Vinegar has been touted as a cheap, effective weed killer. Although vinegar has shown promise as a broad-spectrum herbicide, it does not work equally well on all weeds. Its effectiveness depends on the type of weed, the age of the weed and the concentration of the acetic acid in the vinegar. Household vinegar is a 5 percent acetic acid solution. Stronger concentrations of 15, 20 and 30 percent acetic acid are also available and work better at killing weeds, but should be used with care. They can burn surrounding plants and even your skin or eyes.
All concentrations of acetic acid, including household vinegar, should cause treated leaves to brown within 24 hours. Young, tender weeds and annual weeds like crabgrass are susceptible to treatment with household vinegar. However, the roots are often not killed entirely and the weeds may reappear within a few weeks. Repeated applications, usually three, are more effective and stronger concentrations of acetic acid work even faster and longer.
Acetic Acid vs. Vinegar
Acetic acid is created by fermenting alcohol. Household vinegar has a 5 percent solution of acetic acid made from the fermentation of plant products like grapes and apples. There are stronger concentrations of acetic acid available and even synthetically created acetic acid. All vinegar contains acetic acid, but not all acetic acid is vinegar.
Is Vinegar an Organic Control?
If the acetic acid in the product is created by the distillation or freeze-evaporation of plant sources, like household vinegar or the stronger concentrations sold for home canning, it is considered organic. Acetic acid made by synthetic processes is not.
In a research experiment conducted during the Spring and Fall 2001 by the Sustainable Agricultural Systems Laboratory (SASL), a member of the Animal and Natural Resources Institute (ANRI) located in Beltsville, Maryland, they hand-sprayed common lambsquarters, giant foxtail, smooth pigweed and Canada thistle with 0.0, 5.0, 10.5, 15.3 and 20.2 percent vinegar. The results showed two things: the effectiveness of acetic acid on weeds depends on both the concentration and the plant growth stage.
- Weaker concentrations (5 and 10 percent) worked well on young, more tender weeds.
- Stronger concentrations (15 and 20 percent) were more effective on mature weeds.
Effects of Vinegar on Soil
Being an acid, it can lower the soil’s pH a little. This is a temporary effect. Acetic acid breaks down quickly in water, so any residue will be pretty much gone after the first watering or rain.
Using Acetic Acid as a Herbicide
Vinegar is not labeled for use as a pesticide, so Cooperative Extension agents aren’t able to recommend its use. But a homeowner can experiment. Household vinegar works well on young weeds. Repeated applications improve its effectiveness.
- Spray it directly on the leaves and try for total saturation without too much run-off.
- Don’t apply if rain is expected or before watering, since water breaks down acetic acid.
- Avoid spraying any herbicide on windy days, to prevent it from drifting onto plants you don’t want to kill.
Household vinegar often works well on the weeds between the cracks in a sidewalk—the heat from the pavement helps the process along. It's less effective with the perennial weeds that grow in a flower bed. It helps to apply the vinegar every two or three days until the weeds die off.
Another option is to use one of the commercial herbicides on the market. Look for them in the weed killer section of your local garden centers, under names like: Fast Acting Burn Out (St. Gabriel Labs), Nature's Glory Weed and Grass Killer and Blackberry and Brush Block (Greenergy).
You can find stronger concentrations of plain vinegar sold for canning purposes. These work faster and longer than household vinegar, but are caustic and should be handled with caution. Keep it off your skin and away from your eyes.