Onion sets offer home gardeners a shortcut to harvesting bulbs, bypassing the lengthy germination phase required by seeds. An onion set is a small, immature bulb grown the previous season, harvested, and stored while dormant. This pre-grown material gives the plant a significant head start once replanted, making it a popular method for achieving a quick and dependable harvest.
Understanding Onion Sets
Using sets, rather than starting from seeds or transplants, simplifies the gardening process. Onion sets are small bulbs, typically less than an inch in diameter, that were intentionally crowded during their first year of growth to keep them small. This early harvest and subsequent storage puts the onion into a dormant state, ready to resume growth upon planting. The main advantage of this method is the ease of planting and a reduced risk of early failure compared to delicate seedlings.
Sets are already in their second year of the biennial life cycle, meaning they are biologically primed for bulb production after replanting. While this provides a growth advantage, it also slightly increases the risk of bolting, or sending up a flower stalk, compared to first-year seedlings. However, the convenience and speed of sets often make them the preferred choice for gardeners in regions with shorter growing seasons.
The Standard Timeline for Maturation
Onion sets generally reach a harvestable size within 60 to 90 days from planting. Some storage varieties may require up to 120 days to fully mature, but most common sets fall within the three-month window. This accelerated schedule is achieved because the plant is already established and bypasses the initial 10 to 12 weeks required for seeds to reach a transplantable size. The actual time to maturity is heavily influenced by the variety’s response to day length, a phenomenon called photoperiodism.
Onion cultivars are grouped based on the hours of daylight needed to trigger the bulbing process. Short-day onions require 10 to 12 hours of light and are suited for southern latitudes. Long-day varieties need 14 or more hours and thrive in northern regions. Intermediate-day onions fall between these two, needing 12 to 14 hours of light to start forming a bulb. Planting a variety unsuited to the local latitude can cause the onion to bulb too early, resulting in a small final product.
Key Factors Affecting Growth Duration
The 60 to 90-day timeline relies on providing optimal growing conditions that support continuous development. The quality of the soil plays a significant role, as onions are heavy feeders and require nutrient-rich, well-drained soil to accelerate bulbing. Soil that retains too much water or lacks sufficient nutrients will slow the plant’s metabolic processes and delay maturation. Adequate and consistent watering is also necessary throughout the active growing period to encourage leaf development, as each leaf corresponds to a layer of the final bulb.
Temperature fluctuations can interrupt the growth cycle and potentially cause stress that encourages bolting. Extreme heat or cold can slow the overall growth rate, extending the time needed to reach maturity. Providing a constant supply of nutrients, especially nitrogen early in the season, ensures the plant maximizes its energy toward bulb expansion, keeping the time to harvest shorter.
Knowing When and How to Harvest
The process of maturation is complete when the bulb has finished storing energy. The most reliable visual indicator is when the onion’s foliage, or tops, begin to yellow and naturally flop over at the neck. This falling over signals that the plant is no longer actively growing and that the skin where the leaves meet the bulb has started to dry out. Waiting until about two-thirds of the tops have fallen over is the accepted timing for harvest.
Once this visual cue is present, the bulbs should be gently lifted from the soil, taking care to avoid bruising. After harvesting, onions must be cured to prepare them for long-term storage. Curing involves drying the bulbs in a warm, dry, well-ventilated area, such as a covered porch or barn, for one to two weeks. This process allows the neck to dry completely and the outer skins to become papery, which seals the bulb and improves its storage life.