Preserving a harvest of home-grown onions immediately after they are pulled from the soil is the most important step in extending their shelf life through the winter. Without proper preparation, the bulbs will quickly rot or sprout, wasting the time and effort invested in growing them. The objective is to seal the bulb, reduce its moisture content, and then store it under conditions that suppress sprouting. By following a structured process, it is possible to enjoy firm, fresh onions for many months.
Pre-Storage Preparation: Curing
The first step for long-term storage is curing, which involves drying the neck and outer skins of the onion bulb. Curing forms a protective, papery barrier around the edible layers. This dried outer skin acts as a seal, preventing the entry of moisture and microorganisms that cause decay.
Harvest the onions after their tops have naturally fallen over and begun to yellow, signaling the plant has stopped directing energy to the leaves. The entire plant, including the leaves and roots, should be moved to a warm, dry, and well-ventilated location protected from direct sunlight and rain. Ideal curing conditions involve temperatures between 75°F and 85°F.
The bulbs must be spread out in a single layer, typically on a wire rack or screen, to ensure maximum airflow around all surfaces. Curing usually takes between two and four weeks, depending on the humidity and temperature. A fully cured onion will have a completely dry, thin, and brittle neck, and its outer skins will be crisp. Before moving the bulbs to storage, the dried tops should be trimmed to about one inch above the bulb, and the roots should be clipped close to the base.
Long-Term Storage Methods
Once the onions are properly cured and trimmed, the next consideration is the physical method of containment, which must prioritize continuous air circulation. Storing bulbs in non-breathable containers like plastic bags will trap moisture and lead to rapid spoilage. The goal is to keep the bulbs separate enough to allow air to pass freely around each one.
One traditional and effective method is braiding, which is best suited for soft-neck varieties where the dried foliage remains pliable. The stems of several onions are woven together to form a long rope, which can then be hung vertically, maximizing space efficiency and airflow. For hard-neck varieties, mesh bags or old nylon stockings provide excellent vertical storage.
If using mesh bags, such as those designed for produce, they should not be filled too deeply. The weight of the upper bulbs can bruise those at the bottom and restrict airflow. Another practical option is to use shallow wooden boxes or crates with slats. The cured onions can be placed in these containers in a single or double layer, ensuring no bulb is completely buried, which maintains good circulation.
Selecting the Optimal Environment and Variety
The longevity of a stored onion depends on both the environmental conditions of the storage space and the specific variety grown. Long-term success relies on maintaining a cool, dark, and moderately dry environment. The ideal conditions for storage are temperatures between 32°F and 40°F, paired with a low relative humidity of 65% to 70% to prevent both sprouting and desiccation.
Cool temperatures keep the bulbs dormant, suppressing the metabolic processes that cause them to sprout. A dark location is also important, as exposure to light can trigger greening and sprouting. Basements, cool garages, or root cellars often provide the best combination of these factors.
The choice of variety dictates the potential storage duration, with a clear distinction between pungent and sweet types. Pungent, long-day varieties, such as Copra or Patterson, possess a higher concentration of sulfur compounds and thicker outer skins, allowing them to be stored for six months or longer. Conversely, sweet onions, including varieties like Vidalia and Walla Walla, have higher water and sugar content and thinner skins, meaning they typically only last for one to two months.
Troubleshooting and Handling Damaged Onions
Even with careful curing and storage, some onions may prove unsuitable for winter keeping and should be set aside for immediate consumption. Any bulb with a thick, moist neck, or one that was bruised or cut during harvesting, must be culled because its compromised seal allows moisture loss and pathogen entry. These damaged onions should be used within a few days or processed immediately, as they will quickly initiate rot that can spread to healthy bulbs.
Regular inspection of the stored onions is necessary, as spoilage can begin subtly. Signs of a problem include soft spots, mold, or the start of sprouting, where a green shoot emerges from the neck. Any bulb showing these symptoms must be removed instantly to prevent the release of fungal spores or bacteria that can contaminate the rest of the batch.
For a large yield of onions that cannot be stored long-term, processing methods like dehydration or freezing offer a way to preserve them. Freezing chopped onions is a simple way to preserve their flavor for use in cooked dishes, though they will lose their crisp texture. Dehydrating the onions into flakes or powder creates a shelf-stable ingredient that can last indefinitely.