Harvesting onions requires careful timing to ensure the best flavor and maximum storage potential for the bulbs. Onions are biennials typically cultivated as annuals, shifting their energy from leaf production to bulb formation based on daylight hours and maturity signals. Understanding the approximate growing period and recognizing the visual cues of readiness are necessary for a successful harvest. Proper timing, followed by a thorough curing process, determines how long the bulbs will last in storage.
General Timeframes for Onion Growth
The time it takes for an onion to reach a harvestable size varies significantly based on the starting material. Onions grown directly from seed require the longest duration, typically four to five months to fully mature. This method is economical but demands a longer commitment and often involves starting seeds indoors before transplanting.
Planting from small, dormant bulbs called sets offers a quicker timeline, usually maturing in approximately three months. Sets provide a head start but are often prone to bolting, which can compromise the bulb’s size and storage quality. Using transplants (small seedlings) generally offers a balance, taking about three to four months to develop mature bulbs.
The specific variety also plays a substantial role, categorized by light requirements: short-day, intermediate-day, or long-day types. Long-day onions require 14 to 16 hours of daylight to trigger bulbing and suit northern regions. Short-day varieties need 10 to 12 hours of light and are grown in the South. Total maturity time spans from 85 to 120 days, depending on these factors.
Confirming Harvest Readiness
Relying solely on the calendar can be misleading; the most reliable way to confirm readiness is by observing specific physical changes. The primary sign that an onion is finished growing is when the green tops, or foliage, naturally begin to yellow, wilt, and fall over. This collapse indicates the plant has ceased transferring energy to the leaves and is directing resources into the bulb for final maturation.
Gardeners typically wait until 50% to 80% of the tops have bent over before initiating the harvest. Harvesting too early results in smaller bulbs, while waiting too long increases the risk of splitting or sprouting. Another key indicator is the softening of the neck, where the leaves meet the top of the bulb. A soft, pliable neck confirms that the flow of carbohydrates has concluded, which is necessary for proper bulb development.
As the onion matures, the outer layers of the bulb will start to form a papery, dry skin that is visible above the soil line. This tightening of the skin is a natural pre-curing process that happens while the bulb is still in the ground. If the neck cavity remains sunken or soft, or if the last leaf is still green and moist, the onion is not ready and should not be pulled, as premature harvesting can lead to rot during storage.
Post-Harvest: Curing and Storage
Curing immediately follows harvest and prepares the onions for long-term storage. Curing involves drying the outer skins and the neck of the bulb, which seals moisture inside and creates a protective barrier against decay. A properly cured onion will have a tight, dry neck and papery skin that rustles when touched.
To cure the bulbs, they should be spread out in a single layer in a warm, sheltered, and well-ventilated location. Ideal curing conditions are warm, with temperatures between 75°F and 90°F, and free from direct sunlight, which can cause sunscald. The curing period typically lasts between one to three weeks, depending on the humidity and temperature of the environment.
Once the neck is completely dry and the outer layers are papery, the onions are cured and ready for storage. The dried tops should be trimmed to about one inch above the bulb, and the roots can be clipped close to the base. Cured onions should be stored in a cool, dark, and moderately dry location, such as a cellar or basement. The ideal temperature range is 32°F to 40°F with 65% to 70% relative humidity. Storing bulbs in mesh bags or slatted crates maintains air circulation, preventing moisture buildup and extending storage life for several months.