Long-term storage of bulb onions depends entirely on harvesting them at the precise moment of maturity. Proper timing ensures the bulb has fully developed its protective outer layers and sealed off moisture transfer from the foliage. Harvesting too early or too late compromises the onion’s ability to cure and significantly reduces its shelf life. Visual cues are more reliable than counting days, as growth rates vary widely based on weather and soil conditions.
Visual Signs of Maturity
The most reliable sign that an onion is ready is the softening and collapsing of the neck, the area just above the bulb where the foliage begins. This signals that the plant has completed the transfer of carbohydrates from the leaves down into the bulb. The once rigid, green tops will begin to yellow and then bend over, sealing the bulb off from the stem.
Wait until approximately 50% to 75% of the onion tops in a patch have fallen over before harvesting. Although the plant finishes pulling sugars when the tops are down, skin development continues in the ground. Harvesting after the majority of tops have collapsed ensures the bulb has maximized its size and is preparing for dormancy. Size alone is not a dependable indicator; a large bulb with an upright neck has not completed its growth cycle and will not store well.
Adjusting Care Before Harvest
A few weeks before the anticipated harvest, when the tops begin to show initial yellowing, adjust the plant’s care regimen. Withholding water and fertilizer 7 to 14 days before the tops start to fall signals the plant to prepare for dormancy. The goal is to dry out the surrounding soil, which encourages the outer skin (tunic) to toughen up.
Reducing moisture promotes the natural drying and sealing process necessary for long-term storage. If the soil remains overly saturated, the onion is susceptible to storage diseases like sour skin and black mold. This pre-harvest drying period leads to better skin development and a reduced risk of rot.
Curing for Long-Term Storage
Curing is the post-harvest process that transforms a freshly pulled onion into a storage-ready vegetable. After gently pulling the bulbs, move them to a warm, dry, and well-ventilated location, ideally out of direct sunlight. Optimal curing conditions are temperatures around 75 to 80 degrees Fahrenheit, with sufficient airflow to prevent moisture buildup.
The goal of curing is to thoroughly dry the neck and the outer layers of the onion. This process typically takes two to three weeks, depending on the humidity and temperature. The neck should become completely dry and constricted, and the outer skins should transform into a papery, rustling texture. Trimming the roots and cutting the tops back to one or two inches should only be done after curing is complete. A properly cured onion has a sealed neck, which acts as a protective barrier against moisture and pathogens, allowing it to be stored for many months.