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If your onions have flowered,If your onions have flowered, they have prematurely bolted; the usual reason is usually some type of temperature variation shifting between cold and warm. There's not much you can do. It may still be edible, albeit smaller; use right away. One tip: use only transplants that are pencil-sized or smaller in diameter. Generally, a large onion plant will produce a seed stalk after planting instead of forming a large bulb.