MSU Extension Vegetables

Onion weed control always requires attention and effort, but in cool and dry years, it can be even tougher to keep fields clean. Wide temperature fluctuations and dry weather in the last couple of weeks has resulted in uneven onion stands and unanticipated weed problems. Emergence in some fields is erratic, with some onion plants in the one leaf stage, cotyledon, flag, loop or just emerging.

Cold temperatures increase risk of crop injury because plants can’t metabolize herbicides as fast. Rapid degradation of herbicides under warm conditions allows crop plants to escape herbicide injury. At the same time, weed growth slows down and cell walls harden which can reduce herbicide uptake and translocation, and consequently reduced weed control. Therefore, waiting a few days for warmer weather and safer application conditions will not change the weed control situation much, and may improve success. Overall, it is best to avoid applications of postemergence herbicides during periods of very cool temperatures (less than 40 degrees Fahrenheit at night; less than 55 F during the day).

So where might growers be at? Quick emergence of early-planted onions may have made it difficult to apply Prowl H2O plus bromoxynil just before emergence. In other fields, there may be onion plants at several young stages, plus many weeds. If nothing has been applied yet, and only a few onions have emerged, it may be prudent to apply the Prowl H2O plus bromoxynil now, rather than waiting until the onion one leaf stage for the first postemergence application to kill small weeds. Regardless of whether bromoxynil is used, Prowl H2O should still be applied now if it hasn’t been already. This will prevent more weed germination, even if it slows onion growth slightly. Onions will grow rapidly when soil temperatures increase.

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. Growers may have to sacrifice a few onion plants to follow a herbicide plan of applying GoalTender at the onion one leaf stage, followed by another pre application and GoalTender at the two-leaf stage. Onions can lose 10% of the intended stand without decreasing yield by producing bigger bulbs. Without good weed control, yield reduction can be much greater than 10%, so it normally is most effective to kill the weeds early and completely. If common lambsquarters and ladysthumb are not killed in the cotyledon to one leaf stage, they may survive all season. High rates of GoalTender (5-6 fluid ounces) can injure soft onion leaves, especially in wet, cool years.

Cold weather generally is not a concern for pre-emergence herbicides and can actually help to increase herbicide longevity by reducing herbicide breakdown by soil microorganisms. In an average year, Prowl H2O provides four to six weeks of pre-emergence activity. Under cool weather, it may remain active in the soil slightly longer. The label allows 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 given 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 a total of 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.

Grasses can be controlled with Fusilade, Poast or Select Max. The most critical early grass problem in onion is cover-crop barley, which may 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.

Complete weed control is very important to obtain maximum yields of onions. There are several herbicides registered for onions, which when used properly will help maintain an acceptable level of weed control, even in cool and dry years.

Complete onion weed control recommendations can be found in the Michigan State University Extension bulletin E-433 2021 Weed Control Guide for Vegetable Crops, available at the MSU Bookstore or county MSU Extension offices.