Onion weed control during cold growing conditions

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. 

The prolonged cool growing conditions this spring have complicated onion weed control plans. Most seeded onions are growing very slowly with some just emerging, and others in the 2 leaf stage, often in the same field. Weeds are germinating, and small grain cover crops continue to grow. Growers face a dilemma regarding what to do next.

An old physician (Greek?) adage is “Do no harm.” That is good advice for onion weed control under these conditions as well. Whatever we do, we don’t want to thin the onion stand too much or inflict permanent injury that will result in yield loss. A good rain and a couple of warm days will encourage most of the onions to reach the 2 leaf stage soon. A few lagging seedlings won’t be missed.

Once the onions are growing well, the remaining small grain (primarily barley) cover crops should be killed immediately with a high dose of one of the grass killers. Two to three days after a killing application, you should be able to pull the youngest growth out of the heart of the barley plant. There will be brown discoloration at the point that it breaks. If a number of growing points do not pull out easily and do not demonstrate the brown color, you have not killed the cover crop and you will have to spray it again. Use the highest recommended dose of any of the registered graminicides for onions.

If broadleaf weeds have germinated and are growing rapidly, the onions should be sprayed with GoalTender as soon as possible after the onion 2 leaf stage. Use 2-4 fl oz of GoalTender if weeds are larger than 0.5 inch tall and have true leaves. Once the onions have reached the full 2 leaf stage and the third leaf is coming, they can be sprayed with Chateau at 2 oz per acre. It will extend preemergence control for most weeds, and also burn off most weeds surviving the GoalTender applications. Do not tank mix Chateau with any other chemicals.

If yellow nutsedge is a problem, apply Outlook or Dual Magnum as soon as the onions reach the 2 leaf stage. Application earlier than that can result in stand reduction, especially in cool weather. These herbicides have no effect on emerged nutsdege, and need to be watered in to be effective.

If you have specific questions about your onion weed control program, please call me for advice. (517-353-6637).

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