Jul 23, 2015
Onion weed control in a cool, wet year

Onion weed control always requires attention and effort, but in wet years it becomes very difficult to maintain clean fields. Weeds grow more rapidly than onions, herbicides lose activity sooner and wet fields make subsequent herbicide applications difficult. Michigan State University Extension recommends several actions growers can take to stay ahead of weeds in wet years.

Onion growers should have a weed control plan on paper with approximate dates of application and chemicals to be used. By checking the calendar frequently, they can remind themselves of the next anticipated action. If the season is earlier or later than normal, treatments can be moved ahead or back.

The most frequent weed control problem in wet years is failure to maintain pre-emergence herbicide activity in onion fields. Herbicides wash and leach out of soil with excessive moisture. In an average year, Prowl H2O provides four to six weeks of pre-emergence activity. In a wet year, it may be gone in three to four weeks. A subsequent application needs to go on soon after heavy rain. We are allowed to use three applications of 2 quarts (1.9 pounds active ingredient) of Prowl H2O (6 pounds active ingredient total per year) on onions on organic soil. Under normal conditions, the last application should go on in early to mid-July for the 45-day pre-harvest interval (PHI), assuming harvest around Sept. 1.

Dual Magnum and Outlook may be applied with Prowl H2O or alone, after the two-leaf stage. Both have good yellow nutsedge activity. Dual Magnum may be applied twice and has a 60-day PHI. One application of Outlook is allowed with a 30-day PHI, so Outlook normally would be the last pre-emergence application of the season. All of these herbicides tend to be weak on common ragweed and mustards.

Chateau is registered for up to three applications and 3 ounces of product per acre per year. It may be applied with Prowl H2O, but not with Dual Magnum or Outlook, because their solvents make the Chateau more toxic to onion. Adding Chateau at 1-2 ounces per acre to the two applications of Prowl H2O to emerged onions will improve pre-emergence control of mustards (shepherdspurse, marsh yellowcress, Virginia pepperweed), smartweeds (ladysthumb, Pennsylvania smartweed, prostrate knotweed) and composites (common ragweed, common groundsel, pineapple weed). Chateau also has some pre-emergence and post-emergence activity on spotted spurge.

Once weeds have emerged, they need to be controlled with post-emergence herbicides. GoalTender at 2-6 fluid ounces per acre (0.063-0.188 pounds active ingredient) may be applied to Michigan onions at the onion one-leaf stage (1 LS) and later until 45 days before harvest. With an annual total of 16 fluid ounces of GoalTender per acre, it can be applied three to four times at 4-5 fluid ounces per acre, which will kill most small broadleaves. It will not kill ladysthumb that is 3 inches or taller and common lambsquarters 3-4 inches or larger. It does not control spotted spurge. High rates of GoalTender (4-6 fluid ounces) can injure soft onion leaves, which may occur in wet, cool years. But onions have an amazing ability to outgrow GoalTender injury.

The situation with Buctril is different. Buctril will kill most broadleaves that GoalTender misses. It has no effect on common purslane, which is killed easily with GoalTender. Unfortunately, onions usually do not outgrow Buctril injury and yields are often reduced. It is only safe to use post-emergence on established onions during hot, dry years. If weeds are so thick that the crop is in danger of being lost, Buctril at 10-12 fluid ounces per acre will burn most broadleaves back and allow most of the onions to grow. Apply Buctril on warm, sunny days after three to four sunny days. Be prepared for reduced yields.

Starane can also help clean up weedy onion fields, but it often flattens the onions to the ground. At low rates, it will kill common chickweed and ladysthumb that have merged and survived GoalTender application. It should only be used as a rescue treatment.

Grasses can be controlled with Fusilade, Poast or Select Max. The most critical grass problem in onion is cover-crop barley, which is allowed to grow too tall. This often happens when heavy rains prevent timely herbicide applications. Barley should be planted at 0.75-1 bushel per acre and killed when it is 4-5 inches tall. It can grow past that stage rapidly during cool, wet conditions, and adversely affect the small onions, often causing stand and yield loss. It may be necessary to apply post-grass herbicides by airplane to kill barley before it causes crop injury, if the soil is too wet to support a ground sprayer.

Complete weed control is very important to obtain maximum yields of onions. We have several herbicides registered for onions, which when used properly will help maintain an acceptable level of weed control, even in wet years.

— By Bernard Zandstra, Michigan State University Extension, Department of Horticulture

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