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Using Vinegar as a Herbicide

August 2, 2007

Over the years, I’ve been told that vinegar is being used for weed-control. Because it is a food product, it is perceived as being safer than traditional synthetic herbicides. There are various recipes out there for mixtures with box salt and liquid dish soap and others with orange oil and molasses. I’m always amazed at the different combinations people come up with and how they swear that their recipe is the only one to use.

Does vinegar work, and how does it work? Vinegar contains acetic acid. That’s the weed killer. According to the EPA “Fact Sheet on Acetic Acid”:

Acetic acid is found in all living organisms. It is readily broken down to carbon dioxide and water. Vinegar consists of approximately 5% acetic acid and 95% water. This is also the concentration of acetic acid when applied as a pesticide product. To be effective, acetic acid needs to contact the plant leaves; the acidity of the spray solution damages and dries out the leaves.

Vinegar is nonselective and may damage any plant tissue. It does not move within treated plants, so only top growth is killed. This means perennial weeds return. Vinegar is fast acting and most effective on young, actively growing annual weeds, and good spray coverage is critical.

Studies have shown that for vinegar to be most effective, the percent of acetic acid should be 10 to 20%. That means that the vinegar in your kitchen cabinet isn’t quite strong enough to do the trick. Most commercially available vinegars are only 5%. In 2002, USDA research found that control of smaller weeds was variable with 5% acetic acid, while concentrations from 10 to 20% provided 80 to 100% control. Weed species included foxtails, lambsquarters, pigweed, velvetleaf, and Canada thistle. Study details can be found at http://www.ars.usda.gov/main/site_main.htm?modecode=12=65-04-00. They issued this in their research report: “WARNING: Note that vinegar with acetic acid concentrations greater than 5% may be hazardous and should be handled with appropriate precautions.”

Not your grandmother’s vinegar. There are commercial food-grade formulations of vinegar available that are stronger than what you can buy at your supermarket. However, their use for weed control is not recommended. It’s important to note that unless the product you are using for weed control is registered with the EPA as a herbicide, its use is illegal and a violation of FIFRA. Registered herbicides come with label directions, including use rate and required PPE (personal protective equipment). These products have been studied extensively.

We splash vinegar on our lettuce salads and think nothing of it. However, stronger forms of vinegar can be hazardous to humans. Acetic acid concentrations over 11% can cause burns upon skin contact. In fact, eye contact can result in severe burns and permanent corneal injury. This is why reading and following the label is so important.

Herbicidal vinegar finally available. Good news! Herbicidal vinegar with 20% acetic acid is now labeled for use in Illinois. The product is sold as Weed Pharm and more information is available at http://pharmsolutions.com/weed_pharm.html. At the time of this writing, their Web page does not include Illinois in their list of states where the product is registered. However, a quick phone call yesterday to the Illinois Department of Agriculture assured me that it is indeed registered for use.

Users are strongly encouraged to read and follow the label carefully. I was a little surprised to see that the Weed Pharm label signal word is DANGER. It’s corrosive and causes irreversible eye damage, so goggles or a face shield is needed when handling this product. If the product is swallowed or splashed on the skin, a poison-control center should be called. Another note is that it cannot be applied directly to water. Also, the label reads, “Keep unprotected persons out of the treated area until spray residues have dried.” Desirable plants need to be protected from potential spray drift. For more information, consult with the product label.

(Michelle Wiesbrook; Sources: “EPA Fact Sheet on Acetic Acid,” http://www.epa.gov/oppbppd1/biopesticides/ingredients/factsheets/factsheet_044001.htm; “Fact Sheet for Vinegar/Acetic Acid Recommendations,” by Oregon Department of Agriculture, http://oregonstate.edu/dept/nursery-weeds/weedspeciespage/acetic_acid_factsheet.pdf.)