W. Thomas Lanini, weed ecologist for the California Cooperative Extension Service, recently investigated the effectiveness of a group of organic herbicides that are all contact herbicides with no residual activity. The active ingredient in these products was either acetic acid (vinegar) or various plant oils that work by stripping away the waxy cuticle on the leaves, causing them to wilt and dry up. They included WeedPharm (20% acetic acid); GreenMatch (55% d-limonene, which is the major component of the oil extracted from citrus rinds); GreenMatch Ex (50% lemongrass oil); Maratec (50% clove oil); and WeedZap (45% clove oil and 45% cinnamon oil).
As detailed in an article in the January/February 2011 issue of TPI Turf News, Lanini found that these products gave reasonably effective control of broadleaf weeds such as pigweed if weeds were treated when they were still young and small. In his study, treatment 12 days after weed emergence gave anywhere from 100% to 61% control; waiting until 26 days after emergence brought the percentage of control down to 0% to 38%. Organic herbicides were much less effective on grassy weeds, most giving about 25% control on 12-day old weeds and virtually no control on 26-day old ones.
He also found that good spray coverage was essential and that adding organic surfactants or spreaders to the herbicides improved control. Most of the organic herbicides also worked best at temperatures above 75 degrees F.
More information on the evaluation of organic herbicides.