Onions are a garden favorite, and for home gardeners seeking the fastest path to a harvestable bulb, onion sets are the preferred starting material. An onion set is a miniature, partially grown bulb produced from seed the previous year, giving the plant a head start on the growing season. This method avoids the lengthy germination and fragile seedling stage required when starting from true seed, resulting in a significantly quicker turnaround time.
The Typical Growth Timeline
The general timeframe for an onion to reach full maturity when planted from a set is between 60 and 100 days. The speed of development depends on the variety chosen and the specific growing conditions.
The maturation process begins with an initial phase focused on root establishment and foliage growth. In the first two to three weeks after planting, the set develops a robust root system and pushes out the green, leafy tops. Once the plant has developed sufficient foliage, the second, longer phase begins: the swelling of the bulb itself. This final stage is the period of weight gain and enlargement that culminates in the mature onion.
Key Factors Influencing Maturation
Photoperiodism
The most significant factor controlling when an onion set begins its final bulbing stage is photoperiodism, the plant’s response to the length of daylight. Onions are categorized into three main groups based on the minimum hours of daylight required to initiate bulb formation. Long-day varieties, suited for northern latitudes, require 14 to 16 hours of daylight. Short-day varieties, grown in southern regions, only need 10 to 12 hours of light. Intermediate or day-neutral onions fall between these two, needing 12 to 14 hours of daylight. Planting the wrong type for your region will cause the plant to bulb too early, resulting in tiny, underdeveloped bulbs.
Soil Temperature
Soil temperature also plays a role in the speed of maturity, as onions prefer cool soil for planting but warm soil for bulbing. Sets should be planted in early spring as soon as the soil is workable, allowing roots to establish before the summer heat arrives. If the ground remains too cold, the initial growth phase will be delayed, pushing back the final harvest date.
Watering
Consistency in watering influences both the timing and quality of the final harvest. Onions have shallow root systems, making them susceptible to stress from uneven soil moisture. Periods of drought followed by heavy watering can cause the forming bulb to split or produce multiple centers. This damage halts proper development and effectively ends the growth cycle prematurely.
Recognizing When Onions Are Ready
The most reliable indicator that the bulb is finished enlarging is the “necking” process. This occurs when the green leafy tops soften, turn yellow, and naturally fall over. This collapse signifies that the plant has stopped producing new foliage and has redirected all stored energy into the bulb.
Once roughly 50 to 75 percent of the tops have fallen over, the onion is ready to be lifted from the soil. Harvesting too early, when the tops are still fully erect, results in a smaller bulb with a thick, stiff neck that will not store well.
After the bulbs are pulled, they require curing to prepare them for long-term storage. Curing involves drying the outer papery skin and the neck to seal the bulb, preventing moisture loss and protecting against decay. This process typically takes between two and four weeks in a warm, dry, and well-ventilated location.