Acid Herbicides

U.S. Department of Agriculture Studies

• Acetic acid is one of the main ingredients in household vinegar, which averages 5 percent acidity. In 2002, the U.S. Department of Agriculture conducted tests that showed that a 5 percent solution was strong enough to kill young weeds up to 14 days after germination. The target weeds in the study were Canada thistle, a perennial, and lamb's quarters, velvetleaf, giant foxtail, and smooth pigweed, all of which are annuals. Stronger solutions, ranging from 10 to 20 percent, killed 85 to 100 percent of weeds of any age, but Canada thistle always grew back from the root.

How Acetic Acid Works

• Acetic acid works by disrupting the cell membrane of the weed. It does not translocate to the root, as with some synthetic herbicides, which is why perennial weeds such as thistle and dandelion grow back after the upper leafy growth has been killed. Because acetic acid kills on contact, full coverage is necessary to kill the weeds. Perennial weeds may succumb if they are sprayed repeatedly, a process that robs the plant of the nutrient reserve necessary to spring back to life.

Product Registration

• Herbicides based on acetic acid are available at garden centers, but while some may be labeled as "horticultural vinegar," most are not registered as herbicides with the U.S. Environmental Protection Agency (EPA). Those who sidestep registration, a lengthy and costly process, are forbidden by the EPA from including any information on labels and directions that suggest the product is an effective herbicide. In addition, county extension offices may be reluctant to recommend a product that has not been registered, even though government studies have shown it to be effective.

Hazards

• Although it is an essential ingredient in the vinegar we use for cooking, acetic acid is far from benign if concentrations exceed 5 percent. Concentrations of 11 percent or higher can cause skin burns and are extremely caustic to the eye. Gloves, long sleeves and safety glasses are as necessary with higher concentrations of this organic product as they would be with any synthetic herbicide.