Growing onions from seed is a rewarding process that allows a gardener to select from a much wider range of varieties than is available through pre-grown sets. Starting onions from seed requires a longer lead time but results in a more robust and higher-yielding harvest with better storage potential. This method gives the plant a full season to develop a strong root system and ample foliage, which are the primary determinants of final bulb size.
Essential Preparation Before Planting
The most important decision before sowing any seed is selecting the correct onion variety for your geographical location, which is governed by photoperiodism, the plant’s response to day length. Onion bulb formation is triggered when the hours of daylight reach a specific threshold.
Gardeners in the northern latitudes (above the 40th parallel) should choose long-day varieties, requiring 14 to 16 hours of daylight to initiate bulbing. Intermediate-day varieties are suited for central latitudes, bulbing when daylight reaches 12 to 14 hours. Short-day onions are necessary for southern regions, forming bulbs when exposed to only 10 to 12 hours of light. Planting the wrong type means the plant will not receive the necessary light signal to form a large bulb, yielding only small, underdeveloped onions.
Beyond variety selection, gather necessary supplies: sterile seed-starting mix, shallow trays or flats, a thermostatically controlled heat mat, and a full-spectrum grow light system.
Starting Onion Seeds Indoors
The timing for indoor sowing is critical, generally 8 to 10 weeks before the average last spring frost date in your region. Fill the planting trays with a sterile, fine-textured seed-starting mix, moistened to the consistency of a wrung-out sponge. Onion seeds should be planted shallowly, approximately one-quarter to one-half inch deep, and can be sown densely.
Place the trays on a heat mat set to 70 to 77 degrees Fahrenheit to encourage rapid germination. Once the seeds sprout (typically within a week), remove the heat mat and move the seedlings to a cooler environment, ideally 55 to 65 degrees Fahrenheit. Maintain grow lights four to six inches above the emerging seedlings for 12 to 14 hours daily to prevent them from becoming weak and spindly. When the hollow leaves reach about six inches tall, use sharp scissors to trim them back to three or four inches; this encourages thicker, stronger stems better prepared for transplanting.
Transitioning Seedlings to the Garden
Before planting outside, seedlings must undergo “hardening off” to acclimatize them to the harsher outdoor environment. This gradual transition toughens the plant structure, preparing them for direct sunlight, wind, and fluctuating temperatures. Over 7 to 10 days, move the trays outside, starting with a few hours in a sheltered, shaded location.
Slowly increase the time the seedlings spend outdoors, gradually exposing them to direct sun and cooler nighttime temperatures. Protect them from strong winds and heavy rain. Monitor the weather closely, as consistent exposure to temperatures below 50 degrees Fahrenheit for more than 10 days can cause the plants to prematurely bolt.
Once hardened, transplanting should occur four to six weeks before the last expected frost, as soon as the soil is workable. The garden bed requires a loose, crumbly soil structure, as dense or compacted earth inhibits bulb growth. Amending the bed with aged compost and ensuring a soil pH between 6.0 and 6.8 promotes optimal nutrient availability and drainage.
Plant the seedlings no deeper than one to one and a half inches, setting the base of the bulb just beneath the soil surface. Space the transplants approximately three to six inches apart in the row; closer spacing yields smaller, but still marketable, bulbs.
Maintenance for Mature Bulb Development
Onions are shallow-rooted and demand consistent moisture and zero competition from weeds to develop large, healthy bulbs. The soil should be kept evenly moist, requiring about one inch of water per week, delivered through deep, infrequent soakings. Overhead watering should be done in the morning to allow foliage to dry quickly, minimizing the risk of fungal diseases.
Weed control is paramount, as the onion’s non-aggressive root system cannot effectively compete with weeds for water and nutrients. Any cultivation should be shallow to avoid damaging the delicate, fibrous roots near the surface.
Onions are heavy feeders, requiring a nitrogen-heavy fertilizer applied every two to three weeks during the vegetative growth phase to maximize leaf production. Since each leaf layer corresponds to a ring in the final bulb, maximizing foliage size correlates directly to a larger harvest. Stop all nitrogen fertilization once the bulb begins to swell and push the soil aside, approximately four weeks before the estimated harvest date. Late-stage nitrogen encourages thick necks and soft bulbs, which compromises the onion’s ability to cure and store long-term.