There's never a time when onions aren't ready for harvesting. They can be picked and eaten at any stage. No matter how many onions you use during the season, though, it's nice to have a crop of big onions mature at the end of summer to store for the fall and winter months.
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.
Pull your onions up on a sunny day if you can, then let them sit in the sun for another day or so to dry (in hot climates this usually takes just a few hours). This drying kills the root system at the bottom of each bulb. The roots will be like little brittle wires when they're dry.
Picking the right day to pull the onions can determine how well the onions will keep. If you harvest them after some rainy weather they'll have a lot more moisture in them and won't dry out as well.
After drying the onions in the open for a day or so, it's time to bring them under cover for a second, longer drying or "curing" process.
Some people cut the tops off the onions before curing, but that's not strictly necessary. However, if you do trim the top leaves, don't cut them any closer than one inch from the bulb. Otherwise the neck won't dry out, and the onion could rot in storage.
To cure the onions, spread them out in any warm, airy place out of the sun, such as on a porch. If you find you have too many onions for your available porch space, try spreading them out near the edge of your driveway, covering them with a light cotton (not plastic) sheet to provide shade. The sheet, held in place by stones along the edge, keeps the sun from burning the bulbs but still allows a lot of air circulation. Turn the bulbs a couple of times to promote even drying.
Heavy coverings like canvas or plastic trap moisture inside, so the onions will never get really dry. With the sheet system you won't have to worry about a few scattered rains. The sheets and the onions dry out rapidly together after a shower.
You don't want any wet spots on the onions when you put them in storage, so cure them really well. This can take two to three weeks. After curing them, hang the onions in mesh bags in your garage and dry them some more before putting them in your root cellar. It doesn't take this long in the South, but wherever you live, the longer you cure your onions, the better they'll keep.
Here are the basics of curing: