At least two possibilities 1. The onions were not fully matured.The
tops need to fall over and dry. 2. The variety that you were growing
is not a storage onion. Many of the sweet table onions have little
storage life.
Compostman wrote:
I planted onions for the first time this Spring. They grew very fast and I
harvested them when the leaves began to turn brown and the onions were
baseball, or larger, size. I let them dry some in the attic for a few days
then put them in a dry basement. Even while in the attic, some of them
began to rot in the center. Any idea why?
Note: Although this is my first onion crop, I've been very successful with
garlic and shallots.

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John Henry Wheeler
Washington, DC
USDA Zone 7