Proper post-harvest care is the most important factor determining how long harvested onions will last in storage. The process begins immediately after the bulbs are pulled, focusing on minimizing moisture and creating a protective outer layer. Correctly handling the onions through curing, cleaning, and storage significantly extends their shelf life. The goal is to maximize the duration the bulbs remain firm and flavorful, preventing premature sprouting or rot.
The Essential Process of Curing Onions
Curing is a necessary step that dries the outer layers of the onion bulb, along with the neck, forming a protective seal against moisture loss and microbial entry. This process converts the outer scales into the papery wrapper characteristic of a storage onion, which is essential for long-term preservation. Without a complete cure, moisture remains in the neck, creating a soft entry point for decay organisms like neck rot.
The ideal environment for curing is a warm, dry location with excellent air circulation, avoiding direct sunlight which can scald the bulbs. Aim for temperatures between 75°F and 85°F, with a relative humidity around 60% to 75% for optimal results. Spreading the onions out in a single layer on a screen or rack ensures that air moves freely around all sides of the bulbs.
Curing typically takes between one and three weeks, depending on the climate and the size of the onions. The bulbs are fully cured when the neck is completely dry, feeling thin and papery to the touch, and the outer skin rustles when handled. A simple test is to cut the stem about an inch from the bulb; the center of the cut area should be dry all the way through.
Cleaning and Preparing Bulbs for Storage
Once the curing process is complete, the onions require physical preparation before being moved to their final storage location. Start by gently brushing off any loose soil clinging to the bulbs, taking care not to bruise the onion. It is important not to wash the onions, as adding moisture back to the skin will compromise the protective layer you just created.
The next step involves trimming the dried foliage and roots. Use sharp scissors or shears to cut the roots off close to the bulb. The dried tops should be cut back, leaving a stem of approximately one to two inches above the bulb’s neck. Leaving this short, dry stub helps maintain the bulb’s seal and prevents decay from entering the core.
During this preparation stage, closely inspect each bulb for any signs of damage, bruising, or a thick, soft neck. Onions with any imperfections, such as those bruised during harvest or those with necks that failed to dry fully, will not keep well. Set these aside for immediate use, as they will spoil much faster than perfectly cured bulbs.
Optimal Long-Term Storage Conditions
Achieving the longest storage life requires maintaining a specific environment that slows the onion’s metabolic processes and inhibits the growth of pathogens. The most successful long-term storage occurs at a cool temperature range, ideally between 32°F and 40°F, which is just above freezing. This low temperature prevents the bulbs from sprouting and minimizes moisture loss.
Maintaining low humidity is equally important; a relative humidity of 60% to 70% is recommended. High humidity encourages root growth and bulb rot, while excessively low humidity can cause the onions to shrivel. Storage containers must allow for continuous air circulation.
Storage Containers
- Mesh bags
- Open-weave crates
- Braided strings of onions hung from a ceiling
The inherent storage potential depends on the onion variety. Pungent, long-day varieties typically store much longer than milder, sweet onions. Pungent yellow and white onions, often called storage onions, can last for many months due to their higher solids content and tougher outer skins. Sweet varieties like Vidalia or Walla Walla have a much higher water content and thinner skins, meaning they should be consumed within a few weeks to a couple of months.