ONION HEALTH MANAGEMENT & PRODUCTION Table 6b. Common Weeds in Southern and Southeastern U.S. Onion Production Regions. Common Name J F M A M J J A S O N D Bermudagrass Bluegrass Bristly starbur Broadleaf signalgrass Carolina geranium Carpetweed Common chickweed Common cocklebur Common fumitory Common purslane Common ragweed Corn spurry Crabgrass Cudweed Curly dock Cutleaf evening primrose Dandelion Florida beggarweed Florida pusely Henbit Mayweed chamomile (dog fennel) Weed Effects Because onions develop relatively little leaf area or ground cover during the growing season, weeds cause greater yield and quality losses in onions than in most other crops. Onion yields are reduced from very early season weed competition as well as late emerging weeds. Onions compete poorly with weeds because they grow slowly early in the season compared to cool-season weeds such as kochia and lambsquarters. Weeds reduce onion net economic return by reducing both total yield per acre and the size of individual onions. Smaller sized onions reduce crop value. Heavy weed pressure in an onion field can reduce the amount of insecticide and fungicide that reaches the onion canopy, thereby reducing the performance of these pesticides. Weeds also can serve as alternate hosts for insect and disease pests of onions. Additionally, heavy weed pressure may hinder harvest operations by clogging machines or otherwise disrupting undercutting of the crop. Heavy weed pressure can also prevent onion tops from falling which hinders proper field curing of the bulbs prior to harvest and long term storage. Avoid planting onions in fields with a history of heavy weed pressure. A high weed seed bank level combined with poor onion competition against weeds dictates that an onion producer will need to develop a very intensive weed management program. Crop rotations that provide good weed control one or two seasons before planting onions will set the stage for more effective weed management in the onion crop.
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