Acetic acid, commonly known as vinegar, has been used for thousands of years as a condiment, a pickling or preserving agent and even as a household cleaner. It has long had a word-of-mouth reputation among organic gardeners as an herbicide. In 2002, the United States Department of Agriculture's Agricultural Research Service released a study on its efficacy.
Household Vinegar or Something Stronger?
Common, household vinegar has a concentration of about 5 percent acetic acid. This concentration is sufficient to kill very young weeds, but is generally ineffective for more mature specimens. Ten percent concentrations are somewhat more effective, but still higher concentrations--20 percent or more--are required to kill mature weeds.
Commercial Vinegar Products
To obtain acetic acid at high enough concentrations to kill weeds effectively, it is best to buy commercially available products at nurseries or garden centers. These are labeled as herbicides and come with complete directions. Like other acids, vinegar at high concentrations can burn exposed skin or irritate eyes.
Vinegar kills weeds by burning the plant tissue. It is a non-selective weed killer that does not distinguish between weeds and more desirable plants. This makes it less useful for broad applications and more effective for spot treatment of weeds, like those that sprout in sidewalk cracks or at the edges of driveways. Applying vinegar solutions in this way will not acidify the soil sufficiently to change its pH level for more than a short time, if at all.
Vinegar should not be used on rainy days, as it may be washed off before it can kill weeds. Warm, dry weather is best for application.
To avoid violating federal pesticide laws, use only vinegar-based solutions that are labeled specifically as weed killers.