Homegrown onions can be preserved long after harvest by managing specific environmental factors. Storing them is a distinct post-harvest process that requires specific preparation to maximize shelf life and prevent premature spoilage. This careful handling ensures that the bulbs remain firm, flavorful, and suitable for culinary use over many months.
Preparing Onions for Storage
The most important step following harvest is curing, which dries out the outer layers and the neck of the bulb. Curing seals the onion, creating a protective barrier against moisture loss and the entry of fungi or bacteria that cause rot. This protective layer forms when the outer skins become papery and the neck completely dries and tightens.
The ideal conditions for curing are a warm, dry, and well-ventilated location, such as a covered shed or garage, out of direct sunlight. Temperatures between 60°F and 80°F are effective. Onions should be laid out in a single layer on a clean surface or rack to allow maximum air circulation around each bulb. Curing typically lasts for two to three weeks, or until the tops and necks are completely dry and the outer scales rustle when touched.
Once thoroughly cured, onions must be trimmed before moving to storage. Clip the dry roots back to about a quarter-inch. Cut off the dried tops, leaving at least one inch of stem attached to the bulb to maintain the protective seal at the neck. Onions with thick, fleshy necks, bruises, or split outer skins should be set aside for immediate use, as they will not store well long-term.
Optimal Storage Environments
Long-term preservation depends on maintaining a cool, moderately dry, and dark environment. After curing, the storage temperature must be significantly reduced to slow the onion’s metabolism and prevent sprouting. The ideal range for most storage onions is between 32°F and 40°F, just above freezing. Temperatures higher than 40°F encourage the bulbs to break dormancy and sprout.
Controlling humidity is equally important; the storage area should maintain a relative humidity between 65% and 70%. If the air is too dry, onions will lose moisture and shrivel. Excessive humidity leads to condensation on the bulbs, inviting mold and fungal infections. Consistent airflow is also necessary. Proper ventilation prevents moisture buildup and ensures the air surrounding the onions remains fresh and cool.
The storage location should be completely dark to inhibit sprouting, which is accelerated by light exposure. Onions must be kept separate from other produce, especially potatoes and apples. Both release gases and moisture that cause the other to spoil faster. A cool, dark, and well-ventilated basement, cellar, or unheated garage that stays above freezing are suitable locations for long-term storage.
Long-Term Storage Techniques
After curing and trimming, the storage method must prioritize air circulation and protection from physical damage. Mesh bags, such as burlap or onion sacks, are effective because their loose weave allows air to flow freely around the bulbs, preventing moisture accumulation. Small quantities can be stored in baskets or crates with slatted sides for similar ventilation. Avoid using non-perforated plastic bags, as they trap moisture and accelerate decay.
Another traditional method is braiding the onions, particularly if the tops were left long during curing. Braiding the dried foliage into long strings allows the onions to be hung from rafters, maximizing ventilation and saving space. This technique keeps the bulbs separated, preventing them from resting against each other, which limits airflow and promotes spoilage.
Storage duration is determined by the specific variety of onion. Pungent, dry-skinned varieties, typically yellow storage onions with higher sulfur content, are bred for longevity and can last many months, sometimes up to a year under ideal conditions. Sweet onions, such as Vidalia or Walla Walla, contain more moisture and less preservative sulfur compounds. This gives them a significantly shorter storage life of only a few weeks to a couple of months, even when properly cured. Regularly inspect the stored crop for signs of softening or decay to remove compromised bulbs before they affect the others.