Technical Abstract: Vinegar (acetic acid) is a non-selective contact herbicide used in organic crop production. Research was conducted to determine if vinegar could be successfully applied over the top of onion plants to control broadleaf weeds. The experiment included 6 weed control treatments (2 application volumes, 2 hand-weeding levels, an untreated weedy-check and an untreated weed-free) with 4 replications. The two weed control treatments within each application volume (50 and 100 gpa) involved either no hand-weeding, where the uncontrolled weeds were allowed to grow, or a season-long hand-weeding, where all weeds were removed. Broadcast applications of 50 and 100 gpa of 20% acetic acid (vinegar) initially provided excellent (>95%) weed control, but decreased over time, especially for the 50 gpa application volume. Crop injury at 3 DAT was significantly greater for the 100 gpa compared to the 50 gpa application volume, but not significantly different at the later evaluation dates. Broadcast applications of vinegar and the resulting crop injury significantly reduced yields for both cultivars compared to the untreated weed-free control indicating that it should be applied with caution.