Onions should be harvested when most of the tops have fallen over and begun to dry. Carefully pull or dig the bulbs with the tops attached.
After harvesting, dry or cure the onions in a warm, dry, well-ventilated location, such as a shed or garage. Spread out the onions in a single layer on a clean, dry surface. Cure the onions for 2 to 3 weeks until the onion tops and necks are thoroughly dry and the outer bulb scales begin to rustle. After the onions are properly cured, cut off the tops about 1 inch above the bulbs. As the onions are topped, discard any that show signs of decay. Use the thick-necked bulbs as soon as possible as they don't store well. An alternate preparation method is to leave the onion tops untrimmed and braid the dry foliage together.
Place the cured onions in a mesh bag, old nylon stocking, wire basket, or crate. It's important that the storage container allow air to circulate through the onions. Store the onions in a cool, moderately dry location. Storage temperatures should be 32 to 40 degrees Fahrenheit. The relative humidity should be 65 to 70 percent. Possible storage locations include a basement, cellar, or garage. Hang the braided onions from a rafter or ceiling. Since the temperature in an unheated garage will likely fall below 32 degrees Fahrenheit in winter, an alternate storage site will be needed when cold winter temperatures arrive.
The storage life of onions is determined by the cultivar and storage conditions. When properly stored, good keepers, such as 'Copra,' 'Red Zeppelin,' and 'Stuttgarter,' can be successfully stored for several months. Poor keepers, such as 'Walla Walla' and 'Sweet Spanish,' can only be stored for a few weeks. If the storage temperatures are too warm, the onions may sprout. Rotting may be a problem in damp locations. Inspect the stored onions on a regular basis in fall and winter. Discard any that are starting to rot.