Field experiments were conducted in 2008 and 2010 to determine crop tolerance and weed control efficacy of the POST herbicides bentazon, flumioxazin, and oxyfluorfen applied to direct-seeded dry bulb onions on organic soil. Postemergence application of oxyfluorfen at 0.071 kg ai ha-1 resulted in less than 20% onion injury when applied at the 2 and 4 onion leaf stages and provided good control of ladysthumb and common lambsquarters. Oxyfluorfen EC caused slightly higher visual injury than oxyfluorfen SC, but there was no difference in onion yield among the treatments. Application of flumioxazin at 0.036 of 0.072 kg ai ha-1 alone or in combination with pendimethalin ACS resulted in minimal onion injury and no yield reduction. Combining flumioxazin in a tank mix with pendimethalin EC, dimethenamid-P EC, or S-metolachlor EC resulted in significant onion injury and yield reduction. Flumioxazin plus S-metolachlor, dimethenamid-P, or pendimethalin improved ladysthumb control in one of two years. Bentazon applied at 0.56 kg ai ha-1 produced moderate onion injury and did not control yellow nutsedge adequately. Bentazon applied at 1.12 kg ai ha-1 provided good control of yellow nutsedge but caused serious onion injury and yield loss.
Nomenclature: Bentazon, dimethenamid, flumioxazin, oxyfluorfen, pendimethalin, S-metolachlor, common lambsquarters, Chenopodium album L., hairy nightshade, Solanum physalifolium Rusby, ladysthumb, Polygonum persicaria L., redroot pigweed, Amaranthus retroflexus L., yellow nutsedge, Cyperus esculentus L., onion, Allium cepa L.