Curing is a necessary post-harvest process for garden onions that involves carefully drying the outer layers of the bulb and the neck tissue. This procedure is distinct from simply allowing the onions to dry out, as its primary purpose is to seal the bulb’s neck, which is a common entry point for moisture and decay organisms. A properly cured onion develops a protective, papery outer skin, or wrapper scales, that acts as a natural barrier against microbial activity and prevents excessive moisture loss. This preparatory step ultimately extends the onion’s shelf life, allowing the bulb to enter a state of dormancy suitable for months of storage.
Knowing When to Harvest
The success of long-term onion storage is determined by the timing of the harvest, as curing begins while the bulb is still in the soil. Onions are ready when their foliage begins to yellow and 50 to 80 percent of the tops have naturally bent or flopped over. This bending signals that the connection between the leaves and the bulb is starting to dry and soften, which transfers the last nutrients into the swelling bulb.
The neck of the onion, where the leaves meet the bulb, should feel soft or crimped rather than stiff, confirming the transfer of carbohydrates is complete. Once the tops have fallen, the onion is no longer actively growing and is ready to be lifted to prevent a second growth cycle in the soil. When harvesting, carefully loosen the soil around the bulbs and gently pull them, ensuring the tops and roots remain fully attached to aid in the initial drying phase.
The Curing Environment and Duration
Curing requires a controlled environment to promote rapid drying of the outer tissues. Ideal conditions require a warm temperature, maintained between 75 and 90 degrees Fahrenheit, which encourages the outer skin to dry quickly into its protective, papery state. This heat should be paired with low to moderate relative humidity, ideally around 60 to 75 percent, to prevent the buildup of surface moisture that can foster mold and decay.
The most important environmental factor during curing is continuous airflow, which removes moisture as it evaporates from the bulbs. Suitable locations include a well-ventilated garage, a covered porch, or an open shed, providing shelter from direct sun and rain while allowing air to circulate freely. Onions must be arranged in a single, non-overlapping layer on wire racks, screens, or slatted surfaces to maximize air exposure to all sides of the bulb and neck.
The drying phase usually ranges from two to four weeks, depending on the prevailing temperature and humidity levels. The process is complete when the outer skins are completely dry and the neck has shrunk down to a thin, tight seal that no longer slides when squeezed. Successful curing is characterized by the formation of two to four dry outer scales, which minimize water loss and provide mechanical protection for the bulb.
Preparing Cured Onions for Storage
After the curing period is complete, the onions require physical preparation before moving to their final storage location. The bulb is ready when the outer skin is fully papery and rustling, and the neck has shriveled to a dry seal. This dried neck confirms the seal is secure and the risk of neck rot is reduced.
The first step is trimming the roots, which should be clipped back to about a half-inch length using clean shears. Next, the dried foliage must be trimmed down. The dry neck should be cut, leaving a stub of one to two inches attached to the bulb to maintain the integrity of the protective seal.
Any bulbs that show signs of bruising, soft spots, or a thick, undried neck should be separated from the storage batch. These bulbs will not store reliably and should be designated for immediate consumption to prevent spoilage from spreading. The prepared onions can then be placed into mesh bags, slatted crates, or braided together by their dried foliage for efficient storage.
Long-Term Storage Conditions
Once cured and trimmed, the onions require a shift in environment to maintain dormancy for several months. The ideal storage temperature is cool, maintained between 32 and 40 degrees Fahrenheit, which slows the metabolic processes that trigger sprouting. Keeping the temperature consistently low is important, as temperatures above 40 degrees Fahrenheit increase the risk of premature sprouting.
Unlike the curing phase, the relative humidity during storage should be moderate, maintained between 65 and 70 percent. This moderate humidity prevents the bulbs from shriveling due to excessive moisture loss while still deterring condensation, which can encourage fungal growth. Storage vessels like mesh bags, onion sacks, or crates are appropriate because they allow for continuous air circulation around the individual bulbs.
The storage location should be dark, such as a cold cellar, basement, or unheated pantry, as light exposure can accelerate greening and sprouting. Regular inspection is necessary to quickly identify and remove any bulbs that begin to soften, sprout, or show signs of decay. Stored properly, high-quality keeper varieties can retain their freshness for six to nine months.