Late planting and wet conditions exacerbate early season onion weed problems

Early season mustard weed control is important to obtain maximum onion yield.

Many acres of Michigan onions were seeded after May 1, 2011. May 1 is the normal “plant-by” date for northern pungent onions in Michigan. Onions can be planted up to May 15, but usually bulbs are smaller and yields are reduced at harvest. Onions initiate bulbing in mid-June in response to long day length, and the size of the plant at that time is an important factor in final bulb size and total yields.

Growers normally wait to apply their first preemergence herbicide (usually Prowl H2O) after seeding until just before onions emerge. This allows the inter-seeded barley to become well-established and adds two to three weeks of preemergence weed control into the season. Buctril (bromoxynil) usually is applied with the preemergence herbicide to kill emerged broadleaf weeds. If the soil becomes too wet to apply herbicides, the onions may begin to emerge before the Prowl and Buctril have been applied. These conditions are occurring currently.

Growers should monitor their onion fields closely between seeding and emergence. If some onion cotyledons poke through the soil surface and preemergence herbicides have not been applied, growers need to make a decision about whether to include Buctril. By two weeks after final tillage, many mustard weeds will have germinated. These include yellow rocket, shepherdspurse, marsh yellowcress, and Virginia pepperweed, among others.

Ladysthumb (smartweed) is another early germinating broadleaf weed that needs to be controlled early on. These weeds can be controlled at the cotyledon or one to two true leaf stage with Buctril or Goal, but they are not easily controlled with Goal (oxyfluorfen) when they are larger. Buctril kills them at this early stage, but Buctril also may kill emerged onions. If very few onions are up, it probably is wise to spray Buctril and sacrifice a few onion plants. Buctril causes more onion injury under cool, wet conditions, so use the lowest labeled rate during those conditions. If too many onions have emerged, apply Prowl H2O alone. Goal should not be applied before onion emergence because of potential for severe stand reduction.

If the mustards and ladysthumb are not killed by Buctril before onion emergence, the next effective application to kill them will be Chateau at the onion three-leaf stage. By then, all of these weeds may be 4 to 5 inches tall and very difficult to kill with Goal. If weeds are large and well-established, use 2 oz. of Chateau. Chateau will kill many of the emerged weeds and also provide four to five weeks of preemergence suppression.

If you apply Prowl and Buctril before onion emergence, but these weeds germinate later, the next opportunity for control is the first Goal application at the onion two-leaf stage. By applying the highest labeled rate (4 fl oz., 0.125 lb ai) of GoalTender, growers may be able to suppress or kill these weeds sufficiently to maintain control until the first Chateau application. Using Goal XL instead of GoalTender also will improve control of these weeds, but may cause slightly more onion foliar burn. However, if these weeds are not controlled early in the season, they will depress onion yields considerably.

Complete weed control throughout the season is essential for effective insect and disease control, as well as optimum use of nutrients and water by onions. Weed-free fields allow onions to dry down normally and to attain maximum size at harvest. Complete weed control recommendations are contained in MSU Extension Bulletin E-433, Weed control guide for vegetable crops.

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