Like other root vegetables, onions store for a long time. Unfortunately, if they aren't stored correctly they sprout and begin growing roots, a stalk or both. Onion bulbs are meant to grow into a new plant, so sprouting is a natural occurrence. The bulbs lay dormant until conditions are right for sprouting, and once they are, new growth begins. Once sprouted, the bulbs rot more quickly. Provide storage conditions that keep the bulb dormant and prevent most sprouting issues while they are stored.
Spread out recently-harvested onions in a single layer in a dry, warm location. Leave the onions to dry for three weeks or until the neck of the bulb is completely dried.
Fill a mesh bag or wire basket with the dried onions. Avoid plastic containers or any storage device that doesn't allow air to circulate around the onions. Improper air circulation leads to moisture build-up, which causes onions to sprout or rot.
Place the onions in a dry location, at 0 to 4.44 degrees Celsius. Warm temperatures can encourage sprouting or rot. Avoid areas where temperatures drop below freezing.
Store other ripening fruits and vegetables in a location away from the onions. Ripening fruits produce ethylene gas, which helps encourage onions to sprout.
Most onion varieties store well for up to three months when properly dried and stored. Sweet onion varieties, such as Walla Walla, only store well for three to five weeks.