Efficacy and cost of four plant‐derived, natural herbicides for certified organic agriculture

Weed management is the greatest production challenge for most certified organic farmers, with few herbicides allowed on organic farms. Here, we compared the efficacy and cost of handhoeing (MECH) with a control (no weed management) and four commercially available registered organic herbicides under United States Department of Agriculture organic standards in managing Canada thistle (Cirsium arvense (L.) Scop.) and total vegetative cover in two apple (Malus domestica Borkh.) orchards. The four herbicide treatments had the following active ingredients: capric and caprylic acid (CAP), d-limonene (LIM), acetic and citric acid (ACET), and clove and cinnamon oil (CIN). In separate greenhouse trials, Canada thistle response to CAP (at two concentrations), LIM, glyphosate (GLY), and MECH were also studied. All materials reduced weed cover by 48% or more 72 h after treatment in the orchard trials; CAP performed best, reducing weed cover by 88% in 1 h and 98% in 72 h. CAP and LIM reduced early season, perennial weed cover after 3 years of repeated applications in an organic orchard; a single application of CAP and LIM would cost on average US$769.50 ha-1 less and US$203.50 ha-1 less than MECH at US$12.00 h-1 wage respectively, with similar efficacies. In greenhouse trials, CAP mixed to 7.11% v/v and 4.74% v/v active ingredient were equally effective at removing weed cover compared with MECH, further reducing the cost of a single application of CAP by US$122. Adding effective, next-generation, natural herbicides, such as CAP and LIM, to existing weed management strategies may allow organic producers to reduce weed pressure, till less, and improve profitability, while providing conventional producers options to combat herbicide-resistant weeds. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.