Efficacy and nontarget effects of glyphosate and two organic herbicides for invasive woody vine control.
Mat-forming vines constitute half of invasive plant cover in eastern United States forests. Although glyphosate provides effective control, it has been garnering waning public acceptance due to its potential for harming human health and nontarget organisms, and due to the evolution of plant resistance. Since 67% of eastern Unites States forests are owned by private individuals, finding more acceptable chemical controls for invasive plants is important. Organic herbicides have been used for herbaceous weed control in agriculture. However, there have been no published studies of their efficacy in controlling woody plants or of their potential nontarget effects in natural areas. We compared the ability of two commercially available, organic herbicide formulations (pelargonic acid and cinnamon plus clove oils) against glyphosate to suppress growth of four woody vines, Akebia quinata, Euonymus fortunei, Hedera helix, and Vinca minor, in an urban woodland. We also tested whether these herbicides affected soil nematodes and the germination of moss and fern spores from soil. We found that glyphosate killed these vines after two spray treatments, but that a third treatment was needed the next year for the organic herbicides to kill or reduce vines. This reduction lasted into a third summer. We detected no herbicide effects on nematode densities and functional feeding groups, nor on abundance and species richness of moss and fern germinants. Although these organic herbicides cost 5-6.5 times more than glyphosate at dosages used, they greatly reduced these woody vines and can expand choices for chemical plant control for natural areas managers.