If you intend to keep your harvest for more than a few weeks, you must cure your onions. Curing is a post-harvest drying process that transitions the living bulb into a dormant state, preparing it for long-term storage. This managed dehydration allows the outer layers to form a protective shell. Understanding this necessity ensures the crop remains usable for many months.
The Core Purpose of Curing
Curing serves a dual biological function focused on preventing spoilage during storage. The first action is the drying and sealing of the neck, the most vulnerable point for pathogens to enter. The neck, where the leaves attach to the bulb, is thick and moist at harvest and must shrivel and close off completely to block moisture and microorganisms.
The second function involves drying the outermost layers of the bulb, transforming the fresh skin into a papery, protective tunic. This dry outer skin acts as a physical barrier against disease and reduces the rate of water loss from the bulb’s interior. By drying the outer scales and sealing the neck, curing induces a state of dormancy.
Without this hardened shell and sealed entry point, the moist bulb would quickly become soft and susceptible to rot and fungal infections, making it unusable in a matter of weeks.
The Step-by-Step Curing Process
The curing process begins when the onion tops naturally start to fall over, signaling that the bulbs are mature. Once about half of the tops have collapsed, the bulbs can be gently lifted from the soil, taking care not to bruise the outer layers. Bruising compromises the onion’s ability to cure properly and leads to premature decay.
The harvested onions, still attached to their tops, must be moved to a warm, dry location with excellent air circulation. Ideal conditions involve temperatures between 75°F and 85°F, but the area must be out of direct sunlight to prevent sunscald. The onions should be laid out in a single layer on a screen or rack, or hung in small bunches to maximize airflow.
Curing typically takes between two and four weeks, depending on the climate and humidity levels. A properly cured onion will exhibit a tight, shriveled, and completely dry neck that no longer feels thick or slippery to the touch. The outer skin should be uniformly dry, forming a rustling, papery shell around the firm bulb.
Distinguishing Storage Onions from Fresh Onions
Not every onion variety needs to be cured for long-term storage, as their inherent characteristics determine their post-harvest requirements. Storage onions, which include many pungent yellow and white varieties, have a naturally higher sulfur content. This sulfur contributes to their strong, tear-inducing flavor and acts as a natural inhibitor against rot and sprouting, making them suitable for curing.
In contrast, sweet or fresh onions, such as Vidalia or Walla Walla types, contain a much higher water content and lower sulfur levels. These varieties are prized for their mild flavor but have thinner, less protective skins. Because they lack the biological defenses for extended keeping, sweet onions are not meant for long-term storage and should be consumed within a few weeks of harvest.
Post-Curing Storage and Longevity
Once the curing is complete and the necks are thoroughly dry, the onions must be prepared for their final storage environment. The dry tops and roots should be trimmed using sharp scissors, leaving about an inch of stem above the bulb and clipping the roots close to the base. Any onions that appear soft, damaged, or have begun to sprout should be separated and used immediately, as they will not store well.
The final storage location should be cool, dark, and have low humidity, ideally between 35°F and 40°F. Cured onions should never be stored in plastic bags, as this traps moisture and encourages rot. Instead, they should be placed in mesh bags, braided together, or kept in open crates to ensure continuous air circulation.
This allows storage varieties to maintain their quality for six to twelve months. This longevity increase is significant compared to uncured bulbs, which spoil in a few weeks.