16650 State Route 536, Mount Vernon, WA 98273. (Email:


Cover cropping is a suggested soil conservation practice widely investigated in cropping systems. Cover crops suppress weeds and often are part of an integrated weed management plan that could lead to reduced herbicide use and possibly reduce the weed seedbank. Winter brassica cover crops are popular in the eastern Washington potato (Solanum tuberosum L.) production region, but in western Washington, the production of brassica seed crops presents disease issues along with the risk of cross-pollination, which limits the use of brassica cover crops. Research for this article was conducted in two trials from 2018 to 2020 and 2019 to 2021in Mount Vernon, Washington, to identify winter cover crops compatible with regional restrictions and climatic challenges in western Washington cropping systems. Treatments including a no-cover control, eight single species (including brassicas, grasses, and legumes), and a grass–legume mixture were investigated. Cover crop and weed biomass production were measured, and percent ground cover for cover crops and weeds by species was estimated. Cover crop biomass and weed suppression varied by year due to variable environments, but annual ryegrass (Lolium multiflorum Lam.) and the mixture were most consistent in producing large amounts of biomass and reducing weed biomass and cover in all years. The variability of percent weed cover response to environment was ameliorated when weed cover was normalized within each year’s control.

Research Article
© The Author(s), 2023. Published by Cambridge University Press on behalf of the Weed Science Society of America

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Associate Editor: Nicholas Basinger, University of Georgia


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