Growing onions from seed, rather than sets, provides access to a wider array of varieties and often results in larger, more uniform bulbs. Choosing seeds allows gardeners to select varieties specifically adapted to their local climate, maximizing the potential for a successful harvest. However, onion growth is highly sensitive to external cues, meaning that timing the planting correctly is the most important factor for success. Missing the narrow window for starting seeds indoors can lead to small, underdeveloped bulbs or crop failure, making a precise schedule necessary.
Understanding Onion Day Length Types
The fundamental factor determining when to plant is the specific onion variety chosen, categorized by the amount of daylight needed to trigger bulb formation (photoperiodism). Onions are classified into three types: short-day, intermediate-day, and long-day, each requiring a different number of daylight hours to shift from foliage production to bulbing. Planting the wrong day-length type for a specific latitude is the most common reason onions fail to form large bulbs.
Short-day onions initiate bulbing at 10 to 12 hours of daylight and suit southern latitudes (typically between 25° and 35°). They are planted in the fall to allow vegetative growth through the mild winter before the bulbing trigger occurs in late winter or early spring. Intermediate-day onions (day-neutral) require 12 to 14 hours of daylight and thrive in mid-latitudes (generally between 32° and 42°).
Long-day onions require 14 to 16 or more hours of daylight to begin forming a bulb. These varieties are the correct choice for northern states and higher latitudes, typically above 37°. Planting a short-day variety in the north results in premature bulbing and very small bulbs because the plant lacks sufficient time for top growth.
The Optimal Time to Start Seeds Indoors
Once the correct day-length variety is chosen, starting seeds indoors provides the necessary head start. The general rule is to sow seeds 8 to 10 weeks before the average last expected spring frost date. This early indoor start is crucial because onions require a long period of vegetative growth before the day-length trigger initiates bulbing.
Starting seeds too late results in small seedlings lacking enough leaf growth to produce a large bulb. Starting them too early can lead to overly large seedlings that may suffer transplant shock or bolt (prematurely send up a flower stalk) if exposed to cold temperatures outdoors. The ideal size for transplanting is a robust seedling that is pencil-thick at the base.
Indoor Seed Starting Requirements
To encourage strong germination, sow seeds shallowly (about a quarter to a half-inch deep) in a sterile seed-starting mix. The optimal temperature for germination is around 70°F, often requiring a heat mat. After germination, seedlings need intense light for 12 to 16 hours daily, typically provided by grow lights, to prevent them from becoming thin and weak (“legginess”). Direct sowing is discouraged for bulbing onions because early spring soil is often too cold for good germination.
Hardening Off and Transplanting Outdoors
The second timing milestone is the transition from the controlled indoor environment to the outdoor garden. Before transplanting, seedlings must undergo “hardening off,” which gradually acclimatizes the plants to harsher outdoor conditions like sunlight, wind, and fluctuating temperatures. This process prevents transplant shock.
Hardening off should begin one to two weeks before the planned transplant date. Start with one hour of exposure in a sheltered location, then gradually increase the duration and introduce the plants to more direct sunlight and wind daily. Onions are cold-tolerant; transplant the seedlings outdoors about four weeks before the last expected frost date, or as soon as the soil is workable.
The ideal size for transplanting is a seedling 6 to 8 inches tall with a pencil-diameter stem. Set the seedlings into the soil about 1 to 1.5 inches deep, ensuring the roots are buried and the white base is partially covered. Proper timing ensures the plants are mature enough to take advantage of lengthening days, maximizing their potential to form large bulbs.