Progress report for GNC23-383
Project Information
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. Although, 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 herbicide may be an alternative weed management method, adding a tool to the toolbox for growers to manage weeds. Very little research has been published on the efficacy of organic herbicides on multiple species in a greenhouse setting. Most organic herbicides are a non-selective, post-emergence product, used commonly 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 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. 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.
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. This research's target audience is organic growers. However, current conventional herbicides have shown a decrease in efficacy due to herbicide-resistant weeds. Alternative options need to be explored, and this study explores 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, an extension publication, a thesis chapter, 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. Year 1 results of this project have been presented at an academic conference, sparking interest in collaborative learning/interaction among other researchers. In addition, year 1 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 to furthering the research for year 2. An extension publication available to growers, a thesis chapter, and a scientific journal article are in progress upon completion of Year 2 results.
Cooperators
Research
The study is occurring at the Iowa State University greenhouses in Ames, IA. The project aim is to evaluate the efficacy of five organic herbicides: Avenger® (citrus oil), AXXE® (ammonium nonanoate), Green Gobbler® (acetic acid), HomePlate® (caprylic acid + capric acid), and Weed Zap® (clove oil + cinnamon oil). All herbicides were Organic Materials Review Institute-approved when designing this experiment. Water was used as a control, and Ranger Pro® (glyphosate) was used as a positive control. All herbicides were 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). The experiment is a randomized complete block design with four replications. The study will be repeated two times. This report shows the results from year one of the experiment.
Weed species were seeded in a soilless media mix, which was a four-part soilless media mix to a one-part sand mixture, two tablespoons of Osmocote (flower and vegetable fertilizer) per 32-gallon amount of substrate was added. Weeds were seeded in 10-inch x 10-inch x 2-inch-deep plastic flats on February 2, 2024. Seeds were broadcast seeded into substrate-filled trays with a light sprinkle of the soilless media mix on top and kept moist until germination. After seedling emergence, weeds were thinned to 10 weed seedlings per tray, ensuring the 10 plants were of similar average heights. Weed species were sprayed once the average plant reached 2 inches tall. Broadleaf weed species were measured at the top of the growing point, and grass species were measured at the average top height of the grass blade.
All herbicide products were sprayed according to label recommendations using a calibrated spray chamber at the Iowa State University greenhouses. See Table 1 for herbicide application rates. Figure 1 shows herbicide labels.
Table 1. Herbicides trialed, including active ingredients, rate (gal per acre), and concentration (%V/V) of herbicides applied in the study.
| Product | Active Ingredient | Product per 300 mL H2O (mL) | Rate (Gallons per Acre) | Concentration (%Volume/Volume) |
| Avenger® | Citrus Oil | 37.5 mL | 60 GPA | 12.5% |
| AXXE® | Ammonium Nonanoate | 30 mL | 60 GPA | 10% |
| Green Gobbler® | Acetic Acid | 300 mL | 30 GPA | At Concentration |
| HomePlate® | Caprylic Acid + Capric Acid | 9 mL | 60 GPA | 3% |
| Weed Zap® | Clove Oil + Cinnamon Oil | 15 mL | 60 GPA | 5% |
| Ranger Pro® | Glyphosate | 3.75 mL | 20 GPA | 1.25% |
| Control | Water | 0 mL | 60 GPA | N/A |
Herbicide efficacy data was taken 1 day after treatment (DAT), 3 DAT, 10 DAT, 14 DAT, and 21 DAT. Percent visual injury data was assessed by graduate student PI. Percent visual injury data was taken objectively by identifying the percent damage from herbicide treatment of each experimental unit (tray of weed species) compared to the control (water-applied) treatment. This data was taken throughout all replications. High-resolution digital images of each experimental unit were taken at 1 DAT, 3 DAT, 10 DAT, 14 DAT, and 21 DAT. Photos were taken with a digital camera using the photo box (Figure 2). The photo box was made using a plastic storage container with battery-powered LED lights attached inside at the top. A black fabric cloth skirt was added to block any excess light from entering from the side. At the top of the photo box, a circular hole was drilled to the size of the digital camera's lens to capture the image. The photo box allows for consistent lighting across all images. Images will be analyzed with a computer software called Turf Analyzer. This program can detect healthy green color pixels compared to unhealthy dead tissue. The Turf Analyzer results show each organic herbicide's percent efficacy on each weed species. Our final data parameter was weed biomass. To evaluate the long-term efficacy of the herbicides, at 21 DAT, above soil weed biomass was taken, dried to consistent weight at 67˚C, and weighed for final weed biomass. 21 DAT allows enough time for glyphosate efficacy to be observed and allows rebound time of herbicides that initially appeared effective but later bounced back from the damage.
