Onions are biennial plants. They don’t set seed until their second year of growth. During the first year, the leaves gather energy through photosynthesis and use the energy to develop the bulbs underground.

As autumn rolls around, the leaves wither and the onions go dormant. We usually harvest onions at the end of their first year, when their leaves start to die back. But if left in the ground, the bulbs survive on the stored energy throughout winter, and new leaves and roots re-emerge in the spring.

Having gone through the complete growing cycle, onions then send up flower stalks (a process called bolting) and produce seed in this second year.

How the weather can trick an onion into flowering early

If the weather is warm during the early stages, onion seedlings will grow vigorously until cold weather slows their development.

If a cold snap suddenly occurs, the onions will have grown to sufficient size to sustain flowering, and thus be forced into early dormancy.

As the weather warms again and then moves into winter, they may be confused into thinking they’ve already gone through two growing seasons. Just like they’re programmed to do in their second year of life, they prepare to set seed the following spring for the next generation of onions.

Young seedlings that are less than the diameter of a pencil by the time temperatures drop aren’t affected by their first winter. Their size isn’t significant enough to realize the cold is upon them and they have to initiate flower stalk production.

Timing is everything when it comes to planting onions

Plant too soon, and your whole onion crop could flower the following spring as the bulbs will have grown enough to think they’ve completed their cycle after winter. Plant too late, and you could end up with smaller bulbs, if the delicate seedlings are strong enough to withstand winter at all.

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