Early season complicates onion weed control

Many weeds are emerging in onion fields and there are few choices for control.

The unusual weather this spring has resulted in serious weed problems in onion fields. Many acres of onions were planted earlier than normal because of the warm, dry conditions in March. Subsequently, the weather has turned cooler and, in many places, wetter, which has slowed onion development and growth. Herbicides applied early have not been very effective, and many weeds have emerged in onion fields. The onions range from just emerging to two true leaves. Barley has grown well and needs to be killed soon before it reduces the onion stand.

Ladysthumb smartweed has been the most serious problem thus far. It is moderately suppressed by Prowl under good conditions, but control has not been good this spring. After ladysthumb has two true leaves, it is very difficult to control with GoalTender. Chateau is more active against ladysthumb, but Chateau is labeled for use only after the 3 leaf stage. Common lambsquarters also is a problem in many fields. Chateau is quite active against lambsquarters.

Onion growers should be killing barley soon before it is 6 inches tall to avoid crop thinning. As soon as onions are showing two true leaves, apply GoalTender at 2 fluid ounces to kill broadleaves. As soon as the onions show three true leaves, apply 1 to 2 ounces of Chateau. The Chateau may be applied in a tank-mix with Prowl H2O. It should not be tank-mixed with Dual Magnum or Outlook.

The second application of Prowl H2O should be applied four to six weeks after the first application. For many fields, that is now. Prowl H2O has been safe on onions at all stages. For broadleaf suppression through the season, the preemergence herbicides should be applied every four to five weeks. GoalTender is normally safe in tank-mixes with Prowl H2O, Dual Magnum and Outlook. A grass herbicide (Fusilade, Poast, Select) may also be included. Chateau should only be tank-mixed with Prowl H2O.

Buctril is usually too active for routine use in onions. However, under very weedy conditions, it may be needed to kill common lambsquarters and ladysthumb. It is safe to apply before onions emerge, but it should not be applied after emergence and before the two leaf stage. Buctril should be applied postemergence after two sunny days, and in at least 50 gallons water per acre. Buctril may thin onion stands and should be used only under serious weed pressure.

Complete weed control is essential to achieve good onion yields. Growers need to be aware of their options and proceed appropriately to stay ahead of the weeds this year.

Dr. Zandstra’s work is funded in part by MSU‘s AgBioResearch.

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