PERCENT VISUAL INJURY
Two grass species were trialed: green foxtail and large crabgrass; these two species showed herbicide injury most strongly one day after treatment. As the days after treatment increased, the original herbicide injury proved not to be enough to set back the grass weed species for long. Many grass species’ growing point is at or below the soil surface. With this in mind, if the herbicide cannot adequately damage or even contact the growing point, the grass weed species are shown to rebound quickly.
Organic herbicides in this experiment were all contact herbicides. Due to this, organic herbicide species 1 DAT had significantly higher efficacy than the positive control, Ranger Pro®. Glyphosate is a systemic product and takes time to translocate within the plant. This is why Ranger Pro® had significantly higher injury levels as DAT increased. Table 2-7 represents visual injury (%) taken through the study for all weed species. Weed Zap® was somewhat effective 1 and 3 DAT but weed species quickly bounced back as DAT increased.
Table 2. Common lambsquaters % visual injury at 1 DAT, 3 DAT, 10 DAT, 17 DAT, and 21 DAT.
| Visual Injury (%) | |||||
| Herbicide | 1 DAT | 3 DAT | 10 DAT | 17 DAT | 21 DAT |
| Avenger® | 47 b | 36 c | 8 c | 20 bc | 13 |
| AXXE® | 82 a | 98 a | 97 a | 92 a | 73 |
| Green Gobbler® | 41 b | 36 c | 10 c | 0 c | 10 |
| HomePlate® | 33 b | 29 c | 16 c | 6 c | 9 |
| Weed Zap® | 80 a | 79 b | 52 b | 44 b | 45 |
| Ranger Pro® | 0 c | 48 c | 54 b | 48 b | 52 |
| Control | 0 c | 0 c | 0 c | 0 c | 0 |
| P-value | <0.0001 | <0.0001 | <0.0001 | <0.0001 | 0.0586 |
Table 3. Common Purslane % visual injury at 1 DAT, 3 DAT, 10 DAT, 17 DAT, and 21 DAT.
| Visual Injury (%) | |||||
| Herbicide | 1 DAT | 3 DAT | 10 DAT | 17 DAT | 21 DAT |
| Avenger® | nd | 61 bc | 17 cd | 6 c | 36 b |
| AXXE® | nd | 99 a | 99 a | 99 a | 89 a |
| Green Gobbler® | nd | 33 d | 0 e | 0 c | 0 b |
| HomePlate® | nd | 47 cd | 7 de | 0 c | 0 b |
| Weed Zap® | nd | 67 bc | 25 c | 6 c | 20 b |
| Ranger Pro® | nd | 72 b | 82 b | 86 b | 85 a |
| Control | nd | 0 e | 0 e | 0 c | 0 b |
| P-value | nd | <0.0001 | <0.0001 | <0.0001 | 0.0013 |
Table 4. Green foxtail % visual injury at 1 DAT, 3 DAT, 10 DAT, 17 DAT, and 21 DAT.
| Visual Injury (%) | |||||
| Herbicide | 1 DAT | 3 DAT | 10 DAT | 17 DAT | 21 DAT |
| Avenger® | 46 b | nd | 8 c | 0 c | 4 c |
| AXXE® | 90 a | nd | 74 b | 49 b | 42 b |
| Green Gobbler® | 29 c | nd | 0 c | 0 c | 0 c |
| HomePlate® | 21 c | nd | 6 c | 0 c | 4 c |
| Weed Zap® | 50 b | nd | 15 c | 0 c | 0 c |
| Ranger Pro® | 4 d | nd | 94 a | 98 a | 99 a |
| Control | 0 d | nd | 0 c | 0 c | 0 c |
| P-value | <0.0001 | nd | <0.0001 | <0.0001 | <0.0001 |
Table 5. Large crabgrass % visual injury at 1 DAT, 3 DAT, 10 DAT, 17 DAT, and 21 DAT.
