Late spring presents weed problems for onion growers
Onions are growing slowly, but weeds are growing well.
All dry bulb onions should be planted by now. Planting onions after May 10 results in smaller bulbs and smaller total yields. If fields intended for onions have not been planted yet, growers should consider other crops such as carrots, red beets, parsnips, other vegetable crops, corn or soybeans. Onions begin to form bulbs in response to hours of sunlight and size of plant. If onions are planted too late (after May 10), they do not develop sufficient plant to support the bulbs that are initiated in mid-June. The result is fewer and smaller onions. The market for boilers has not been good.
Most onions in Michigan have emerged, but for those that have not emerged, growers have the option of applying Buctril (bromoxynil) to kill emerged weeds. Buctril is active against most annual broadleaf weeds. It is weak against some of the composite weeds and common chickweed, and it has no activity on common purslane. The application rate is 0.13-0.25 pounds active ingredient per acre (8-16 fluid ounces of Buctril 2EC). If there are common chickweeds or larger mustards, use the full 0.25 pound per acre rate. As long as the onions are not up, Buctril is safe. Do not apply Buctril if any onions are appearing above the soil surface. Do not apply Goal or GoalTender before onions emerge. Goal (oxyfluorfen) remains active on the soil surface and may kill emerging onion plants. A typical symptom of herbicide injury on emerging onions is a brown ring around the seed leaf. If the seed leaf is killed, the plant usually dies.
Between onion emergence and the one true leaf stage, there is little growers can do to control emerged weeds. Interplanted barley may be killed with a post-emergence grass herbicide, which will also kill emerged annual grasses. When onions have one extended (3 inches) true leaf, GoalTender may be applied at rates of 1-4 fluid ounces (0.031-0.125 pound active ingredient) per acre. GoalTender has been very safe and effective at this stage of growth. Do not use Goal 2XL, which can cause crop injury this early in the season. GoalTender controls most annual broadleaf weeds post-emergence. It is weak against common chickweed, mustards, composites and ladysthumb.
Another herbicide registered for use post-emergence in onion early in the season is Nortron. Nortron has been safe on onions in our experiments when applied post-emergence. It has fair to good activity against common chickweed, yellow rocket, shepherdspurse, nightshade and ladysthumb. Nortron may be applied at one pint per acre (0.5 pound active ingredient) four times during the season.
Growers should reapply Prowl H2O about four to six weeks after the first application. Suppressing weed germination is the easiest way to maintain weed control. Dual Magnum, Outlook and Chateau may be applied at various stages of onion growth after the two leaf stage for pre-emergence weed control. See Michigan State University Extension bulletin E0433, “Weed Control Guide for Vegetable Crops,” for complete onion weed control recommendations.
Dr. Zandstra’s work is funded in part by MSU’s AgBioResearch.