A reader recently asked me if a solution of vinegar, epsom salts and a small amount of liquid dish soap, mixed and sprayed on weeds was as effective as using glyphosate (the active ingredient in Roundup® and similar products) and safer. The reader went on to ask, “If this solution is as effective as Roundup® and much cheaper, is there anything in the mixture that would/could be dangerous to humans?”
Yes, vinegar (acetic acid) can be hazardous. For example, according to University of Maryland Extension (UME) in a 2014 publication, the hazards of using vinegar include severe eye irritation and injury, including possible blindness; burns and severe skin irritation and possible allergic reactions. “Prolonged or repeated exposure may cause dermatitis, chronic bronchitis and erosion of teeth.” In addition, UME reports vinegar can damage fencing, outdoor furniture, aluminum, iron and masonry. Drift during application can harm desirable plants.
UME notes that vinegar can be a contact herbicide, especially when applied in bright sunlight, killing the leaves of plants, but not affecting the roots. Weeds grow back from the roots within a short period of time. On the other hand, glyphosate is a systemic herbicide and moves from the leaves to the roots, killing the whole plant. This provides longer, more effective weed control. With vinegar, multiple applications can give some control of very young weeds, but it rarely works on grasses or perennial weeds.
Herbicidal vinegar is not the same as household vinegar. The herbicidal concentration is usually ten percent to 20 percent, while household vinegar has only a five percent concentration. However, according to the UME publication, research has found that five percent to ten percent acetic acid herbicide products can give viable control of small, young weeds with only one to two leaves (or within two weeks of germination). Larger weeds (with greater than three to four leaves) are likely to survive treatment. Using higher (20 percent) concentrations of acetic acid and increasing the application volume can improve weed control. Applying acetic acid on weeds with tap roots (dandelions, Canada thistle, etc.), may only result in top kill unless the weed is very young.
Commercially available acetic acid herbicide products are available. Their labels are required to include safety warnings, plants the product works on and designated amounts for effective weed management. While a homemade mixture may be cheaper than glyphosate, it is not as effective and could be harmful.