Efficacy and Cost of Four Plant-Derived, Natural Herbicides for Certified Organic Agriculture

Project Director: Aaron Appleby, Washington State University

Project Overview

Weed management poses a significant challenge for organic producers. While several organic herbicides are available on the market, these can be expensive, and may have a shorter residual activity than conventional herbicides. Nevertheless, organic herbicides are an important component of integrated weed management programs and can reduce the need for frequent/intense tillage.

Organic farmers need real-world, research-based information about the efficacy and cost of organic herbicides within specific production systems. In this study, researchers at Washington State University established a series of field and greenhouse trials evaluating the efficacy of several commercial organic herbicides in high-density apple production systems.

Farmer Takeaways

All organic herbicides reduced weed cover when compared to the untreated control.
Capric + Caprylic Acid and D-limonene were the most effective herbicides, reducing weed cover by >80%. They also reduced early-season perennial weed cover in subsequent growing seasons, suggesting a longer-term benefit.
Capric + Caprylic Acid was the most cost-effective treatment.
Organic herbicides may provide rapid, short-term management for perennial weeds, but additional research is needed for longer-term management strategies.

Project Objectives and Approach

Evaluate the efficacy and cost of several organic herbicides in apple production systems

Researchers at Washington State University established field trials in two high-density apple orchards. Trials were conducted for three consecutive years (2016, 2017, 2018) at one site, and for one year (2018) at the other site.
Organic weed management treatments included:
1. 1. Capric + Caprylic Acids (Suppress Herbicide EC)
  2. D-limonene (Avenger Weed Killer)
  3. Acetic + Citric Acids (Summerset AllDown)
  4. Clove + Cinnamon Oil (Safergro Weed Zap)
  5. Hand Hoeing: No organic herbicides were applied; however, plots were routinely hand-hoed to manage weeds.
  6. Control: Plots did not receive any form of weed control.
All organic herbicide treatments were applied using a backpack sprayer to spot-spray visible/emerging weeds directly beneath apple trees. Orchard alleys were not sprayed, but were mowed to maintain resident vegetation. Treatments were applied as-needed throughout the growing season (typically on 2-week intervals), and researchers recorded the exact amount of each herbicide applied.
Herbicide efficacy in both orchards was assessed by visual determination of the percent necrosis* of weed canopy cover 1-hour, 24-hours, and 72-hours after application. The types of weeds evaluated for cover reductions included: Canada thistle and field bindweed (perennial weed species), as well as broadleaf and grasses (annual weed species).
- - *Percent necrosis: the proportion of dead tissue relative to living weed tissue.
Treatment costs were estimated from the total amount of herbicide applied, as well as estimated labor costs (for the hand hoeing treatment).

Evaluate the efficacy of organic herbicides on Canada thistle (a perennial weed) in a controlled, greenhouse setting

Researchers established a separate, greenhouse study at Washington State University to assess the efficacy of organic herbicides on Canada thistle, an aggressive perennial weed.
Canada thistle roots were harvested from vegetable plots at the Washington State University Eggert Family Organic Farm (Pullman, WA). Roots were cut, planted in pots, and watered. Upon emergence, Canada thistle plants were replanted (1 plant/pot) and categorized by size (small/medium/large). Plants from each size category were placed in the greenhouse and received one of the following treatments:
- 1. Low-Dose Capric + Caprylic Acid
  2. High-Dose Capric + Caprylic Acid
  3. D-limonene
  4. Glyphosate (non-organic herbicide)
  5. Mechanical: Biomass growing above the soil surface was removed mechanically, via hand shears.
All herbicide treatments were applied at medium-to-high concentrations (per individual label recommendations) using a pressurized sprayer.
Herbicide efficacy was visually estimated at 1, 24, 72 (3 days), 168 (7 days), 336 (14 days), and 504 hours (21 days) post-application using the same procedure outlined above in the orchard field trials. At 21 days post-application, sample thistle plants were cut at the soil surface, weighed, and analyzed for above- and belowground root biomass.

Key Findings

All organic herbicide treatments reduced weed cover, with longer-term reductions in total and perennial weed cover with capric + caprylic acid and D-limonene herbicides

All of the organic herbicides (Capric + Caprylic Acid, D-limonene, Acetic + Citric Acid, and Clove + Cinnamon Oil) reduced weed cover when compared to the untreated control.
The Capric + Caprylic Acid herbicide was the most effective herbicide, consistently reducing weed cover at all orchard site-years by >95%. D-limonene also performed well, with both D-limonene and Capric + Caprylic Acid treatments achieving weed reductions comparable to mechanical weed removal (hand-hoeing).
- - The Acetic + Citric Acid and Clove + Cinnamon Oil treatments were slightly less effective, reducing weed cover by ~30-70% across site-years.
At the longer-term trial site, early-season weed cover and composition began to diverge among treatments after multiple years of herbicide application.
- - At the beginning of the study, total weed cover, Canada thistle cover, and field bindweed cover were similar among all treatment plots.
  - By Year 3, total early-season weed pressure and perennial weed pressure (by Canada thistle and field bindweed) were significantly lower in the Capric + Caprylic Acid and D-limonene herbicide plots than in other treatment plots.
These findings indicate that Capric + Caprylic Acid and D-limonene herbicides may be particularly effective organic weed management options for orchards, reducing weed cover beneath trees by 80-95% and reducing early-season perennial weed growth in subsequent growing seasons.

Certain organic herbicides, including capric + caprylic acid, provide cost savings compared to mechanical weed management

Field treatment costs were fairly costly, ranging from $100 to $600 per acre.
Of the evaluated treatments, the Capric + Caprylic Acid and Cinnamon + Clove Oil treatments were the least expensive, costing less than the mechanical (hand-hoeing) treatment.
- - Other herbicide treatments (i.e., D-limonene, Acetic + Citric Acid) were more expensive than the mechanical (hand-hoeing) treatment.
These findings highlight the importance of conducting cost-benefit analyses for management practices. In this study, Capric + Caprylic Acid herbicide emerged as a highly cost-effective organic weed management option in apple orchards, providing weed cover reductions similar to mechanical weed control, yet costing less.

Organic herbicides may provide short-term management of perennial weeds (i.e., Canada thistle) in orchard production systems, but effects wear off quickly

In the greenhouse trial, the organic herbicide treatments and mechanical weed removal resulted in faster, albeit shorter-acting, Canada thistle necrosis than the glyphosate treatment.
- - After 72 hours (3 days), Canada thistle plants treated with glyphosate continued to necrose, whereas plants in the organic herbicide or mechanical treatments began to regrow.
After 21 days, Canada thistle aboveground dry weights were similar among all treatments. These findings indicate that organic herbicides and mechanical weed removal may provide rapid, short-term management for aggressive perennial weeds such as Canada thistle, but that frequent additional applications will likely be needed to prevent regrowth. Additional research is needed on effective organic management strategies for perennial weeds.

Resources

Appleby, A. B., Reganold, J. P., & Carpenter-Boggs, L. A. (2025). Efficacy and cost of four plant-derived, natural herbicides for certified organic agriculture. Pest Management Science, 81(6), 3391–3402.

Location

Washington

Collaborators

John Reganold, Washington State University
Lynne Carpenter-Boggs, Washington State University

Region

Northwest

Topic

Weed Management

Year Published

2025