Gardeners typically start onions in one of three ways: from seeds, from small bulbs called sets, or from young plants known as transplants. Each method has a distinct schedule, and the earliest possible planting date is directly tied to the last expected frost in your region.
Selecting the Right Onion for Your Climate
Selecting an onion variety suited to your geographical location is essential. Onions are highly sensitive to the duration of daylight, a phenomenon known as photoperiodism, which dictates when the plant switches from growing leaves to forming a bulb. Planting the wrong type will result in a failure to bulb, regardless of how perfect your timing may be.
The three main types are categorized by the number of daylight hours required to trigger bulbing. Long-Day onions need 14 to 16 hours of daylight and are best suited for northern latitudes, roughly above the 40th parallel. Conversely, Short-Day onions require only 10 to 12 hours of light to bulb and thrive in the southern United States, generally below the 35th parallel.
Intermediate-Day, or Day-Neutral, varieties bridge the gap between these two extremes, initiating bulbing when daylight reaches 12 to 14 hours. These types are well-suited for the central regions of the country, where the growing seasons have moderate day lengths. A geographical line near the 36th parallel often serves as a general guide for where to switch from Long-Day to Short-Day varieties. Failing to match your onion type to your latitude will prevent the plant from ever storing energy in a large bulb.
Ideal Planting Timelines by Method
The earliest possible planting date is determined by the last expected spring frost, with the goal of establishing a robust root system before the warm temperatures trigger bulbing. The method you choose—seeds, sets, or transplants—will push this outdoor planting date further back into the cold weather.
Planting Onion Sets
Onion sets, which are small, marble-sized bulbs, offer the earliest opportunity for outdoor planting. They are the most cold-tolerant of the three starting methods and can be planted as soon as the soil is workable. This generally means four to six weeks before your area’s last expected frost date, allowing the bulbs to break dormancy and begin root growth in the cool soil.
Planting Onion Transplants
Transplants are young, pencil-sized seedlings, offering a middle ground between the speed of sets and the variety selection of seeds. These are typically ready to be planted outdoors two to four weeks before the last expected frost date. Because they are already actively growing, they are slightly less tolerant of extreme cold than dormant sets but still benefit from the early planting window. Before planting, it is necessary to harden them off by gradually introducing the young plants to outdoor conditions over a period of seven to ten days.
Planting Onion Seeds
Starting onions from seed offers the greatest choice of varieties but requires the longest lead time, demanding an early start indoors. To produce a transplant-sized seedling, seeds must be sown indoors 10 to 12 weeks before the intended outdoor transplanting date. For direct-sowing seeds into the garden, the timeline must be adjusted to sow as soon as the soil can be worked in early spring, approximately a month before the last frost. However, direct-sown onions mature later and are more susceptible to cold-weather stress than the other two methods.
Preventing Bolting and Other Early Planting Issues
Pushing the planting date as early as possible carries the risk of bolting, which is the premature formation of a flower stalk. Bolting is a survival mechanism triggered by severe temperature stress; once it occurs, the plant diverts energy from bulb production to seed creation. This results in a woody center, a thick neck, and a bulb that will not store well.
The primary trigger for bolting is the young plant experiencing prolonged periods of cold temperatures, specifically a cold snap below 45°F after the plant has begun active growth. While onions are cold-hardy, this temperature shock can trick the biennial plant into believing it has already completed a full growing season, prompting it to reproduce. This risk is highest for onion sets and transplants, which are already in a more advanced developmental stage when planted.
To mitigate temperature stress and the risk of bolting, gardeners can use techniques focused on stabilizing the soil temperature. Applying a layer of straw or other organic mulch around the young plants helps insulate the root zone from sudden cold snaps. Additionally, using row covers or low tunnels can provide several degrees of protection from freezing night air and minimize drastic temperature fluctuations.
If an onion does bolt, the stalk will emerge as a thick, rigid spear from the center of the plant. At this point, the best course of action is to harvest the onion immediately, even if the bulb is small. It can still be consumed fresh, but its shelf life will be severely compromised. Choosing heat-treated onion sets or using smaller, younger transplants can also help reduce the plant’s susceptibility to this cold-induced stress response.