Submitted to: Onion World
Publication Type: Trade Journal
Publication Acceptance Date: October 1, 2000
Publication Date: December 1, 2000
Citation: BOYDSTON, R.A. KEEPING WHITE ROT OUT OF THE COLUMBIA BASIN AMONG THE TOPICS: WEED MANAGEMENT. ONION WORLD. P 4-8. DEC 2000. Interpretive Summary: Weed management in most row crops includes a critical period from planting to about 6 to 8 weeks after planting where weeds are managed with herbicides and/or cultivation. Weed control the reminder of the season is accomplished primarily through crop competition with weeds. However, onions germinate slowly and have upright narrow leaves that provide much less late season shading to suppress weeds. As a result, weed management in onions requires additional cultivation, herbicide applications, or hand weeding later in the season compared to many crops. Herbicide use and cultivation at various onion growth stages are discussed. Recent research results on weed control and volunteer potato control in onions are discussed.
Technical Abstract: Weed management in onions requires extenstive cultivations, herbicide applications, or hand weeding throughout the season to prevent yield loss. Onions germinate slowly so weeds that emerge before the crop can be killed with flaming or nonselective herbicides. Bensulide and DCPA may be applied after planting but before onion emergence to control early season weeds. Pendimethalin applied preemergence to sprinkler-irrigated onions slightly reduced onion stand at rates above 1 lb ai/acre. However, a delayed pre- emergence application of pendimethalin did not significantly reduce onion stands. All preemergence and delayed preemergence pendimethalin treatments slightly reduced onion yield compared to hand weeded checks. Once onions are in the flag leaf stage, pendimethalin may be applied to control weeds that haven't emerged. When onions have two or more true leaves, trifluralin may be applied for weeds that have not emerged. Bromoxynil and oxyfluorfen are labeled for control of emerged broadleaf weeds when onions have at least two true leaves. Late emerging grass weeds can be controlled with cultivation or postemergence applications of sethoxydim, fluazifop, and clethodim. In research trials conducted in 1996 and 2000 volunteer potatoes were controlled well in onions with three applications of oxyfluorfen alone or tank mixed with bromoxynil at the 2-, 3-, and 4 to 5-leaf stage of onions folled by a cultivation 7-10 days after each application. This was the only treatment that controlled volunteer potatoes without injuring onions or reducing onion yield.