The common onion, Allium cepa, is a widely cultivated vegetable that develops a fleshy, underground bulb. The time it takes for an onion to reach maturity is not fixed, depending heavily on the starting material: seed, small bulb (set), or pre-started transplant. This variability means the time from planting to harvest can range from approximately three months to over half a year.
Timeline Based on Starting Material
Starting onions from seeds represents the longest commitment, requiring 150 to 175 days before a full-sized bulb is ready for harvest. Seeds must be started indoors, typically 10 to 12 weeks before the final expected frost date, to give seedlings a necessary head start. This method is often preferred for producing large, high-quality bulbs that store well over the winter.
Using transplants, which are young seedlings started by a commercial grower, offers an intermediate timeline. These seedlings are planted outdoors in early spring, bypassing the initial indoor care required for seeds. From transplanting, the onions generally require 100 to 140 days to reach full maturity. This method balances a reduced time commitment with the ability to grow a variety of onion types.
The fastest route to harvest is planting from sets, which are small, immature bulbs from the previous season. Sets typically mature in the shortest time, requiring only 80 to 90 days in the ground. While this quick turnaround is popular, onions grown from sets are sometimes more prone to bolting and may not produce bulbs as large as those grown from seed.
Key Environmental and Genetic Variables
The most significant factor influencing an onion’s growth timeline is its genetic sensitivity to day length, known as photoperiodism. Onions are classified into three major groups based on the number of daylight hours required to trigger bulb formation. Short-day varieties are suited for Southern regions, beginning to bulb when daylight reaches 10 to 12 hours.
Intermediate-day types need 12 to 14 hours of daylight to initiate bulbing, thriving in middle latitudes. Long-day varieties, typically grown in Northern areas, require 14 to 16 hours of sunlight to swell into a bulb. Planting a variety mismatched to the local peak day length will result in small, undeveloped bulbs or plants that fail to bulb entirely.
Temperature also plays a role in growth duration and quality. Onions are a cool-season crop, but prolonged cold temperatures after establishment can trigger bolting. Bolting is the premature formation of a seed stalk, which diverts energy away from bulb development. This results in a smaller bulb with a thick neck that does not store well.
Soil conditions are another important environmental variable. Onions thrive in loose, well-drained, and highly fertile soil, such as sandy loam, with a pH between 6.0 and 6.8. Heavy clay soils, which hold excessive water, can slow bulb development and increase the risk of disease. Consistent moisture and nutrient availability are necessary for the leaves to support a sizable bulb, affecting the final harvest date.
Recognizing Bulb Maturity and Harvest
The completion of the onion’s growth cycle is signaled by clear visual changes in the foliage, indicating the transfer of carbohydrates into the bulb is finished. The first sign is the softening of the neck, where the green leaves meet the top of the bulb. This softening is followed by the natural bending and toppling of the entire green top, a process that should occur on its own.
Most growers wait until 85 to 90% of the foliage has fallen over before beginning the harvest. Pulling the bulbs too early, while the tops are still upright, means the onion has not reached its maximum size or developed its protective outer skin. Waiting too long, until the tops are completely brown and dry, can increase the risk of neck rot and compromise storage life.
Once the bulbs are pulled, the final stage is curing, which is necessary for long-term storage. Curing involves drying the onions in a warm, dry, and well-ventilated location, such as a covered porch, for two to four weeks. During this period, the outer layers and the neck dry completely, forming the papery, protective skin that prevents moisture loss and decay. A properly cured onion will have a tight, dry neck and brittle outer scales.