There is an urgent need to accelerate the development and implementation of effective organic-compliant herbicides that are environmentally safe and that help the producer meet increasing consumer demand for organic products. Therefore, greenhouse experiments were conducted to evaluate the effectiveness of acetic acid (5%), acetic acid (30%), citric acid (10%), citric acid (5%) + garlic (0.2%), citric acid (10%) + garlic (0.2%), clove oil (45.6%), and corn gluten meal (CGM) compounds as natural-product herbicides for weed control. The herbicides were applied to the broadleaf weeds stranglervine, wild mustard, black nightshade, sicklepod, velvetleaf, and redroot pigweed and to narrowleaf weeds crowfootgrass, Johnsongrass, annual ryegrass, goosegrass, green foxtail, and yellow nutsedge. The herbicides were applied POST at two weed growth stages, namely, two to four and four to six true-leaf stages. CGM was applied PPI in two soil types. Citric acid (5%) + garlic (0.2%) had the greatest control (98%) of younger broadleaf weeds, followed by acetic acid (30%) > CGM > citric acid (10%) > acetic acid (5%) > citric acid (10%) + garlic (0.2%), and clove oil. Wild mustard was most sensitive to these herbicides, whereas redroot pigweed was the least sensitive. Herbicides did not control narrowleaf weeds except for acetic acid (30%) when applied early POST (EPOST) and CGM. Acetic acid (30%) was phytotoxic to all broadleaf weeds and most narrowleaf weeds when applied EPOST. Delayed application until the four- to six-leaf stage significantly reduced efficacy; acetic acid was less sensitive to growth stage than other herbicides. These results will help to determine effective natural herbicides for controlling weeds in organic farming.

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Organic crop production, whether for export or local consumption, is increasing to avoid the residual effects of synthetic herbicides in foods, soil, and water, toxicity to other nontarget organisms, and herbicide-resistant weed populations. Organic farmers consistently ranked weed management as one of their most important production problems. Therefore, a 2-year study was conducted under 15-year-old mandarin trees to compare the effects of rice straw mulch, cattail mulch, black plastic mulch, hand hoeing, cultivation, glyphosate, and unweeded control treatments on weed control, fruit yield, and fruit quality. The greatest control (94%–100%) of weeds occurred with the plastic mulch (200 or 150 μm) and three mulch layers of rice straw or cattail. Covering soil with cattail or rice straw mulch (two layers) gave 85% to 98% control of weeds. Uncontrolled weeds in the weedy control caused significant reduction in yield and fruit quality and decreased the yield/tree by 62% compared with hand hoe treatment. Plastic mulches of 200 and 150 μm, cattail (Cyprus articulatus L.) mulch (two or three layers) and two mulch layers of rice (Oryza sativa L.) straw treatments significantly increased the fruit yield/tree by 24%, 18%, 20%, 11%, and 12% more than cultivation treatment, respectively, without significant differences among these superior treatments. Soil mulching with three layers of rice straw, cultivation, glyphosate, and 80-μm plastic mulch treatments caused a significant reduction in weed density and weed biomass, but gave lower yield than superior treatments. Total soluble solids of fruits was unaffected by any of the weed management strategies, whereas values of total acidity and vitamin C were significantly lower in the unwedded control than most weeded treatments. These results demonstrate that two layers of cattail or rice straw mulch could be used effectively for controlling weeds in citrus groves. Their effectiveness in controlling weeds may increase their use in agriculture systems with a concomitant decrease in the need for synthetic herbicides. Further studies are needed to evaluate their side effects on beneficial organisms, diseases, and insects.