| Visual Injury (%) | |||||
| Herbicide | 1 DAT | 3 DAT | 10 DAT | 17 DAT | 21 DAT |
| Avenger® | 8 c | 12 cd | 4 c | 6 b | 6 |
| AXXE® | 78 a | 72 a | 30 b | 12 b | 31 |
| Green Gobbler® | 9 c | 11 cd | 0 c | 4 b | 34 |
| HomePlate® | 10 c | 24 c | 7 c | 8 b | 6 |
| Weed Zap® | 62 b | 56 b | 22 b | 12 b | 20 |
| Ranger Pro® | 0 c | 61 b | 78 a | 90 a | 62 |
| Control | 0 c | 0 d | 0 c | 0 b | 0 |
| P-value | <0.0001 | <0.0001 | <0.0001 | <0.0001 | 0.1633 |
Table 6. Redroot pigweed % visual injury at 1 DAT, 3 DAT, 10 DAT, 17 DAT, and 21 DAT.
| Visual Injury (%) | |||||
| Herbicide | 1 DAT | 3 DAT | 10 DAT | 17 DAT | 21 DAT |
| Avenger® | 10 d | nd | 4 d | 6 d | 9 d |
| AXXE® | 95 a | nd | 86 b | 70 b | 69 b |
| Green Gobbler® | 27 c | nd | 7 d | 5 d | 12 d |
| HomePlate® | 27 c | nd | 10 d | 6 d | 9 d |
| Weed Zap® | 92 a | nd | 71 c | 41 c | 44 c |
| Ranger Pro® | 47 b | nd | 97 a | 98 a | 98 a |
| Control | 0. d | nd | 0 d | 0 d | 0 d |
| P-value | <0.0001 | nd | <0.0001 | <0.0001 | <0.0001 |
Table 7. Velvetleaf % visual injury at 1 DAT, 3 DAT, 10 DAT, 17 DAT, and 21 DAT.
| Visual Injury (%) | |||||
| Herbicide | 1 DAT | 3 DAT | 10 DAT | 17 DAT | 21 DAT |
| Avenger® | nd | 18 d | 29 cd | 4 c | 4 c |
| AXXE® | nd | 96 a | 98 a | 100 a | 100 a |
| Green Gobbler® | nd | 35 c | 4 cd | 4 c | 0 c |
| HomePlate® | nd | 21 cd | 9 cd | 4 c | 10 c |
| Weed Zap® | nd | 62 b | 36 c | 8 c | 6 c |
| Ranger Pro® | nd | 64 b | 79 b | 95 b | 94 b |
| Control | nd | 0 e | 0 d | 0 c | 0 c |
| P-value | nd | <0.0001 | <0.0001 | <0.0001 | <0.0001 |
TURF ANALYZER IMAGES
These results are still in the process of being analyzed and will be included in the final grant report. Figure 3 shows an example of how Turf Analyzer detects green pixels. This data will be used to show percent weed cover within the flats.
WEED BIOMASS
AXXE® herbicide had significantly lower weed biomass than all other organic herbicides trialed in green foxtail, common lambsquaters, large crabgrass, purslane, redroot pigweed, and velvetleaf weed species. AXXE® was statistically as effective as Ranger Pro® (Glyphosate) on common purslane and velvetleaf (p<0.0001). Ranger Pro® was less effective than AXXE® on common lambsquaters (p=0.0002). However, Ranger Pro® was more effective than AXXE® on redroot pigweed species (p<0.0001).
Common purslane and large crabgrass were further analyzed to represent one grass and one broadleaf species. Figure 4 displays weed biomass 21 DAT comparing the efficacy of all herbicides on both species. Visual comparison of purslane (Figure 5) and large crabgrass (Figure 6) before weed biomass sampling at 21 DAT.
Grass species proved extremely difficult to injure using any of the organic herbicides at the rates we applied. I believe this can be attributed to the growing point of grass being beneath the surface. After spraying, grass species showed minimal damage, and the grasses quickly bounced back and saw little to no impact from the herbicide pass at 21 DAT.
Table 8. Dried weed biomass (grams) taken 21 days after treatment (DAT).
| Dried weed biomass (grams) | ||||||
| Herbicide | Common Lambsquaters |
Common Purslane |
Green Foxtail |
Large Crabgrass | Redroot Pigweed | Velvetleaf |
| Avenger® | 16.9 a | 16.2 a | 25.5 a | 27.1 a | 17.3 a | 11.4 a |
| AXXE® | 2.0 c | 1.0 b | 12.1 b | 18.0 b | 7.5 b | 0.1 b |
| Green Gobbler® | 16.9 a | 18.1 a | 32.2 | 25.2 a | 19.0 a | 11.7 a |
| HomePlate® | 13.9 ab | 16.8 a | 25.7 a | 24.0 a | 17.1 a | 11.8 a |
| Weed Zap® | 8.8 b | 16.0 | 26.2 a | 23.5 a | 10.9 b | 10.1 a |
| Ranger Pro® | 8.1 b | 2.3 b | 0.6 c | 2.9 c | 0.2 c | 1.6 b |
| Control | 18.3 a | 18.7 a | 26.3 a | 24.1 a | 20.6 a | 11.4 a |
| P-value | 0.0002 | <0.0001 | <0.0001 | <0.0001 | <0.0001 | <0.0001 |
Educational & Outreach Activities
Participation Summary:
I presented a poster (a 10-minute presentation) of year 1 results at the American Society of Horticultural Sciences Annual Conference (September 24, 2023) in Honolulu, HI. Collaboration among researchers from multiple universities was fostered at the presentation. In addition, I also participated in the poster competition at the conference.
