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Temperature fluctuation isTemperature fluctuation is the most common cause for onions to flower. If an onion plant is exposed to temperature changes from cold to warm to cold again it will go dormant before resuming growth again and this will cause the onion to flower prematurely. The entire row of onions will experience the temperature changes and  probably produce flowers. Flowering can be prevented by planting the onion sets or seeds at the right time, not too early in the season.

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