Smartweed and mustard 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.

Ladysthumb smartweed, marsh yellowcress and Virginia pepperweed are germinating in large numbers in onion fields. We see these weeds every year, but they seem to be doing exceptionally well this spring. Unfortunately, none of the preemergence herbicides registered for onions give good or better control. Onions that were planted early are in the 2 leaf stage (2 LS) or larger, and can be treated with postemergence herbicides. Late planted onions that are still in flag or one leaf stages present a greater problem.

In general, onions in the full 2 LS (i.e., the leaves are at least four inches tall), are quite tolerant of oxyfluorfen (GoalTender) at rates including 4 fl oz/ acre (0.125 lb active ingredient (ai). The recent sunny, hot weather has toughened the onion leaves so they can withstand a high rate of oxyflurofen. Growers should use GoalTender under normal conditions because of its better safety on onion. However, the old formulation (Goal 2 XL) is an EC formulation, and the carrier increases the activity against the weeds. Goal 2 XL gives slightly better kill of larger and more tolerant weeds, such as we currently are facing. Goal 2 XL also will cause more curling and twisting of onion leaves. Onions will survive this injury, but may not survive the weed pressure if the weeds are not controlled.

I usually do not recommend use of Buctril in the spray mix at this stage, although it is labeled for use on onions with two to five leaves. I have seen too much onion injury and yield reduction from use of Buctril. However, Buctril is more effective against these weeds than Goal, and if growers believe that they have no other alternative, they should use it alone and in a minimum of 50 gallons of water per acre. The labeled rate is 0.125-0.25 lb ai/acre (0.25-0.5 pt/acre of the 4EC formulation). Be sure to check the formulation, because there are generics available with the old 2EC formulation. You will need to double the amount per acre with the 2EC formulation.

Chateau gives fair to good preemergence and postemergence control of smartweeds and mustards. It is labeled for use on onions at the 3-6 LS. At 2 fl oz per acre (0.064 lb ai), it will kill most of these weeds up to two inches tall. When there is a huge infestation as we have this year, there will be some escapes. It might require hand weeding to remove any weeds that escape. Otherwise, they will continue to grow throughout the season.

If onions are not up to the 2 LS stage, be very careful with any herbicide applications to avoid stand loss and permanent yield reduction.

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