The Potential for Injury to Horticultural Crops from Herbicide Carryover and Material Contamination

Many herbicides applied to crops or ornamental areas have the potential to persist in soil and to injure desirable vegetation or rotationally grown crops. Active ingredients such as atrazine, halosulfuron, dichlobenil, and clomazone are just a few examples of herbicides with extended half-lives that, under certain conditions, may persist in soil for more than one growing season. Sometimes purposeful applications of selective herbicides have unintended effects upon sensitive horticultural plant species, as was recently observed by applications of aminocyclopyrachlor to residential turf in the Midwestern US, resulting in severe damage to nearby trees and shrubs. Clopyralid, picloram, and aminopyralid are similar auxinic herbicides that have been shown to not only persist in soil, but also to resist metabolism by microbes when treated plant residues are composted. Mandatory curbside recycling programs of lawn waste and grass clippings in Spokane, Washington in the early 2000s resulted in clopyralid-contaminated compost that caused severe damage to backyard and commercial crops in Solanaceae, Asteraceae, and Fabaceae. In the late 2000s, aminopyralid was found in finished compost in northwestern Washington state that similar injured sensitive vegetable crops. Aminopyralid had been legally applied to forage and fed to cattle, but dairy operators failed to quarantine the manure and it entered the composting stream. In a subsequent greenhouse trial, aminopyralid residue as low as 10 ppb caused significant injury to pea, sunflower, and bell pepper seedlings. Label modifications by manufacturers have greatly reduced the likelihood of similar damage to horticultural crops from application of herbicide-contaminated compost, but local growers are advised to perform a bioassay using a sensitive plant species when they are unsure whether soil or compost contains potentially harmful herbicide residues.