Onions can be harvested at any time for fresh eating or cooking. Onions that will be stored need to reach full maturity before harvest. For storage onions, wait until at least half of the tops have fallen over, then pull the remaining plants. Allow them to dry in a warm, dry, and well-ventilated area for 1-2 weeks until the outer skins become papery. Afterward, trim the roots and (optionally) cut the stems to 1 inch, remove any damaged or diseased layers, and store them in a cool, dry, and dark place with good air circulation, such as a mesh bag or crate. Check the storage recommendations for your variety and use or freeze before the onions have spoiled. Some varieties may last 6 months or more in storage while others, particularly the very large varieties, may store for only one month.