Many organic vegetable growers find weed control a problem. There are few registered organic herbicides and many are not very effective and are expensive. One chemical that has been discussed as an organic herbicide is acetic acid – vinegar. Does it really work as well as an established herbicide like glyphosate? Jacob Domenghini at Eastern Kentucky University wanted to find out.
Jacob ran an experiment twice, in 2016 and 2017, for 132 days of growing season. His weed control treatments were glyphosate, 5% acetic acid (typical household grade vinegar), 20% acetic acid (horticulture grade vinegar), 30% acetic acid (horticulture grade vinegar), and a plain water control. Treatments were applied in the fall and spring (weed knock down the season before planting) or spring only both years to ungrazed pasture that was mown to 3 inches high before treatment. Percentage of weed cover within the plots was evaluated on a scale of 0-10 with 0=no weeds and 10=covered in weeds. All plots started with a rating of 10 and evaluations were based on the number of days after treatment it took to have 50% and 100% weed regrowth, and the number of follow-up treatments required through the growing season to keep the weeds down.
Fall + spring treatments received a treatment in October two days after mowing and were sprayed to wet, not to runoff. In the spring, plots were once again mown to 3 inches and all treatments applied two days after mowing in March, again spraying until wet. Plots were rated daily for the first 10 days then all plots were tilled to a depth of 6 inches with a rototiller to simulate garden bed preparation. After tilling, plots were rated twice a week. When a plot was rated as 5 or above, the treatment was reapplied.
The vinegar treatments all declined to a rating of 0 within 2 days of treatment because of the burn-down effect of acid. Plots treated with glyphosate required 6-7 days to get to a rating of 0. Glyphosate with both fall and spring treatment required a reapplication at an average of 78 days. Both 20% and 30% horticulture grade vinegar required an average of 79 days before reapplication was required. The spring only application of 5% vinegar required reapplication in an average of 63 days. Application of any of the herbicides in the fall did not reduce weed competition in the spring. Glyphosate plots only required one retreatment each year. The 5% vinegar required 5-9 retreatments in a year and the 20% and 30% acetic acid required between 3 and 5 retreatments in a year.
The 20-30% horticulture grade acetic acid is an effective herbicide that can be used by organic growers to replace glyphosate, but will require more retreatments through the season. The 5% acetic acid was not as effective. Preparing the garden in the fall with herbicide before spring planting was not worthwhile.
J.C. Domenghini. 2020. Comparison of Acetic Acid to Glyphosate for Weed Suppression in the Garden. HortTechnology. February 2020 30(1) 82-87.