Onions should be stored in a dry, cool area at a temperature between 45 to 55 degrees Fahrenheit. The storage area should be well-ventilated and away from direct sunlight exposure and heat sources, advises the National Onion Association. Additionally, onions should be stored separately from potatoes and produce that release moisture. Storing onions in plastic bags prevents air circulation and reduces the shelf life of the onions.
Cooler temperatures slow decaying, and exposure to high levels of humidity causes onions to rot and develop mold. An unheated garage or basement is a good place to store onions, providing the temperature doesn't drop to freezing, advises Gardening Know How. Boxed onions should be positioned at least a foot away from walls to allow for adequate air circulation.
The length of time onions keep depends on storage conditions and the specific variety of onion. Sweet onions contain more water than other varieties, so they have a shorter shelf life. Peeled whole onions should be stored in a refrigerator set to 40 degrees Fahrenheit or less. Chopped or cut onions should be stored in a sealed container and refrigerated; they typically keep for seven to 10 days in a refrigerator.
Onions are fat-free, low in calories, and a good source of fiber, vitamin C and other nutrients.