Effect of pesticide-treated grass clippings used as a mulch on ornamental plants

With increasing pressure to reduce disposal of yard waste in landfills, many homeowners are seeking alternative methods for grass clipping disposal. When turf is treated with pesticides, the collected grass clippings become a potential source of injury to susceptible plants that come in contact with the clippings. In this study, grass clippings were collected at 2, 7, and 14 days after pesticide treatment from a turf treated with chlorpyrifos, clopyralid, 2,4-D, flurprimidol, isoxaben, or triclopyr. The clippings were used as a mulch around Lycopersicon esculentum Mill. (tomato), Phaseolus vulgaris L. (bush bean), Petunia xhybrida Hort. Vilm.-Andr. (petunia), and Impatiens wallerana Hook. f. (impatiens). Beans were planted 4 weeks prior to mulching, whereas the other plants were grown in the greenhouse for 6 weeks and transplanted into the field 2 weeks prior to mulching. Clippings containing residues of clopyralid, 2,4-D, or triclopyr killed tomato, bean, and petunia plants when used 2 days after pesticide treatment (DAPT) and severely injured these same species when mulched 7 and 14 DAPT. Flurprimidol injured tomato, impatiens, and bean plants when present on mulch collected 2, 7, and 14 DAPT, but was not lethal. Flurprimidol slowed plant growth, caused darker green leaf color, and reduced flowering when mulched at 2 DAPT. Isoxaben injured tomato and bean plants when present on mulch used 2, 7, and 14 DAPT but was not lethal. Injury was not as severe in the second year of the study, indicating different environmental stresses and climatic conditions make predicting pesticide injury for all growing seasons difficult; however, grass clippings from a turf treated with herbicides or plant growth regulators should not be used for mulch around sensitive plants for at least 14 DAPT. Chemical names used: O,O-diethyl O-(3,5,6-trichloro-2-pyridinyl) phosphorothioate(chlorpyrifos);3,6-dichloro-2-pyridinecarboxylic acid, triethylamine salt (clopyralid); 2,4-dichlorophenoxyacetic acid, dimethylamine salt (2,4-D); α-(1-methylethyl)-α-[4-(trifluromethoxy)phenyl]-5-pyrimidinemethanol (flurprimidol); N-[3-(1-ethyl-1-methylpropyl)-5-isoxazolyl]-2,6-dimethoxybenzamide and isomers (isoxaben); 3,5,6-trichloro-2-pyridinyloxy acetic acid, triethylamine salt (triclopyr).