The common onion, Allium cepa, can be grown from seed. Seeds are used for most onion varieties cultivated globally. This method allows growers access to a wide genetic selection, encompassing different colors, shapes, and day-length requirements. Seeds are one of three primary methods used for propagation, each offering a distinct balance of convenience, variety, and time to harvest.
Starting Onions Directly from Seed
Growing onions from seed offers the greatest control over the plant’s life cycle. The process typically begins indoors during late winter, 8 to 12 weeks before the final expected frost date. Onion seeds require a long vegetative growth period before increasing day length triggers bulb formation, making an early indoor start necessary for a successful harvest.
Seeds should be sown shallowly, about a quarter to a half-inch deep, into a sterile seed-starting mix. Once germinated, seedlings benefit from a cooler environment (50–65°F) to prevent them from becoming leggy. Providing 14 to 16 hours of bright light daily supports strong initial growth.
The goal is to produce thick, vigorous seedlings about the diameter of a pencil before transplanting. Before moving them outside, the young plants must undergo “hardening off” to acclimate them to outdoor conditions. Over seven to ten days, seedlings are gradually exposed to direct sun, wind, and cooler temperatures.
Once the soil is workable in early spring, plant the seedlings outdoors, spaced 3 to 4 inches apart. Starting with seeds provides access to unique cultivars, including specific long-day, short-day, or day-neutral varieties, often unavailable otherwise. Careful control over the seedling phase also reduces the risk of bolting, which halts bulb development.
Understanding Onion Sets and Transplants
Beyond seeds, gardeners commonly utilize onion sets and onion transplants. Onion sets are small, immature bulbs grown from seed the previous season, harvested while small, and forced into dormancy. These bulbs are ready to be planted directly into the garden in early spring.
The advantage of using sets is convenience and speed, as they bypass the lengthy indoor seed-starting phase. Sets are planted shallowly with the pointed tip just below or at the soil surface, spaced a few inches apart. They establish quickly and often mature earlier than onions started from seed.
Onion transplants are young, actively growing plants started by a nursery. They are typically shipped in bunches. Transplants offer a middle ground between the long commitment of starting seeds and the limited variety of sets.
When planting transplants, place them in the soil so the white bulb portion is just covered, ensuring the roots are spread out below. Using transplants gives the gardener a head start, often six or more weeks compared to direct seeding. Both sets and transplants allow gardeners to skip the delicate early stages of growth, providing a reliable path to harvest.
Choosing the Best Planting Method
The selection among seeds, sets, and transplants depends on a gardener’s specific goals and available resources. Seeds are the most economical option, costing less than purchasing pre-grown sets or transplants. This method is the choice for gardeners needing large quantities or wishing to grow specialty varieties, as seed catalogs offer the broadest selection.
The trade-off for cost savings and variety is time, as seeds require the longest commitment, 100 to 120 days to mature. Sets are the most convenient and fastest route to harvest but carry the highest risk of bolting. Bolting occurs when the immature bulb experiences cold followed by warmth, causing it to prematurely send up a flower stalk, diverting energy from bulb growth.
Transplants provide a good compromise, offering a balance of speed and low bolting risk. Since they are past the fragile seedling stage, they establish quickly and are less susceptible to environmental stresses than sets. However, variety selection is limited compared to seeds, and they represent the highest initial cost per plant.
For a beginner seeking an easy, reliable first harvest, sets or transplants are often recommended. Experienced gardeners focused on specific flavor profiles or long-term storage varieties usually prefer to start from seed.