How to Grow Onions from Seed. You can start seeds indoors, sow seeds directly outside, or use the winter sowing method to grow onions from seed. It is simple to plant seeds directly outside. There will be no need to fuss with an indoor seed-starting setup or transplant seedlings later. Starting seeds indoors during the winter, on the other hand, will put you several weeks (if not months) ahead of schedule.
In the spring, you’ll have onion seedlings ready to transplant into the garden. This is a huge benefit for northern gardeners with short growing seasons! Because the direct sowing method will not allow the onions enough time to mature, seeds should be started indoors.
Sowing Onion Seeds Directly Outside:
Plant onion seeds outside in late winter to early spring, once the ground in your area is no longer frozen and can be worked. Before you begin, read the soil preparation and site selection tips below. Then, sow onion seeds about 1/4 to 1/2 inch deep and 4 inches apart.
To promote germination, lightly cover the seeds with soil and keep them damp. To avoid crowding, thin to one onion seedling every 4 inches once they sprout. (Note that this method will result in slower and less consistent germination rates, and it is not recommended for northern climates.)
Growing an Onion from Seed Indoors:
Use a sterile seedling potting medium, keep the soil damp but not soggy, and provide at least 12 hours of bright light per day after germination to achieve common best practices for indoor seed-starting. Seedling trays, small containers or pots, and a grow light or other supplemental light fixtures are all required. Onion seeds will sprout most quickly in consistently warm soil (70-75°F, a seedling heat mat will help!) but can then be moved to a cooler location in the 60s.
The warm, comfortable indoor conditions will encourage onion seeds to sprout and grow quickly. Trim the tips of their greens to 3 or 4 inches when they reach 5 or 6 inches (or taller). This promotes thickening, and stronger root development, and keeps the plants from flopping over. Consume the trimmed bits as green onions!
I do not thin my onion seedlings until they are ready to be transplanted outside. Allowing a handful of seedlings to grow together in one cell or small pot, I gently pull them apart to plant separately when it’s time to transplant. Onions respond well to this treatment, so don’t be afraid to plant multiple seeds per container!
Method of Winter Sowing:
The ‘winter sowing method’ is a final option for growing onions from seed. When you sow seeds inside a covered container but do so outside, you are doing it outside. As the name implies, start your onion seeds outside during the winter, which is typically between late December and February in most locations. The seeds will remain dormant in cold outdoor conditions and sprout in the spring when the conditions are favourable. The seedlings that have already hardened off can then be transplanted to their final location in the garden. Winter sowing combines the advantages and disadvantages of the other options: you won’t get the same early advantage as starting indoors, but it’s almost as low-maintenance as sowing seeds directly outside.
To grow onions in the winter, choose a wide shallow container with drainage holes that can be covered with a clear or opaque dome or lid (or, one that already has one). Add at least a couple of inches of seed-starting soil to the bottom, evenly sprinkle or sow onion seeds inside, and then lightly cover the seeds with 14 to 12 inches of soil. Add one or two air holes to the cover for ventilation.
Winter Sowing:
Choose a wide shallow container with drainage holes that can be covered with a clear or opaque dome or lid to grow onions in the winter (or, one that already has one). Fill the bottom with at least a couple of inches of seed-starting soil, evenly sprinkle or sow onion seeds inside, and then lightly cover the seeds with 14 to 12 inches of soil. For ventilation, add one or two air holes to the cover.
Two to four weeks before the final date of frost in your location, plant onion sets in the garden. Bury them deep enough so that the bulb is mostly covered by earth, but the pointed tip is still level with or barely protrudes from the soil. Depending on the projected mature size of the cultivar you are planting, place onion sets 4 to 6 inches apart. Speaking of diversity, don’t assume that all of the onion sets you buy at your neighbourhood grocery shop will thrive there. Large-format garden centres are well-known for carrying varieties that are terribly zone-inappropriate. Small, locally operated nurseries ought to provide a far wider variety.
Onion Production from Sets:
Two to four weeks before the final date of frost in your location, plant onion sets in the garden. Bury them deep enough so that the bulb is mostly covered by earth, but the pointed tip is still level with or barely protrudes from the soil. Depending on the projected mature size of the cultivar you are planting, place onion sets 4 to 6 inches apart. Speaking of diversity, don’t assume that all of the onion sets you buy at your neighbourhood grocery shop will thrive there. Large-format garden centres are well-known for carrying varieties that are terribly zone-inappropriate. Small, locally operated nurseries ought to provide a far wider variety.
