Growing onions from seed allows gardeners to select from a wider range of varieties, often resulting in larger, more uniform bulbs than those grown from sets. This method requires a significant head start indoors to accommodate the onion’s long growing season. Starting seeds early ensures the plants develop substantial green tops, which directly relates to the final size of the bulb they produce.
Calculating Start Time and Preparing Materials
Onions need a long growth period, typically between 85 and 120 days, before they begin forming a bulb. To achieve large, mature bulbs, start seeds indoors approximately 8 to 12 weeks before your area’s average last expected spring frost date. Starting too late results in smaller bulbs because the plants lack sufficient leaf growth before the summer heat triggers bulbing.
Selecting the correct onion variety depends on your geographical location. Onions are categorized by the daylight hours required to trigger bulb formation: short-day, intermediate (day-neutral), or long-day types. Long-day onions need 14 to 16 hours of light and are best for northern latitudes. Short-day varieties require 10 to 12 hours and suit southern regions. Intermediate types need 12 to 14 hours and are versatile for middle latitudes.
For sowing, gather shallow trays or containers that are at least two inches deep, as onions do not transplant well from very shallow cells. Fill these containers with a sterile seed-starting mix, which is lighter and finer than potting soil. Garden soil is too heavy and compacts easily, hindering delicate root growth.
Detailed Steps for Indoor Sowing
Thoroughly moisten the seed-starting mix until it is uniformly damp but not saturated, preventing seeds from being washed away during initial watering. A fine mist or bottom-watering technique is ideal for keeping the surface undisturbed. Onion seeds are planted very shallowly, about one-quarter inch deep, or simply sprinkled on the surface and lightly covered.
The recommended technique is dense or broadcast planting in a tray, rather than individual cells. Scatter the seeds generously across the pre-moistened soil. After scattering, cover the seeds lightly with a thin layer of starting mix or vermiculite.
To encourage germination, cover the tray with a clear plastic dome or plastic wrap to create a humid microclimate. Place the covered tray in a warm location, such as on a heat mat, as seeds do not require light until they sprout. Once germination occurs, typically within 7 to 10 days, remove the cover immediately to allow air circulation and prevent fungal issues.
Essential Care for Seedling Development
Once the slender, grass-like shoots emerge, move them immediately under a bright light source to prevent “legginess.” A sunny windowsill is generally insufficient for the 14 to 16 hours of light that onion seedlings require daily. Utilize a dedicated grow light system, keeping the light source a few inches above the seedlings to provide the necessary intensity.
Temperature management is important for strong development, as seedlings prefer cooler conditions after germination. Aim for an air temperature between 50 and 60 degrees Fahrenheit to encourage robust growth without forcing early bulbing. Maintaining consistent moisture is also important; the soil should remain damp but never waterlogged, as onions have shallow roots and are susceptible to drying out or rotting.
Seedlings may form a characteristic “hairpin” shape as they push out of the soil, which is normal. If the green tops grow taller than about six inches and begin to flop over, trim them with scissors down to about four inches. This “haircut” encourages the plant to put energy into developing a thicker base and stronger root system, resulting in sturdier transplants. Trimming can be done every week or so until they are ready to be moved outside.
Hardening Off and Transplanting
The transition to the outdoor garden requires a gradual adjustment period known as “hardening off.” This process acclimates seedlings to harsher conditions like direct sunlight, wind, and fluctuating temperatures. Begin hardening off one to two weeks before the planned transplant date, which should be after the danger of a hard frost has passed and the soil is workable.
On the first day, place the seedlings in a shaded, sheltered outdoor location for just one to two hours, then bring them back inside. Over the next seven to ten days, progressively increase the amount of time they spend outside and gradually expose them to more direct sunlight and wind. This slow exposure prevents severe transplant shock, which can stunt growth or cause the plants to die.
Once fully hardened off, the seedlings are ready for their permanent garden bed. Onions tolerate light frost, so they can typically be transplanted a few weeks before the last expected frost date. Gently separate the densely sown seedlings, taking care not to damage the fine roots. Space the transplants four to six inches apart. Plant the seedlings shallowly, ensuring the white base is barely covered by the soil, as the developing bulb needs to sit mostly above the soil line. Water thoroughly after planting and begin a consistent watering regimen, as onions require constant moisture for optimal bulbing.