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via Food Gardening Guide :: National Gardening Association.
When to Harvest
You can always tell when onions have stopped growing. The leaves will lose their color, weaken at the top of the bulb and flop over. Each year a few new gardeners watch the leaves die and wonder, “What’s wrong?” There’s nothing wrong; it’s Nature’s plan. The leaves’ job is done – they’ve put the last of their energy into the bulbs.
Let most of your onion tops fall over by themselves – maybe 80% or 90% of them – then bend over the rest of the tops. Once they’re down, leave the bulbs in the ground for another 10 days to two weeks to mature fully. It’s not good to leave the onions in the ground for longer than two weeks after the tops die because they become open to organisms that can cause rot in storage, or they might even start growing again.
Country Traditions 