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

Abstract

Background: 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.

Results: 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.

Conclusion: 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.

Keywords: Canada thistle; herbicide; natural herbicide; orchard; organic herbicide; weed management; weeds.

Figure 1

Efficacy of natural herbicides with the active ingredients capric and caprylic acid (CAP), d‐limonene (LIM), acetic and citric acid (ACET), clove and cinnamon oil (CIN), or mechanical removal (MECH), at 72 h after each application at Eggert in 2016 (A), Eggert in 2017 (B), Eggert in 2018 (C) and Tukey in 2018 (D). Data are means of six plots at each site and application. Error bars signify $\overline{x}\pm$ SE. Within a treatment application time, treatments with the same letter are not significantly different at P < 0.05, Tukey's HSD.

Figure 2

Efficacy of natural herbicides with the active ingredients capric and caprylic acid (CAP), d‐limonene (LIM), acetic and citric acid (ACET), or clove and cinnamon oil (CIN) at 1, 24, and 72 h after treatment application at Eggert in 2017 (A), Eggert in 2018 (B), and Tukey 2018 (C). Data are means of all application times across each season. Error bars signify $\overline{x}\pm$ SE. Within an observation time, treatments with the same letter are not significantly different, P < 0.05, Tukey's HSD.

Figure 3

Weed cover before applying treatments with the active ingredients capric and caprylic acid (CAP), d‐limonene (LIM), acetic and citric acid (ACET), clove and cinnamon oil (CIN), mechanical removal (MECH), or a do‐nothing control at the beginning of each growing season. (A) Total weed cover, (B) Canada thistle and field bindweed cover, (C) grass weed cover. Data are means of six plots at the beginning of each year. Error bars signify $\overline{\mathrm{x}}\pm$ SE. Because there was no treatment × time interaction, treatment marginal means are compared. Treatments with the same letter are not significantly different, P < 0.05 (A and C) or P < 0.06 (B), Fisher's LSD.

Figure 4

Efficacy of herbicides with the active ingredients capric and caprylic acid 4.74% (LoCAP) or 7.11% (HiCAP), d‐limonene (LIM), or glyphosate (GLY), and mechanical removal (MECH) in a greenhouse at 1, 24, 72, 168, 336, and 504 h after treatment application at greenhouse A (A) and greenhouse B (B). Data are means (n = 6). Error bars signify $\overline{x}\pm$ SE. Within an observation time, treatments with the same letter are not significantly different, P < 0.05, Tukey's HSD.

Figure 5

Canopy cover by Canada thistle after treatment with the active ingredients capric and caprylic acid 4.74% (LoCAP) or 7.11% (HiCAP), d‐limonene (LIM), or glyphosate (GLY), and mechanical removal (MECH) in a greenhouse at 1, 24, 72, 168, 336, and 504 h after treatment application at greenhouse A (A) and greenhouse B (B). Data are means (n = 6). Error bars signify $\overline{x}\pm$ SE. Within an observation time, treatments with the same letter are not significantly different, P < 0.05, Tukey's HSD.