Eliminating knapweed with a common kitchen ingredient, such as vinegar, may seem like a safe enough solution. After all, vinegar has been touted as a natural herbicide for many years. While acetic acid, the active component in vinegar, does have herbicidal uses, knapweeds (Centaurea spp.) are tenacious adversaries. Before trying household vinegar or stronger horticultural vinegars found in garden stores, there are some important things to understand about knapweed and these natural herbicides.
Many species of Eurasian knapweeds have invaded U.S. lands. Short-lived perennial types such as spotted knapweed (Centaurea stoebe ssp. micranthos ) have proven winter-hardy from U.S. Department of Agriculture plant hardiness zones 1 through 13. Creating havoc for gardeners and farmers, knapweed forms a deep taproot and then spread by prolific seeds. In bloom from June to October, one plant of this invasive weed can produce 400 to 25,000 seeds. Approximately 90 percent of knapweed seed is viable when it is dispersed each autumn. Dormant seeds can remain viable up to eight years.
Household vinegar contains 5 percent acetic acid. As an herbicide, this low concentration is sometimes effective against recently sprouted, immature weeds. Killing mature weeds requires much higher concentrations of acetic acid. Newly germinated seedlings have not yet developed protective stem and foliage coverings. Tender growth can be susceptible to vinegar's effect. Acetic acid works by damaging cell membranes, causing foliage to dry out and die. Vinegar is not transported from plant leaves to roots. For killing weeds such as knapweed with its deep taproots and thick stem coverings, household vinegar does not work. Foliage may be damaged, but the plants regrow quickly from the root.
Ready-to-use herbicides containing 10 to 20 percent acetic acid are often marketed as natural horticultural vinegar. At these levels, acetic acid is effective against many kinds of weeds. However, the acid still works in the same manner as household vinegar. Foliage is killed, but the acid is not transmitted to plant roots. Even at these higher acetic acid levels, horticultural vinegars are ineffective in eliminating mature, taprooted plants such as knapweed. Horticultural vinegar works best when applied to actively growing young weeds in full sun with temperatures above 65 degrees Fahrenheit. Plants must be sprayed thoroughly to cover all foliage. Safety precautions should be a major concern.
The use of the word "vinegar" in product names evokes a feeling of familiarity and safety. But at high acetic acid levels, products are highly toxic pesticides. Twenty-percent formulas can cause irreversible eye damage and burn skin. Always wear protective googles, waterproof gloves, long sleeves, long pants and shoes and socks when using acetic acid products. Treated areas should not be re-entered for 48 hours. Some manufacturers exploit loopholes that exempt these products from regulation. When labeled for pesticide use, 20-percent acetic acid products carry the EPA's most toxic rating and the signal word "Danger" on the label. Never confuse horticultural vinegars with the vinegar in your kitchen cupboard.
- Oregon Department of Agriculture: Fact Sheet for Vinegar/Acetic Acid Recommendations
- New Mexico State University: Vinegar as Herbicide
- UC Weed Science: Natural Herbicides: Are They Effective?
- Pesticide Action Network: Safer Weed and Pest Controls: Low Risk Solutions for Parks, Schools and Homeowners
- Kansas Department of Agriculture Plant Protection and Weed Control: Spotted Knapweed Biocontrol Program
- Photo Credit Creatas Images/Creatas/Getty Images