Greenhouse trials were conducted to determine the response of stevia to reduced-risk synthetic and nonsynthetic herbicides applied over-the-top post-transplant. In addition, field trials were conducted with stevia grown in a polyethylene mulch production system to determine crop response and weed control in planting holes to reduced-risk synthetic and nonsynthetic herbicides applied post-transplant directed. Treatments included caprylic acid plus capric acid, clove oil plus cinnamon oil, d-limonene, acetic acid (200 grain), citric acid, pelargonic acid, eugenol, ammonium nonanoate, and ammoniated soap of fatty acids. Stevia yield (dry aboveground biomass) in the greenhouse was reduced by all herbicide treatments. Citric acid and clove oil plus cinnamon oil were the least injurious, reducing yield by 16% to 20%, respectively. In field studies, d-limonene, pelargonic acid, ammonium nonanoate, and ammoniated soap of fatty acids controlled Palmer amaranth (>90% 1 wk after treatment (WAT). In field studies caprylic acid plus capric acid, pelargonic acid, and ammonium nonanoate caused >30% injury to stevia plants at 2 WAT, and d-limonene, citric acid, acetic acid, and ammoniated soap of fatty acids caused 18% to 25% injury 2 WAT. Clove oil plus cinnamon oil and eugenol caused <10% injury. Despite being injurious, herbicides applied in the field did not reduce yield compared to the nontreated check. Based upon yield data, these herbicides have potential for use in stevia; however, these products could delay harvest if applied to established stevia. In particular, clove oil plus cinnamon oil has potential for use for early-season weed management for organic production systems. The application of clove oil plus cinnamon oil over-the-top resulted in <10% injury 28 d after treatment (DAT) in the greenhouse and 3% injury 6 WAT postemergence-directed in the field. In addition, this treatment provided 95% control of Palmer amaranth 4 WAT.

Weed management is one of the major challenges responsible for growers’ reluctance to switch from conventional to organic vegetable production. Competition from uncontrolled weeds can significantly reduce the yield and water use efficiency (WUE) of vegetable crops. A field study was conducted during the summers of 2019 and 2020 at the Quaker Research Farm of Texas Tech University, in Lubbock, TX, to determine physiology, plant growth, yield, soil water depletion pattern, and WUE of pumpkin as affected by five weed control treatments: 1) ammonium nonanoate 5% ai, 2) ammonium nonanoate 6% ai, 3) clove oil + cinnamon oil 4.5% ai, 4) clove oil + cinnamon oil 9% ai, and 5) an untreated control. Each plot received the same herbicide treatment at 20 and 30 d after planting (DAP). The experiment was conducted in a randomized complete block design with four replications of each treatment. Both herbicides resulted in significant weed suppression compared to the untreated control and weed control was 88% to 98% with ammonium nonanoate and 39% to 69% with clove oil + cinnamon oil. Photosynthesis rate, stomatal conductance, shoot dry biomass, average fruit weight, total fruit yield, and WUE of pumpkin were significantly higher in plants that were treated with ammonium nonanoate compared to the untreated control. All the aforementioned parameters were not significantly different between clove oil + cinnamon oil and the untreated control. Use of higher active ingredient concentration did not improve the performance of either herbicide. Soil water depletion and evapotranspiration were comparable among all the treatments. Based on the results, ammonium nonanoate at its lower concentration could be an option for effective weed control, and for improving fruit yield and WUE of pumpkin.

Ammonium nonanoate is registered for weed control in certified organic cropping systems and may be useful to control cool-season weeds in organic Vidalia® sweet onion production. Ammonium nonanoate combined with tine-weeder cultivation was evaluated for weed control in organic onion in Georgia. There were no statistical interactions between main effects of herbicides and cultivation with a tine weeder for cool-season weed control and onion yield, indicating that ammonium nonanoate does not improve weed control compared with cultivation. Ammonium nonanoate at 4% and 6% did not adequately control weeds and onion yields were reduced. Ammonium nonanoate at 8% and 10% controlled cutleaf evening-primrose and lesser swinecress equal to the standard of d-limonene (14%), but the degree of control did not consistently protect onion yields from losses due to weeds. These results are in agreement with previous studies using clove oil and pelargonic acid. There is no advantage to using ammonium nonanoate for cool-season weed control in organic Vidalia® sweet onion production.