What are the least toxic herbicides?
Answer: Corn gluten meal is a byproduct of the process of milling corn and has been used successfully on lawns and high-value crops as a pre-emergent herbicide that inhibits growth and root development. It is non-selective and must be applied carefully to ensure crop safety. It must be applied just prior to weed-seed germination to be effective. A common rate is 40 pounds per 1,000 square feet, which suppresses many common grasses and herbaceous weeds. Corn gluten meal cannot be derived from genetically modified (GM) corn if being used in an organic operation (Dufour et al., 2013; Quarles, 1999).
Herbicidal soaps made from fatty acids are available from companies like Safer Brand* and Mycogen*. They are fast-acting, broad-spectrum herbicides, and results can often be seen within hours. Herbicidal soaps are used as a post-emergent, sprayed directly on the foliage, and are most effective on annual broadleaf weeds and grasses.
Vinegar is an ingredient in several organically approved herbicides on the market today, with acetic acid levels of 5 to 30% or more (household vinegar is about 5% acetic acid). Vinegar-based herbicides are effective as post-emergent herbicides sprayed onto the plant to burn off top growth—hence the concept “burndown.” Burndown treatment is most effective on small annual weeds and is less effective on grasses than it is on broadleaf weeds.
Researchers in Maryland (Anon, 2002) tested 5 and 10% acidity vinegar for effectiveness in weed control. They found that older plants required a higher concentration of vinegar to kill them. At the higher concentration, they got an 85 to 100% kill rate. A 5% solution burned off the top growth with 100% success. Vinegar is corrosive to metal sprayer parts—the higher the acidity, the more corrosive. Plastic equipment is recommended for applying vinegar. It is also very important to wear hand and eye protection when applying, as acetic acid concentrations of greater than 10% can cause serious damage (Dufour et al., 2013).
Clove oil is another active ingredient in organic post-emergent, non-selective herbicides, and it can be just as effective as acetic acid at controlling broadleaf weeds. Additionally, it can be applied at lower rates than acetic acid while retaining its efficacy (Dufour et al., 2013).
USDA Agricultural Research Service is researching a method called Propelled Abrasive Grit Management, “a tractor-mounted system that uses compressed air to shred small annual weeds like common lambsquarters with high-speed particles of grit made from dried corn cobs” (Suszkiw 2014). The method shows promise for organic weed control, but there are still questions that remain—for example, whether grit made from cobs of genetically modified corn can be used in certified organic systems, and how well the spraying controls grass weeds.
Learn more in the ATTRA publication Sustainable Weed Management for Small and Medium-Scale Farms. This publication discusses several strategies, both proactive and reactive, as alternatives to conventional tillage systems. Options include mulching, competition, crop rotations, and low-toxicity control alternatives.
*Note: The mention of specific brand names or companies is intended for educational purposes only and does not constitute endorsement by NCAT, ATTRA, or USDA.
Anon. 2002. Vinegar wipes out thistles organically. Stockman Grass Farmer. July. p. 1.
Dufour, Rex, Sarah Brown, Ben Bowell, Carrie Sendak, Jennifer Miller, Mace Vaughn, Eric Mader, Jessa Guisse, Jolie Goldenetz Dollar, and Brianna Borders. 2013. Conservation Buffers in Organic Systems. National Center for Appropriate Technology, Butte, MT; Oregon Tilth, Corvallis, OR; and Xerces Society, Portland, OR.
Quarles, William. 1999. Non-toxic weed control in the lawn and garden. Common Sense Pest Control Quarterly. Summer. p. 4-14.
Suszkiw, Jan. 2014. Whacking Weeds Organically. USDA Agricultural Research Magazine. July. p 21.