Organic weed control with vinegar: Application volumes and adjuvants Authors Webber, Charles Shrefler, James - OSU, LANE, OK Submitted to: Proceedings of Horticultural Industry Show Publication Type: Proceedings/Symposium Publication Acceptance Date: February 8, 2007 Publication Date: May 1, 2007 Citation: Webber III, C.L., Shrefler, J.W. 2007. Organic weed control with vinegar: Application volumes and adjuvants. Proceedings of Horticultural Industry Show. 26:149-151. Interpretive Summary: Initial research and anecdotal testimonies have reported that vinegar (water and acetic acid) has potential as an organic herbicide, but further research is needed to better understand the relationship between the percentage of acetic acid applied and the use of spray additives to weed control. The objective of this research was to determine the effect of application volumes and spray additives (aduvants) on weed control. The experiment included three application volumes [20, 80 and 160 gallons per acre (gpa)] of vinegar (20% acetic acid) applied in combination with three additives [orange oil, non-ionic surfactant, and crop oil concentrate (COC)]. The experiment also included two weedy- checks and four replications. The orange oil, non-ionic surfactant, and COC were mixed at a 1.0% volume/volume (v/v), based on each application volume (20, 80, and 160 gpa). The weeds present at spraying included large crabgrass, goosegrass, carpetweed, cutleaf evening primrose, spiny amaranth, eclipta, and yellow nutsedge. Vinegar was more effective controlling broadleaf weeds than grass weeds. Using an additive (orange oil, non-ionic surfactant, and COC) increased the effectiveness of vinegar only at the lowest application volume (20 gpa). There were no weed control differences between the 80 and 160 gpa application volumes; therefore there was no advantage of using the highest application rate. Additional research should focus on the impact of multiple applications of vinegar at lower application volumes on weed control. Technical Abstract: Preliminary results have indicated that vinegar has potential as an organic herbicide, but further research is needed to increase our understanding of the relationship between acetic acid concentrations, application volumes, adjuvants, weed species, and weed maturity on effectiveness of vinegar to control weeds. The objective of this research was to determine the effect of application volumes and adjuvants on weed control efficacy using vinegar with a 20% acetic acid concentration. The factorial experimental design included applications of vinegar (20% acetic acid) applied at three sprayer application volumes [20, 80 and 160 gallons per acre (gpa)] in combination with four adjuvant treatments [none, orange oil, non-ionic surfactant, and crop oil concentrate (COC)]. The experiment also included two weedy-checks and four replications. The orange oil, non-ionic surfactant, and COC were mixed at a 1.0% volume/volume (v/v), based on each application volume (20, 80, and 160 gpa). The weeds present at spraying included large crabgrass (Digitaria sanguinalis L.), goosegrass (Eleusine indica L., Gaetn.), carpetweed (Mollugo verticillata L.), cutleaf evening primrose (Oenothera laciniata Hill), spiny amaranth (Amaranthus spinosus L.), eclipta (Eclipta prostrata L.), and yellow nutsedge (Cyperus esculentus L.). Total weed control ranged from 25% for the 20 gpa application volume with no adjuvant to 96% control for the application volumes of either 80 or 160 gpa used with orange oil. Vinegar was less effective in controlling grasses than broadleaf weeds. Ninety percent, or greater, grass control, primarily crabgrass, was obtained when the vinegar was applied at 80 or 160 gpa, irrespective of the adjuvant used. Broadleaf weed control was 99 or 100% for plots receiving either 80 or 160 gpa. Cutleaf evening primrose and carpetweed were the most susceptible to vinegar applications; however, this response may reflect differences in weed size rather than weed species. When averaged across adjuvants (none, orange oil, non-ionic surfactant, and COC), weed control increased as application volume increased to either 80 or 160 gpa. The selection of the adjuvant or the lack of an adjuvant did not significantly affect broadleaf weed control. The use of adjuvants did increase grass control; adding orange oil provided significantly greater control than using COC. Additional research should focus on the impact of multiple applications of vinegar at lower application volumes on weed control.