Onions are continuously available in grocery stores throughout the year due to modern agricultural practices and storage technology. The true “onion season” refers to the peak local harvest period when bulbs are pulled from the ground at their freshest, often possessing their highest sugar content. This peak timing depends heavily on the specific onion variety and the number of daylight hours in the growing region. A successful harvest relies on matching the onion type to the local environment’s photoperiod, or day length.
Defining Onion Types and Their Harvest Timing
The timing of an onion harvest is controlled by photoperiodism, a genetic response to daylight length. Onions are categorized into three main groups based on the minimum daily sunlight required to trigger the bulbing process. Choosing the correct type for a specific location is necessary for a successful bulb harvest.
Short-day onions are adapted to southern latitudes, requiring only 10 to 12 hours of daylight to begin forming a bulb. They are typically planted in the fall, grow through the mild winter, and are harvested in the late spring or early summer. These varieties often yield the sweetest and juiciest bulbs, such as sweet onions, but they do not store well.
Intermediate-day onions, sometimes called day-neutral, need about 12 to 14 hours of sunlight to start bulbing. These varieties thrive in the middle latitudes and are harvested in the early to mid-summer. They offer a balance between the rapid growth of short-day types and the storage potential of long-day onions.
Long-day onions are best suited for northern regions where summer days stretch to 14 to 16 hours of sunlight. They are planted in early spring and mature over the summer, ready for harvest in mid-to-late summer. Varieties like yellow and red storage onions are bred for pungency and a thick skin, allowing them to be stored for many months. Green onions (scallions) are an exception; harvested before the bulb swells, they can be pulled almost any time during the growing cycle for their fresh leaves and small white base.
The Life Cycle of an Onion
The agricultural cycle of an onion begins months before the harvest, and the starting material dictates the time to maturity. Onions can be started from true seeds, small immature bulbs called sets, or young plants known as transplants. Starting from seeds provides the greatest varietal choice and is the preferred method for long-term storage onions, but it requires the longest maturation period, often 120 to 150 days.
Onion sets are miniature bulbs grown the previous year and held dormant through winter. Planting sets gives the crop a significant head start, allowing for a quicker harvest. However, sets tend to produce smaller mature bulbs and are sometimes prone to bolting (prematurely developing a flower stalk). Transplants are young seedlings that offer a middle ground, providing a quick harvest without the risk of bolting associated with sets.
Planting time is strategically chosen to ensure the plant maximizes leaf growth before the critical day-length threshold is met. For example, long-day onions are planted in early spring to allow for extensive leaf growth before the longest days of the year trigger the bulb to swell. Once the top foliage begins to yellow and collapse (usually between 90 and 150 days after planting), the bulb is considered mature and ready to be pulled from the soil.
Year-Round Availability and Storage
The year-round presence of onions in the marketplace results from specialized post-harvest handling and global sourcing. After harvest, onions intended for long-term storage undergo curing, a drying period lasting two to six weeks. This process involves exposing the bulbs to warm, dry, and well-ventilated conditions, often at temperatures between 75°F and 90°F.
Curing allows the outer layers of the onion to dry into a papery skin while the neck, where the leaves meet the bulb, tightens and seals. This protective layer prevents moisture loss and blocks the entry of decay-causing microorganisms. Once properly cured, storage onions (such as pungent yellow and red varieties) can be kept for four to eight months in a cool, dark, and moderately dry environment, typically around 32 to 40°F.
Sweet onion varieties have a higher water content and thinner skin, making them poor candidates for long-term storage. These fresh spring and summer harvests must be consumed quickly, usually within a few weeks. The combination of regional harvest times, the short shelf life of sweet onions, and the long shelf life of cured storage onions ensures a continuous market supply regardless of the local season.