I presented a poster at the Great Lakes EXPO in Grand Rapid, MI (December 10-12, 2024). I interacted with growers sharing my results of the study.
I presented a poster at the Great Plains Grower conference in Saint Joseph, MO (January 9-11, 2024). I interacted with multiple fruit and vegetable growers to discriminate the results I found and ways organic herbicides may be used on their farms.
I presented a poster at the Marbleseed Organic Conference in La Crosse, WI (February 20-22, 2025). I participated in the poster completion, which allowed me to interact with organic professionals. This opportunity also fostered outreach to growers to learn more about organic herbicide products. During these interactions, I gained further insight into how growers are using these organic herbicide products on their farms.
I will be writing an extension publication covering organic herbicides and products available to growers. I will also be writing a thesis chapter covering this experiment and a scientific journal article for potential publication in Weed Technology in 2025.
Project Outcomes
The use of organic herbicides provides an alternative weed management method for organic growers. It may also be a valuable tool for homeowners wanting a less chemically intensive choice to manage weeds in their yards. Weeds for growers can hinder yields by robbing the cash crop of water and excess nutrients. For many growers, multiple options to control weeds are necessary. Growers who have interacted with my posters and extension publication will have an increased understanding of organic herbicide efficacy, modes of application, and tips for success. Growers will also begin to think about on-farm weed management and how they can add organic herbicides into their integrated weed management plan. If organic herbicides could prove to be effective in a variety of situations, these options could contribute to future sustainability by being an effective option for conventional growers as well. With concerns about herbicide resistance with systemic products, an increase in other options is necessary.
Results from this study provide growers with practical and applied data to make informed decisions regarding the use of organic herbicides.
Overall, this study will work to increase the usage and implementation of organic herbicides. Even if growers may not choose to use these products, they are educated and know its potential as another tool in the toolbox. With these year 1 results and the addition of a surfactant for year 2. This will hopefully increase the success of the herbicide's efficacy so we can provide further recommendations for products that are effective. Year 1 results were significant in showing what rates many herbicides remained relatively ineffective, so our addition of a surfactant will hopefully improve efficacy and increase the chance of potential grower adoption, if they feel confident the product will work. A broader amount of information will be available, impacting grower decision-making. Our impact will expand with year 2 results, providing scientifically based data for growers available via an extension publication and further grower events.
After completing year one of this project, I have gained an insight into organic grower’s challenges with weeds. I have interacted with many growers at conferences, and most reported struggling with weeds. However, many were unfamiliar with the products I was trialing. One grower I interacted with reported using many of the organic herbicide products I used in this study. Other growers shared plans of how they would use organic herbicides. This changed my knowledge of how growers plan or are already using these products. This has allowed me to better target my focus of an extension and scientific journal articles.
With our results showing many of the products being relatively ineffective, my advisor and I have seen a further gap in the research in this area. I think there is a need for trialing on perennial species such as thistles which many organic growers struggle with. Another avenue of increasing efficacy would be to trial repeat application of herbicides or using a surfactant. A more specific struggle this project unveiled was the ability to achieve product coverage on the leaf. Many of these labels advise the product to have complete coverage to the point of the herbicide dripping off the plant. When designing the rate application, we found this very hard to achieve using the herbicide label recommendations. Labels like Green Gobbler did not provide a spray concentration or rate, while some mentioned a rate but also advised complete saturation of the weed. Based on year 1 results, we plan to add an organic surfactant to all herbicide applications to increase the efficacy while still following a general application rate for the product (60 gal per acre). I believe my perspective of what growers are willing to try to get rid of weeds on their farms has broadened. I initially thought many organic growers would have skepticism about spraying these products, but many I interacted with seemed very open to the idea. Many growers are looking for an option to control weeds in non-production areas, such as gravel near greenhouses, fence rows, and areas transitioning to prairie.