Evaluations of driftable fractions of dicamba and 2,4-D to selected herbaceous and woody plant species

Abstract

Commercialization of 2,4-D and dicamba-resistant cotton and soybean has given producers another mechanism of action to control herbicide resistant weeds. As a result of the adoption of this technology, off-target movement of 2,4-D and dicamba onto nearby sensitive broadleaf species is a major concern for specialty crop growers, homeowners, organic producers, and commercial growers of ornamental plant species. Two separate studies were conducted in 2017 and 2018 to determine the sensitivity of select herbaceous and woody plant species to driftable fractions of 2,4-D or dicamba with or without glyphosate. In the herbaceous plant study, three driftable fractions of 1/10, 1/100, and 1/300th of the fully labeled rate of 2,4-D, 2,4-D plus glyphosate, dicamba, and dicamba plus glyphosate were applied to begonia, coleus, geranium, impatiens, marigold, petunia, vinca, and zinnia. Based on visual injury assessments recorded 28 days after treatment (DAT), coleus was found to be the most sensitive species to driftable fractions of all herbicides. It was also found that marigold and geranium had a greater sensitivity to treatments containing 2,4-D but, coleus and zinnia had a greater sensitivity to treatments containing dicamba. In the woody plant study, the same herbicides were applied but at different fractions corresponding to 1/2, 1/20, and 1/200th of the full labeled rates. Herbicide treatments were applied to apple, crabapple, dogwood, elderberry, elm, grape, hydrangea, maple, oak, pecan, redbud, rose, raspberry, strawberry, sweetgum, viburnum, and walnut plants. Visual injury assessments along with shoot length measurements and tree trunk diameter indicate that grapes were the most sensitive species to all herbicide treatments while hydrangea was the least sensitive. In addition, walnut, grape and elm were found to have a greater sensitivity to 2,4-D while apple, maple, and peach had a greater sensitivity to dicamba. Both studies found that as the herbicide rate increased, greater visual injury, and greater reductions in height and dry biomass or trunk diameter and shoot length were observed. Furthermore, when glyphosate was applied with either 2,4-D or dicamba at the high rates, visual injury increased for most plant species. While the lowest rates of herbicides applied in these studies did not cause significant reductions in height, dry biomass, shoot length or tree trunk diameter, visual injury was still observed for some species. Since many of the species investigated in these studies are fruiting or ornamental plants, it is likely that even the low levels of injury would not be tolerated by homeowners or commercial growers.