Extension Educator, Horticulture
We gardeners are quite creative when it comes to killing weeds. We tug, taunt, smother and stomp. And from what I can tell from the common theme of questions I get, we often look in the kitchen cabinets for weed cures. Borax, bleach, salt, dish soap, and disinfectants, I've heard various recipes for all of them. One common ingredient for weed killer concoctions is vinegar. Because it is a food product, we think it must be safer than traditional synthetic herbicides. But is it? And does it really work as a weed killer?
Michelle Wiesbrook, UI Extension pesticide safety educator helped to clear up some of the confusion about vinegar as an herbicide in a recent issue of UI Extension Home, Yard, and Garden Pest newsletter.
Vinegar consists of approximately 5% acetic acid and 95% water. Acetic acid is found in all living organisms. It is readily broken down to carbon dioxide and water. To be effective as an herbicide, 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 whether it is a flower or a weed. It doesn't know the difference. Thorough spray coverage is critical. Vinegar does not move into the roots of treated plants, so only top growth is killed. This means perennial weeds can return. Vinegar is fast acting and most effective on young, actively growing annual weeds.
According to Wiesbrook, studies have shown that for vinegar to be most effective as a weed killer, the percent of acetic acid should be 10 to 20%. That means the 5% vinegar in your kitchen cabinet isn't quite strong enough. Studies showed only 5% of smaller weeds were controlled with 5% acetic acid, while concentrations from 10 to 20% acetic acid provided 80 to 100% weed control. However acetic acid above 5% concentration can severely burn skin and eyes.
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 since they are not labeled and registered with EPA as herbicides.
Herbicidal vinegar with 20% acetic acid is now labeled for use in Illinois. The product is sold as Weed Pharm by Pharm Solutions, Inc. Read and follow all label directions carefully. 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. 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.
The bottom line on vinegar as an herbicide: