Herbicides Derived From Natural Products and Their Role in the Integrated Management of Weeds in Organic Vidalia® Sweet Onion

Timely cultivation with a tine weeder is the primary tool for successful weed control in organic Vidalia® sweet onion.  Vidalia® sweet onion is a cool-season crop and weather conditions frequently arise that delay cultivation.  These delays are often caused by excessive rainfall.  Herbicides derived from natural products are suitable for use in certified organic crop production and may have a role in Vidalia® sweet onion production when conditions delay the initial cultivation.  Herbicides initially evaluated from 2007 through 2010 were clove oil, d-limonene, and lemongrass oil.  Control of cutleaf eveningprimrose and swinecress was better using clove oil or d-limonene compared to control from lemongrass oil.  It was observed in these early trials that weed size directly affected overall weed control with any of these herbicides.  In addition, sprayers were reconfigured with sprayer output changing from 25 gal./a to 60 gal./A and overall weed control was improved by the higher-output sprayer.  There are few OMRI-approved spray adjuvants and these were evaluated for improving clove oil efficacy in 2010 and 2011.  Adjuvants composed of saponins or citric acid plus garlic extract improved weed control from clove oil compared to clove oil alone.  Despite the improved overall performance of herbicides derived from natural products when applied at a high spray volume and used with an appropriate adjuvant, the overall conclusion from all experiments was weed control was strongly influenced by weed size.   Cutleaf eveningprimrose and swinecress at the seedling stage were effectively controlled by clove oil and d-limonene, while control of larger weeds was inconsistent.  When conditions were optimum, applications of clove oil or d-limonene supplemented the tine weeder and were a useful component in an integrated weed control system in organic Vidalia® sweet onion.