Late season weed control in onions
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
This is the middle of July and it is time to make the final herbicide applications of the season to onions. Assuming a September 1 harvest date, July 15 is 45 days before harvest. Several onion herbicides have 45-day preharvest intervals (PHI), including Prowl H2O, Chateau, GoalTender, Fusilade, and Select. Outlook, Nortron, and Poast have 30-day PHI. Dual Magnum has a 60-day PHI, so it may not be used this late except for very late onions. It may be wise to apply a final application of a preemergence herbicide to provide another four to six weeks of suppression of yellow nutsedge and annual weeds. Dual Magnum and Outlook have nutsedge activity. However, the labels limit Outlook to one application per year and Dual Magnum to two applications per year. Chateau may be applied now, and will control nightshades, mustards, composites and smartweeds. Do not exceed 2 oz in an application and 3 oz per acre per year. Do not apply Chateau closer than seven days before or after other pesticides, especially anything with an EC formulation, except Prowl H2O, which is allowed on the label. Application with or near an EC formulation may result in onion injury.
GoalTender may be applied up to 4 fl oz (0.125 lb ai) per acre to kill large weeds. If it is tank-mixed with a grass herbicide, it can cause onion curling and twisting, but onions will grow out of it. That treatment will kill most emerged weeds except mustards and smartweeds.
This is the last opportunity for effective preemergence application on onion. The plants get too large and most spray is intercepted before it hits the ground from now on. Any weeds surviving now will still be there at harvest, so use the opportunity accordingly.