Least-Toxic Herbicides - July 30, 2014
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Herbicides are widely used to kill weeds in agriculture, commercial and residential landscapes, and home gardens and orchards. While many people accept the risks associated with conventional herbicide use, increasing numbers are declaring that use of these compounds is not acceptable-especially for use in public places or where children may be exposed. There are several weed management alternatives to synthetic herbicides. These include: cultivation, pulling, mulching, solarization, competing vegetation, and least-toxic herbicides.

People often use conventional herbicides because they are easy to apply. Least-toxic herbicides are just as easy to apply, but are less toxic to people, pets, wildlife, and other non-target organisms. Herbicides alone are not as effective as when used in combination with other weed prevention and control strategies. An integrated approach is the foundation of successful weed management.

There are two general categories of herbicides: preemergent and postemergent. The only least-toxic product marketed as a preemergent herbicide I am aware of is corn gluten meal (CGM). CGM is a waste product of corn syrup processing. According to researchers, CGM works by allowing the seed to germinate then be desiccated (physically dried out) by the CGM on the soil surface. In practice, if the soil never dries out, the CGM acts as a fertilizer (it has 10% nitrogen) and can cause weeds and crop plants to respond with increased growth. Research has shown CGM to be and ineffective Preemergent herbicide under most conditions. I have included a reference describing CGM’s shortcomings below.

Herbicidal "soaps" are least-toxic postemergent herbicides. These products contain fatty acid salts which break down into carbon dioxide and water. They are most effective on annual weeds because they disrupt the cell membranes only killing the tissue that they contact. One commercially available fatty acid herbicide is called Scythe and contains primarily pelargonic acid: a fatty acid found in seeds and foods. Scythe is most effective against small, annual weeds. It will not kill large annuals and perennials. In cold weather, Scythe is not as effective as conventional alternatives such as diquat. Advantages include lower mammalian toxicity and very rapid symptom development. It also has an odor that some people may find offensive.

When I last wrote about this topic, vinegar (acetic acid) was still recommended for use as a least-toxic postemergent herbicide. Grocery store vinegar is 5% acetic acid, but more concentrated solutions were marketed for a short period. The 5-7% acetic acid formulations were not found to be effective. These products have mostly been discontinued or reformulated. The reason: concentrated acetic acid is dangerous. Concentrations over 11% can cause burns upon skin contact. Eye contact can result in severe burns and permanent corneal injury. Because the public is used to thinking of vinegar as something you can safely splash on your salad and eat, they are generally unaware of potential dangers of higher concentrations.

The least-toxic postemergent herbicide BurnOut II is one product that has been reformulated to contain citric acid and clove oil as active ingredients. According to the product label, BurnOut II will be most effective on annual weeds, but can be repeatedly applied to control some perennials. It is available through organic gardening catalogs and comes in both ready-to-use and concentrated formulations.

Least-toxic herbicides are becoming more widely available and increasingly effective at killing weeds. I hope that some readers research and try some of these least-toxic products. They certainly have their place and offer alternatives to conventional herbicides. Remember these products are pesticides and must be handled according to label directions. "Least-toxic" does not mean non-toxic. I have also included product labels for Scythe and BurnOut II and a discussion on acetic acid herbicides below.

Finally, do not rely solely on herbicides to manage weeds on your property. Weeds thrive on unoccupied or disturbed soil. Organic mulches such as wood chips are excellent at preventing annual weeds. While labor intensive, hand pulling and hoeing are still extremely effective at managing established annual weeds – especially when done prior to seed production. In landscapes, the key to weed management is to occupy available space with desirable woody or perennial plants which displace undesirable weeds. Perennial weeds such as bermudagrass, are more difficult to manage and many gardeners resort to using conventional herbicides to get them under control, then revert to least-toxic strategies for long-term maintenance.

Naming of products is neither meant to imply endorsement by the University of Arizona nor criticism of similar products not mentioned.

Follow the Backyard Gardener on Twitter – use the link on the BYG website. If you have other gardening questions, call the Master Gardener help line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at verdevalleymg@gmail.com and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.

Additional Resources

The Myth of Weed-Killing Gluten: "Corn meal gluten is an effective organic herbicide”, Linda Chalker-Scott, Washington State University Extension

Fact Sheet for Vinegar/Acetic Acid Recommendations
Oregon Department of Agriculture


Scythe Herbicide Product Label
Gowan Company


BurnOut II Herbicide RTU Product Label
St. Gabriel Organics


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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: August 6, 2014
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