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.
Nomenclature: Acetic acid; citric acid; citric acid (5%) garlic (0.2%; Alldown); citric acid (10%) garlic (0.2%; Groundforce); clove oil (45.6%; Matran II); corn gluten meal (CGM); annual ryegrass, Lolium multiflorum Lam.; black nightshade, Solanum nigrum L.; crowfootgrass, Dactyloctenium aegyptium (L.) Willd; goosegrass, Eleusine indica (L.) Gaertn.; green foxtail, Setaria viridis (L.) Beauv; Johnsongrass, Sorghum halepense (L.) Pers.; redroot pigweed, Amaranthus retroflexus L.; sicklepod, Senna obtusifolia (L.) H. S. Irwin & Barneby; stranglervine, Morrenia odorata (Hook. & Arn.) Lindl.; velvetleaf, Abutilon theophrasti Medik; wild mustard, Brassica kaber (DC.) L. S. Wheeler; yellow nutsedge, Cyperus esculentus L; clove, Syzygium aromaticum (L.) Merr. & L. M. Perry; corn, Zea mays L.; garlic, Allium sativum L.