Growing onions from seed for a fall-planted, overwintered harvest produces large, high-quality bulbs earlier than a spring-planted crop. Starting seeds indoors creates robust seedlings that establish strong root systems before winter dormancy. This head start allows plants to focus energy on bulb formation the following spring, resulting in a significantly larger harvest by late spring or early summer.
Selecting the Right Onion Type
Onion varieties are highly sensitive to photoperiod, the duration of daylight hours required to trigger bulb formation. Selecting the correct type based on your geographic latitude is the most important factor for fall planting success. If you choose the wrong type, the plant will either fail to bulb up or prematurely bolt (go to seed) in the spring.
Gardeners in the Southern United States (zones 7 and warmer) should select Short-Day onions, which begin to bulb when daylight reaches 10 to 12 hours. The mild winters allow these plants to grow foliage throughout the fall and winter, switching to bulbing as the days lengthen in early spring. Gardeners in mid-latitude regions (zones 5 and 6) should look for Intermediate-Day or Day-Neutral varieties, triggered by 12 to 14 hours of daylight.
Calculating the Optimal Start Date
The goal for fall planting is to have seedlings that are about the thickness of a pencil, roughly 6 to 10 inches tall, with four to five leaves before the onset of deep winter or a hard freeze. This size is large enough to survive the cold but not so large that the plant is tricked into bulbing too early in the spring. To determine the seed starting date, you must work backward from your target transplant date.
The ideal transplanting window is typically 6 to 8 weeks before the first expected hard frost in your area, or when temperatures consistently drop below 50°F (10°C). Onion seeds take approximately 8 to 10 weeks of indoor growth to reach the desired pencil-thick size. Therefore, you should sow your seeds indoors 14 to 18 weeks before your anticipated hard freeze date. For example, if your first hard frost usually occurs in late November, you would aim to start your seeds in late July or early August.
It is necessary to look up the specific average first frost date for your local climate zone, as this will serve as your anchor point for the calculation.
Starting Seeds Indoors
Starting seeds indoors is the standard method for fall planting, providing the necessary head start and controlled environment for successful germination. Begin by preparing shallow seed trays or containers that are at least three to four inches deep, filled with a sterile, fine-textured seed starting mix. The mix should be pre-moistened before planting to prevent the tiny seeds from being washed away.
Onion seeds should be sown densely, about one-quarter inch deep. Placing the trays on a heat mat (70°F and 75°F) will significantly speed up germination, which typically occurs within 7 to 14 days. Once the seedlings emerge, they need immediate, bright light from a dedicated grow light source for 14 to 16 hours daily to prevent them from becoming leggy.
If the thin, grass-like foliage exceeds eight inches before the transplant date, trim them down to about six inches using clean scissors. This trimming encourages the plant to put energy into root development and thickens the stem. Maintaining consistent moisture is important, but avoid overwatering, as overly wet conditions can promote fungal diseases like damping-off.
Transitioning Seedlings to the Garden
Before planting the seedlings into the garden, they must undergo hardening off, which slowly acclimates them to the harsher outdoor environment. About 7 to 10 days before the planned transplant date, start by placing the trays in a shaded, protected outdoor location for just a couple of hours. Gradually increase the time the seedlings spend outside each day, exposing them to filtered sun, wind, and cooler overnight temperatures.
For transplanting, gently separate the densely grown seedlings, handling them only by their leaves to avoid crushing the delicate stem or damaging the roots. The soil in the garden bed should be loose, well-draining, and rich in organic matter, as onions struggle in heavy clay. Plant the seedlings approximately one to one and a half inches deep and space them three to six inches apart, depending on your desired final bulb size.
After planting, water the seedlings thoroughly to settle the soil around the roots. In colder climates, applying a layer of organic mulch, such as straw or shredded leaves, after the ground freezes will provide insulation and protect the shallow bulbs from freeze-thaw cycles. This protective layer helps ensure the young onions survive the winter and are ready for rapid growth in early spring.