Managing weeds is one of the most significant challenges, especially in organic vegetable production systems. Farmers control weeds in various ways, many of which can have negative environmental impacts. Cultivation is a common way many organic vegetable growers will manage weeds; however, it leads to decreased soil health properties. Hand weeding is extremely time-consuming and labor-intensive. Conventional herbicides have raised public concern for their impact on human health and the environment. Organic herbicide products are used as a burndown, post-emergence product but can be cost-prohibitive. In addition, there is a lack of current research comparing organic herbicide effectiveness on a range of common weed species. This study aimed to explore the efficacy of five Organic Materials Review Institute-approved organic herbicides. These products included citrus oil (Avenger®), ammonium nonanoate (AXXE®), acetic acid (Green Gobbler®), caprylic acid capric acid (HomePlate®), and clove oil cinnamon oil (Weed Zap®). Water was used as a control, and glyphosate (Ranger Pro®) was used as a positive control. Each herbicide was tested on six common weed species: Chenopodium album (common lambsquarters), Portulaca oleracea (common purslane), Setaria viridis (L.) Beauv. (green foxtail), Digitaria sanguinalis (L.) Scop. (large crabgrass), Amaranthus retroflexus (redroot pigweed), and Abutilon theophrasti (velvetleaf). Products were sprayed according to label recommendations using a calibrated spray chamber at the Iowa State University greenhouses. Each weed species, 10 plants per replication, was sprayed after reaching an average height of seven centimeters. Percent weed cover using digital image analysis software (Turf Analyzer) and percent visual injury was recorded. These data parameters were collected 24 hours, 3 days, 10 days, 17 days, and 21 days following herbicide application. Weed biomass was collected and dried 21 days after herbicide application for all species. AXXE® was a fast-acting herbicide on common lambsquarters, common purslane, redroot pigweed, and velvetleaf. These species showed over 85% injury three days after AXXE® application. Weed Zap® stunted the majority of examined weed species soon after application, but the injury effects were less significant 21 days after application. Visual injury assessments showed Avenger®, Green Gobbler®, HomePlate®, and Weed Zap® had no significant injury on green foxtail and large crabgrass 21 days after herbicide application. Results from this study provide growers with practical and applied data to make informed decisions regarding the use of organic herbicides.
