Onions are ready to harvest when the tops are falling over on more than half of them. That’s the sign that the bulbs are done growing and it’s best to get them out of the ground while they are in optimum condition.
Pull the onions if the soil is loose or use a trowel to gently lift them if they don’t come out easily. Always handle onions gently because bruising will cause them to rot in storage.
The next step is to cure the onions, which means to let the tops and roots dry and the skins get papery. In cool climates, when there is no rain forecast, onions can be left lying on the soil for a few days to cure. In extremely hot weather, though, it’s best to remove them from the garden and cure them in the shade.
The ideal place to cure onions is on a table made of wire mesh or a plastic crate with holes, like a bulb or bread crate. The idea is to ensure air circulation around the onions and the tops so they don’t rot. Some people just put them on a porch floor. Whatever you use, spread them in a single layer in a place with good air circulation.
After about a week, the tops should be completely dry. Cut them off, leaving 1-2 inches of stem above the bulb. Trim the roots, too. Rub off any soil or loose skins.
For long-term storage, onions need a place that is cool, dark, and dry with good air circulation. An old-fashioned root cellar is ideal, but few people have those anymore, so a basement room with a dehumidifier will do almost as well. Again, air movement around the stored onions prevents rot, so put them in a basket, crate, or onion bags and don’t crowd them. Or braid and hang them, bringing only enough for a couple of weeks into the brightness of the kitchen.
Check the stored onions often and if you find any with soft spots, use them immediately. With proper curing, trimming, and a little attention, storage onions should last for many months.