Freshly harvested onions need to cure before storing.

Onions are pretty fool proof when it comes to knowing when they are ready to harvest because of the unmistakable sign they give.  An onion is done growing when the green tops lose their color and bend over. If you're looking to harvest all you onions at one time, wait until most of them have fallen over by themselves.  Once that happens, bend over any still-standing tops by hand.

Once all of the tops are fallen, give the onions another 10 -14 days to fully mature while still in the soil.  Leaving them in the ground any longer than two weeks can allow soil organisms to invade the onion and cause the onions to rot in storage.

The only exception to the fallen tops signal is when an onion bolts, which means it sends up a flower stalk. If this happens, harvest immediately because it won't grow any larger. In addition, the quality of the onion will be affected since the flower stalk comes from the center of the onion. You'll also want to use these onions quickly because they don't store well.

How to harvest onions

Harvest onions early in the morning on a sunny day if you can, preferably when the ground is dry versus the day after a soaking rain. If picked after rainy weather, the onions will have a lot more moisture in them and won't dry out or cure as well.

When you're ready to start picking, you'll find that any onions that are practically on the soil's surface can easily be pulled by hand. However, onions that are still underground may require a small shovel or garden fork to pry them from the ground.

Onions may also be harvested by partially exposing the onion by moving removing the soil around it. The onions will begin to dry out and cure while still in place for at least two weeks and up to a full month.  Professional growers in the North often cure onions in this way.

Curing onions

Once harvested, let the onions dry in a warm dry place for several days. This can even be just on top of the soil from which they were just pulled as long as the sun isn't too strong and hot and they won't be exposed to morning dews or rain.  After this first drying period, they should be brought under cover for the rest of the curing process.

Only cure the onions you plan to store. Separate the thick-necked, young, soft bulbs and use them first. For the onions that will be cured, you can cut off the tops with a sharp knife or clean pair of pruners leaving about one inch of the top in place. If they are cut any closer the neck won't dry out and the onion may rot while in storage. If you plan to braid the onions, leave the tops intact.

This is also a good time to check for cuts and bruises. If there's any damage, use those onions first. Any onions with signs of disease should be disposed of so the rest of the harvest isn't contaminated.

Cure the onions thoroughly in a warm, well-ventilated place that's out of the direct sun; and be sure the onions are not crowded into a space. They need to have air between them so they'll cure properly. You'll also want to turn the bulbs every couple of days so they cure evenly and there aren't any wet or damp spots when they are put into long-term storage.  

The curing process is complete when the skins "rattle" and the roots are dry and wiry.  Once they're done, place them in mesh bags and hang them to dry some more.  As a general rule, the longer the onions are cured, the better they'll keep. So if you have the space to hang the mesh bags of onions in a place where the air can continue to circulate around them, it will only help to ensure they store well. More information about growing onions can also be found at http://www.oniongardening.com