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Tennessee Turfgrass – D.E. Carroll & J.T. Brosnan

In certain regions of the United States and Canada, legislation has severely restricted traditional synthetic herbicide use in residential and commercial landscapes. Specifically, residential use of synthetic pesticides deemed “cosmetic use” in the Ontario Province, Canada, has been banned. Other aspects of the turfgrass industry such as sports and golf are able to receive exemptions through government approval, stated reduction goals, integrated pest management certification, and increased transparency requiring reporting and in-person meetings to discuss pesticide application. (Ontario Regulation, 2018). Similarly, in Montgomery County, Maryland, the use of synthetic pesticides perceived for use cosmetically is banned. Only natural, non-synthetic herbicides listed by the Organic Materials Review Institute (OMRI) may be used for this purpose (County Council for Montgomery County, Maryland, 2019). Many local governments, such as the city of New York, banned glyphosate use on public land such as school districts, parks and in municipalities, except for maintenance of invasive plants in native area. (The New York City Council, 2019).

In the United States, many active ingredients in organic pesticides are considered minimum risk, categorized as section 25b products, and are exempt from the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) registration process. Products must meet six criteria for inclusion in this category. Because labeling in this category is typically not federally regulated, labeling of non-synthetic herbicides is not consistent and may include wording such as “organic” or “natural”, despite these products not undergoing typical organic product registration.  Although not required by federal law, independent agencies perform organic standard research to determine which products receive organic seals on labeling (Figure 1). Additionally, individual states may still require a registration process for minimum risk products and may limit herbicide use in some areas to only products listed by independent testing agencies. An example of a natural product research agency that maintains a list of products compliant with internal organic standards is OMRI. OMRI is a non-profit organization, and thus not considered a regulatory agency. Conversely, the Environmental Protection Agency is considered a regulatory agency and although not federally required, also tests natural products and lists those meeting organic criteria.

Active ingredients on these lists include chelated iron, acetic acid, corn gluten meal, and a variety of soaps and oils. Chelated iron, also referred to as Iron HEDTA, is concentrated iron formulated for increased uptake in broadleaved weeds versus grasses. Once inside the plant, the iron is oxidized and causes necrosis (Charbonneau, 2010). Acetic acid, which is concentrated in household vinegar used for cooking at 5%, is generally concentrated in horticultural vinegar at 20 to 40%. Horticultural vinegar applications can kill weeds by removing the waxy cuticle found on leaves, thus drying out the plant. Citrus oil and soaps such as pelargonic acid or ammoniated fatty acid work in similar fashion by stripping the leaf cuticle to facilitate water loss. Unlike many synthetic herbicides, products with these active ingredients are generally fast acting and results may be observed within a few hours of application (Figure 2).

Products containing these active ingredients are widely available to the public and can be purchased at most big box retail stores and online (Figure 3). Due to the lack of regulation in labeling, many manufacturers produce products of differing trade names that have similar active ingredients included at variable concentrations. For example, horticultural vinegar is available as Green Gobbler concentrated at 20, 30, or 40% acetic acid; Eco Garden Pro (8% acetic acid + 5% sodium chloride); or as WeedPharm (20% acetic acid) among others.

A common misconception is that products labeled as “organic” are non-toxic. This is false. Similar to synthetic pesticides, product labels contain the signal words caution, warning, or danger to alert applicators to toxicity. “Caution” labeling indicates the lowest level of toxicity to humans followed by “Warning” meaning moderately toxic and “Danger” denoting high toxicity. An example of organic herbicide toxicity labeling is horticultural vinegar labeled with a “Danger” distinction because the product is a strong irritant and needs to be used with care. It is critically important that end-users read and follow label instructions for both conventional and alternative herbicides before application.

Most of these alternative weed control products are non-selective and are therefore injurious to desirable turfgrass (Figure 2). To mitigate injury concerns, these alternative options should be used to spot-treat individual weeds rather than being applied via broadcast sprays. Research conducted at the Pennsylvania State University (University Park, PA) assessed the efficacy of AXXE (40% ammonium nanonate), horticultural vinegar (30% acetic acid), Fiesta (26.2% Iron HEDTA), Avenger (70% d-limone citrus oil), and A.D.I.O.S. (11.9% sodium chloride) for control of dandelion (Taraxacum officinale G.H. Weber ex Wiggers) and white clover (Trifolium repens L.) in perennial ryegrass (Lolium perenne L.).  Herbicide treatments were applied in late July with sequential applications made on two or four week intervals. Three applications of Fiesta applied at 25.2 fl oz/1,000 ft² every four weeks controlled clover and dandelion within six days of initial application through mid-October. While not considered injurious to the turfgrass sward, increased iron uptake in the desired perennial ryegrass stand resulted in grey discoloration (Figure 4).

Other natural, non-synthetic herbicide treatments including AXXE (15% v/v) and horticultural vinegar (50% v/v) provided some control of both weed populations compared to an untreated check, although both were injurious to turfgrass (Figure 5). Treatments of A.D.I.O.S. (50% v/v) or Avenger (14% v/v) did not control dandelion or clover and were not injurious to desired turfgrass.

Research conducted at the University of Tennessee (Knoxville, TN) in summer 2019 found similar results. A trial initiated in late July assessed the efficacy of eight alternative herbicides (Fiesta, Avenger, Burnout, horticultural vinegar, A.D.I.O.S., Biosafe, Natria, and WeedBeater Fe) for control of white clover. Two weeks after treatment, several alternative products controlled white clover similar to a single application of Trimec Classic (2,4-D, MCPP, and dicamba) at 3 pt/A; these alternative treatments included single applications of Fiesta applied at 12.6, 25.2, or 50 fl oz/1,000 ft²; horticultural vinegar (30% acetic acid; 50% v/v); and Avenger (70% d-limone citrus oil; 20% v/v). The desired fine fescue (Festuca spp.) turfgrass stand was discolored following Fiesta treatment and injured by horticultural vinegar and Avenger. Treatments of Natria (3.7% ammoniated soap of fatty acid; 20% v/v), Biosafe (40% ammonium nanonate;15% v/v), WeedBeater Fe (1.5% Iron HEDTA; 100% v/v) and A.D.I.O.S (11.86% sodium chloride; 33% v/v) did not control white clover. Burnout (24% citric acid + 8% clove oil; 6% v/v) controlled clover ~ 50% compared to a non-treated check.

A second trial was conducted in February 2020 at the University of Tennessee to assess efficacy of alternative weed control products for control of winter annual broadleaf weeds in dormant bermudagrass (Cynodon spp.). Eleven days after treatment, Fiesta applied at 12.6, 25.2 or 50 fl oz/1,000 ft²; AXXE (15% v/v); horticultural vinegar (50% v/v); and Natria (20% v/v) controlled broadleaved weeds such as hairy bittercress (Cardamine hirsuta), henbit (Lamium amplexicaule), mouse-ear chickweed (Cerastium vulgatum), and corn speedwell (Veronica arvensis) ~ 50%. Thirty-three days after initial treatment, the end of the study, Fiesta applied at 25.2 or 50 fl oz/1,000 ft² with a three-week sequential application controlled these broadleaf weeds 60 to 80%, similar to Roundup Pro at 16 fl oz /A and Cheetah Pro at 82 fl oz/A.  At the end of the study, AXXE, horticultural vinegar, and Natria, treated sequentially three weeks after initial application, controlled these broadleaved weeds ~ 25%. The low control provided by these products at the end of the study compared to the 50% control observed 11 days after treatment indicates that sequential applications should be made every two weeks to obtain the greatest weed control.

Results of these research projects show that alternative weed control products can reduce weed populations compared to non-treated turfgrass, although outside of Fiesta, they are generally not as effective as traditional synthetic herbicides. Alternative products are not translocated throughout plant tissue, which often results in regrowth of treated weeds. Therefore, sequential applications will be required for sustained weed control, which can be costly. For example, an application of Fiesta at 50 fl oz/1,000 ft² is approximately $25.40 per 1,000 ft², compared to $0.76 per 1,000 ft²  for an application of 2,4-D + MCPP + dicamba at 4 pt/A.

Changing legislation in certain areas of the United States and Canada may put pressure on synthetic pesticide use in turfgrass. Homeowners and turfgrass managers should be aware that while natural herbicides may be an effective tool compared to not using any weed control product, they are not an equivalent replacement to synthetic herbicides. In scenarios where alternative weed control products are required or preferred, they can be used to control select weed species via spot treatments applied sequentially. Products containing high concentrations of chelated iron, such as Fiesta, or acetic acid can be used for broadleaf control when applied as sequential spot treatments. However, these applications can cause undesirable injury or discoloration to turfgrass. More research is needed to better understand alternative methods of controlling weeds in maintained turfgrass. Investigations pertaining to effects of environmental or meteorological factors on efficacy of these products are warranted.

The authors would like to thank Dr. Frank Rossi, Associate Professor and Extension Turfgrass Specialist at Cornell University, and Vickie Wallace, University of Connecticut Extension Educator and Program Director of UConn’s Sustainable Turf and Landscape Program, for their assistance in developing this article.

Literature Cited

Charbonneau, P. (2010) Efficacy of iron chelate herbicide for turf broadleaf weed control. Sports Turf Manager 23(2):25-27.

County Council for Montgomery County, Maryland (2019) Non-Essential Pesticide Prohibitions, Cosmetic Pesticide Use Restriction Bill. 52-14.

Ontario Regulation (2018) Pesticides Act, R.S.O. p.11 Ontario Regulation 63/09.

The New York City Council (2019) A Local Law to amend the administrative code of the city of New York, in relation to the use of pesticides by City agencies. Committee on Health Int. No. 1524.