Vinegar has a long list of uses around the home and garden, but only in the last decade has research been conducted on using it as weed control. With varied concentrations having different effects, vinegar is now considered an inexpensive and relatively safe method for keeping annual weeds at bay with no permanent damage to the environment.
Three different forms of vinegar are derived from the fermentation of sugars found in grapes, apples or grain, creating white, cider or malt vinegars. The initial concentration of acetic acid in any of the three forms of vinegar is 5 percent, increasing to greater concentrations of acid by distillation and freeze evaporation, among other methods.
According to the United States Department of Agriculture, "acetic acid acts as a contact herbicide, injuring and killing plants by first destroying the cell membranes, which then causes the rapid desiccation of the plant tissues." Therefore, use of vinegar is more productive on plants with broader leaves while grasses are harder to control due to the respective surface area to contact with spray.
Five- to 10-percent concentrations of acetic acid in vinegar generally kill annual weeds within the first two weeks of their emergence from the soil. Higher concentrations of 20 to 25 percent acetic acid yield an 85 to 100 percent success rate in all life stages of most annual weeds. Purdue University Extension Consumer Horticulturist B. Rosy Lerner further states that "even though vinegar is an acid, it breaks down quickly in the soil and, therefore, is not likely to accumulate enough to affect soil pH for more than a few days."
Since the use of vinegar to control weeds is based on destroying the cells in the leaves of plants, it does little to the root systems of most perennial weeds. Repeated applications of reemerging weeds is mandatory to weaken the root system and possibly kill them off. Perennial weeds are not the only plant systems that may not be affected by vinegar--corn plants seem unfazed by it as well.
Unless specified as an herbicide, using vinegar to kill weeds is illegal in the United States. Even though the application of vinegar is benign to the environment, higher concentrations of acetic acid can be hazardous to humans, causing skin burns and eye injuries. Regardless of the concentration, vinegar should be treated as an acid and handled with care.