Onions should be stored in a relatively cool, dry place. Sweet onions will not store as long as more pungent varieties. The maximum length of storage of sweet onions will run from two to three months. Allow your onions to fully mature in the garden before harvesting. Maturity is indicated by the fall of the top of the onion plant. After the tops have fallen, pull and dry the onions in the garden for several days. Some gardeners prefer to pull the onions up partially which allows the onions to dry while still in the ground. After drying, remove the roots and the top, leaving about ¾ to 1 inch of the neck to seal and prevent entrance of decay organisms.
For more information, read: Onions.
