The time it takes for an onion (Allium cepa) to fully mature and be ready for harvest is highly variable, depending primarily on the starting material used for planting, the specific variety, and environmental conditions. The growth cycle is flexible, responding to light and temperature cues, which means the final harvest date can shift by several weeks. Understanding the initial planting method provides the clearest estimate for the minimum time required to produce a mature bulb.
Growth Timelines Based on Starting Material
The growth duration depends entirely on whether the onion begins as a tiny seed, a small dormant bulb known as a set, or a partially grown transplant. Each method represents a different starting point in the plant’s life cycle and requires a different amount of time to reach maturity.
Starting onions from seeds represents the longest commitment, typically requiring 120 to 180 days from sowing until harvest. Because onion seeds germinate slowly and require a long period of vegetative growth before bulbing is triggered, gardeners often start them indoors six to ten weeks before the last expected frost. This head start allows the plant to develop enough foliage to support a large bulb later in the season.
The fastest method is planting from sets, which are small, immature bulbs harvested the previous year and held in dormancy. These pre-grown bulbs bypass the lengthy germination and early growth stages, allowing them to mature in a shorter window of 60 to 90 days. Sets carry a higher risk of prematurely flowering, or “bolting,” which halts bulb development and makes the onion unsuitable for storage.
An intermediate and reliable option is planting from transplants, which are small, actively growing plants often sold in bunches. These seedlings are usually several weeks old when planted out, placing them ahead of direct-sown seed but behind sets in initial maturity. Transplants generally require 90 to 120 days of growth until they reach full bulb size. This method offers a balance between the speed of sets and the lower bolting risk associated with seed-grown varieties.
Environmental Factors Influencing Growth Duration
The timeframes established by the starting material are modified by external factors, primarily the photoperiod, or the specific length of daylight hours. Onions are photoperiodic plants, meaning they will not begin to form a bulb until daylight hours reach a specific threshold. Varieties are grouped into short-day (10 to 12 hours of light), intermediate-day (12 to 14 hours), and long-day (14 to 16 hours).
Planting a long-day variety in a southern latitude with shorter peak summer days will slow or prevent bulbing, as the plant never receives the necessary light signal. Conversely, planting a short-day variety in a northern latitude can cause it to bulb too early while the plant is still small, resulting in tiny, underdeveloped onions. Matching the variety to the local latitude is essential for achieving a timely and successful harvest.
Temperature
Temperature plays a dual role in controlling the growth rate and duration. Onions prefer cool temperatures, ideally between 6 and 20°C, during their initial vegetative growth phase for optimal foliage development. However, temperatures between 25 and 27°C are required to promote the final bulb initiation and enlargement.
Exposure to prolonged cold temperatures (below 10°C after the plant has established four or more leaves) can trigger vernalization and cause the plant to bolt. This premature flowering shifts the plant’s energy from bulb production to seed production, effectively ending the useful growth cycle.
Nutrients and Water
The availability of nutrients and water directly impacts the speed of growth. Onions require high fertility, with a consistent supply of nitrogen important during the early stages to produce large leaves. The leaves produce the sugars needed for the final bulb. However, excessive nitrogen applied late in the season can delay maturity by promoting continued leaf growth instead of bulb formation.
Recognizing When Onions Are Ready for Harvest
Regardless of the initial planting method, visual cues indicate when the growth cycle is complete and the bulb is mature. The most reliable sign of maturity is the natural softening and “necking over” of the foliage. This occurs when the transfer of energy from the leaves to the bulb is complete, and the neck, where the leaves meet the bulb, loses its rigidity.
The tops of the plants will begin to yellow and wither, eventually falling over onto the ground in a process called lodging. The bulbs should be harvested when approximately 50 to 75% of the tops have fallen over naturally. Harvesting before this point means the bulb has not reached its full size or developed the protective outer layers necessary for storage.
After the bulbs are lifted from the soil, the growth process is over, but a mandatory post-harvest step called curing begins. Curing involves allowing the bulbs to dry in a warm, well-ventilated, protected location for one to three weeks. This process dries the outer skin layers and seals the neck, which prepares the onion for long-term storage and prevents spoilage.