Evaluating Efficacy of Organic Herbicides on Common Weed Species in the North Central Region

View the project final report

Commodities

Practices

• Education and Training: extension, technical assistance
• Pest Management: botanical pesticides, chemical control
• Production Systems: organic agriculture

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 manage weeds. However, excess cultivation can decrease soil health and lead to erosion. Hand weeding is effective yet labor-intensive. Conventional herbicides have recently sparked concern over their impact on human health and the environment. Organic herbicides may be an alternative weed management method, providing an additional tool for growers to manage weeds. Very little research has been published on the efficacy of organic herbicides on multiple weed species in a greenhouse setting. Most organic herbicides are non-selective, post-emergence products, commonly used as a burndown herbicide. Organic herbicides can be cost-prohibitive. In addition, many of the products require high rates to achieve the necessary leaf coverage to be effective. Due to the cost and lack of current research on organic herbicide products, many growers are not using them. This research addresses this by comparing OMRI-approved herbicide products on a range of common weed species.

Five Organic Materials Review Institute (OMRI)-approved products were trialed in this experiment: Avenger® (citrus oil), AXXE® (ammonium nonanoate), Green Gobbler® (acetic acid), HomePlate® (caprylic acid + capric acid), and Weed Zap® (clove oil + cinnamon oil). Water was used as a control, and Ranger Pro® (glyphosate) was used as a positive control. Each herbicide was tested on five common weed species: 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. The efficacy of herbicides was evaluated in three ways: percent visual injury, digital images analyzed with Turf Analyzer to find percent green cover, and dried weed biomass 21 days after treatment.

In an attempt to increase the efficacy of the products, based on the results of Trial 1, an organic surfactant was added to each treatment in Trial 2. The surfactant Nu-Film® P was used. This is an OMRI-approved surfactant made of Pinene (polyterpenes) polymers, petrolatum, alkyl amine ethoxylate.

The outcomes of this research project will provide growers with a guide to selecting an organic herbicide to use on their farms. This will allow growers to make economically informed decisions on organic herbicide purchases. The target audience of this research is organic growers. However, current conventional herbicides have shown a decrease in efficacy due to the rise of herbicide-resistant weeds. Alternative options need to be explored, and this study examines potential alternatives. Growers will benefit from access to scientifically based research to support decision-making for weed management methods on their farms. This grower-based approach will strengthen outreach and interaction with growers while helping them adopt a successful weed management plan that works for their farms.

The objectives of this research are 1) to identify active ingredients that are effective against common weed species, 2) to compare herbicide efficacy on multiple weed species, and 3) to disseminate research findings through grower conferences, a thesis chapter, a scientific journal article, and extension visits with growers. 

Scientific research questions: 1) Which organic herbicide provides the highest control over all weed species? 2) Do specific active ingredients work better on specific weeds? 

Outreach efforts have been specifically targeted towards organic growers to foster potential on-farm trials and cooperation for grower-focused outcomes. Results of this project have been presented at an academic conference, sparking interest in collaborative learning/interaction among other researchers. In addition, results have also been shared at three grower conferences. Research from this project is informing growers of organic herbicide product efficacy, and collaborating with them to understand how growers use these products on their farms was very important. The final project results were used to complete a thesis chapter, presented at an academic seminar, and are currently being prepared for publication in a scientific journal article.