Starting onions from seed is the preferred method for home gardeners seeking control and access to a wider selection of varieties compared to purchasing sets or transplants. Onions are slow-growing plants that require a long season, meaning starting them indoors provides the necessary controlled environment for their initial development. This approach allows the gardener to precisely manage temperature, moisture, and light during the critical germination and early growth phases. A successful onion harvest begins by understanding the specific needs of the seed, which this guide will detail step-by-step.
Essential Pre-Planting Decisions
Onions are slow-growing biennials that require time to mature before the high heat of summer arrives. Seeds must be sown indoors approximately 10 to 12 weeks before the average last expected frost date in your region. This early start ensures the seedlings develop adequate foliage and root mass, allowing them to transition outdoors and begin bulb formation during the longest days of the year.
Selecting the correct onion type is crucial for successful bulb development, as it is governed by photoperiodism, the plant’s response to day length. Gardeners above 37 degrees North must choose long-day varieties, which require 14 to 16 hours of sunlight to trigger bulbing. Conversely, those in southern regions must select short-day varieties, which begin bulbing when daylight reaches only 10 to 12 hours. Intermediate-day varieties are suitable for the middle latitudes. Failure to match the onion variety to the geographic latitude is a common reason for a poor harvest, resulting in plants that produce only green tops without forming a usable bulb.
Setting Up the Germination Environment
The physical environment requires careful preparation to maximize germination rates and prevent early-stage diseases. Shallow trays or individual cell packs should be thoroughly cleaned or sterilized with a diluted 10% bleach solution to eliminate pathogens. The growing medium must be a sterile, fine-textured seed-starting mix, which offers better aeration and water retention than traditional garden soil.
Using a sterile mix prevents “damping off,” fungal diseases that attack and kill young seedlings at the soil line. For optimal germination, onion seeds thrive in consistent warmth, ideally between 70 and 75°F (21–24°C). Achieving this temperature consistently usually requires an electrical heat mat placed beneath the trays, as ambient room temperature is often insufficient. The warmth provided by the heat mat helps break seed dormancy and shortens the time required for the seeds to sprout. Maintaining this specific temperature range is more important than light during the initial pre-sprout phase.
Sowing and Initial Germination Steps
Once the optimal environment is established, the sowing process begins by lightly moistening the sterile seed-starting mix before planting. Onion seeds should be sown shallowly, covering them with about 1/4 inch of the medium. They can be placed fairly close together—typically 10 to 12 seeds per square inch—since they will be thinned later, but excessive clumping should be avoided.
The medium must remain consistently moist but never saturated to prevent seed rot. Bottom watering, where the trays sit in a shallow reservoir of water until the surface darkens, is the preferred method, as it prevents disturbing the seeds with a direct stream of water. The trays should then be covered with a clear plastic dome or plastic wrap to create a humid microclimate.
This covering maintains the high relative humidity necessary for uniform emergence and aids in softening the seed coat. Germination typically occurs within seven to ten days. As soon as the first green loops (cotyledons) emerge from the soil, the plastic dome and the heat mat must be immediately removed to prevent overheating and disease.
Post-Sprout Care and Hardening Off
The moment the seedlings sprout, they require immediate and intense artificial light to prevent etiolation, the development of long, weak stems due to insufficient light. Grow lights must be positioned 2 to 4 inches above the tallest leaf tip and kept on for 14 to 16 hours daily. Insufficient light causes the characteristic “floppy” onion seedling, making the plant weak and susceptible to breakage.
When the seedlings have developed their first true leaf and resemble tiny blades of chives, they must be thinned to allow adequate space for root and stem development. Using small, sharp scissors, clip the weaker seedlings at the soil line, leaving the strongest ones spaced about 1/2 inch apart. This thinning ensures the remaining plants receive sufficient light, nutrients, and air circulation, minimizing competition and reducing the risk of fungal diseases.
Before transplanting them into the garden, the young onions require hardening off. Over a period of seven to ten days, the seedlings are gradually exposed to outdoor conditions, including direct sunlight, wind, and cooler temperatures. This process begins with an hour or two in a sheltered, shady spot and slowly increases the duration and intensity of exposure each day. Skipping this slow acclimatization leads to transplant shock, stunting the plants or resulting in death upon moving them into the outdoor environment.