If you planted the last onion to sprout and saved seed from it could one develope a longer storing onion??

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What you describe is selective breeding. I imagine that is how most of the long storage onions have come to be.

One of my family's habits was to store onions upside down to lessen the likelihood of sprouting. Anytime, we took onions from storage, we had to make sure the remaining were upended.

Recently, I read that there is scientific evidence explaining why this works for storage onions. Do not recall details. cella jane