By Bernard Zandstra, Michigan State University Extension, Department of Horticulture
Time to begin post-emergence weed control applications in onions in Michigan.
All onions in Michigan have been planted and most of them are in the flag to two leaf stage. The cool and damp weather has allowed proliferation of many weeds, including yellow rocket, shepherdspurse, ladysthumb (smartweed), horseweed (marestail) and common chickweed, among others. All of these early emerging weeds can cause serious crop competition and onion yield reduction if they are not controlled. They are easiest to control in the cotyledon to two leaf stage.
As of 2014, Michigan has a 24c state label for use of GoalTender at the onion one true leaf stage. The first leaf to emerge is the cotyledon (seed leaf), which emerges as a loop, then becomes a flag leaf, then straightens out to be the seed leaf. The seed leaf is always recognizable by the small bump on one side, which was the point of the loop as it emerged. The first true leaf is the next leaf to emerge. When the first true leaf is 3 inches tall and the second true leaf is visible, the onions may be sprayed with GoalTender at up to 6 fluid ounces per acre (0.188 pounds active ingredient, or a.i.).
To be safe, use a maximum of 4 fluid ounces (0.125 pounds a.i.) at the first application. Onions have been very tolerant to GoalTender at this stage, and the herbicide will kill most emerged broadleaves at this rate. When the onions have two extended true leaves 3-5 inches tall, they can be treated with GoalTender again. Apply when temperatures are above 50 degrees Fahrenheit. The total amount of GoalTender allowed per year is 16 fluid ounces (1 pint) of product, which is 0.5 pounds a.i. with a 45-day pre-harvest interval.
GoalTender may be applied with a post-emergence grass herbicide (Fusilade, Poast or Select Max) to kill inter-planted barley. If tank-mixing the products, do not include an adjuvant to avoid crop injury. It normally is a better practice to apply the grass herbicide plus an adjuvant first, and then apply the GoalTender a couple of days later. Applying during sunny, warm weather increases onion crop tolerance to GoalTender
If these early weeds are not controlled soon, some may survive the early applications and will need to be treated again later or removed by hand. Normally, weed pressure causes more yield reduction than slight herbicide burn. Subsequent pre-emergence herbicide treatments may include Chateau at the onion three leaf stage, which also adds some post-emergence activity against emerged early-season weeds.