Cure onions under dry conditions before you store them. Source: http://www.gardeners.com
Cleanliness is next to godliness, we’re often told. But that doesn’t mean you should wash bulbs and tubers at harvest time, not if you want to store them at least.
Dry Bulbs Out, Don’t Moisten Them
You probably know that you should leave onions and garlic out in the sun for a week or two after you dig them (although do move them to a shed or garage to dry if the weather is rainy). This helps “cure” them (allows them to build up a thicker outer skin so they store better). Then brush off any excess soil with a soft brush and move them to their winter quarters (a cool, dry storage area).
Do not clean them by washing them in water in an effort to get them squeaky clean. The lingering presence of moisture can lead to rot later on, so you want your bulbs to be as dry as possible. It’s better to have a bit of dirt on a healthy bulb (after all, you’ll be peeling them before you serve, them, right?) than a spotless rotting one!
Oops, it’s too late and you’ve already rinsed them off? Well, put them back out in the sun to dry them thoroughly and keep your fingers crossed. Rot doesn’t always set into washed bulbs, but you have just made it that much more likely.
The same applies to potatoes, with the exception that I suggest always curing them indoors (on the floor of a garage or tool shed) in the dark rather than outdoors in the sun, as potato tubers exposed to sun begin to turn green and green potato parts are poisonous. But if you’re worried about dirt, you can carefully brush off any clinging soil rather than washing them. And if they aren’t perfectly clean, don’t worry about it.
Potato tubers too like to be thoroughly dry on the outside before storage, but keep best under more humid conditions that onions and garlic, in a root cellar, for example, where it’s both cool and humid.