How to Grow Onions from Seed

Onions are biennials, so if you decide to plant your onions from seeds, they won’t bloom the first year. This is actually a good thing because it gives the bulbs more time to grow. In the second year, you’ll have a crop of large onion bulbs. Starting your seeds indoors will give you healthy onion transplants for spring planting.


How to Start Onion Seeds Indoors

For spring planting, sow your onion seeds 10-12 weeks before you plan to plant the seedlings. This is usually from late February to early March. The seeds sprout within 5 days. Follow these steps for starting your onion seeds indoors:

  • Plant onion seeds thickly, about three-quarter-inches deep in potting soil made for seed starting.
  • Keep the soil moist but not saturated.
  • When the seedlings emerge, thin the plants to 3-4 inches apart.
  • When the outside temperature reaches 50°F (10° C), they’re ready to transplant. They will tolerate light frosts.
  • Locate the garden in an area that gets full sun for at least seven hours.
  • Turn over your garden soil, removing all stones, weeds and debris.
  • Rake the soil until it’s smooth. Optimal soil conditions include well-drained soil with a 6.5 pH.
  • Carefully loosen the seedlings from the potting soil.
  • Trim the onion bulb roots to one-half inch.
  • Trim the onion stalks from 1-4 inches from the bulb.
  • Plant the transplants 2-inches deep and 3-4 inches apart. Separate the onion transplants in rows 1- 1 ½ feet apart.
  • Keep the soil evenly moist throughout the growing season but don’t let the bulbs sit in standing puddles.

How to Plant Onion Seeds Directly in the Garden

You can also plant onion seeds directly in your garden. Prepare your garden soil as soon as it thaws. This is usually in late March. After clearing and raking the soil, make furrows from one-quarter to one-half inch deep in the soil. Space your rows from 1-1 ½ feet apart. Sow the seeds thickly in the furrows and cover them with soil. When the onion plants produce five or more leaves, thin them out so they’re about 3-4 inches apart. The onion plants you pull from your garden are scallions, so you can use them in salads, soups and other recipes where you want an onion flavor.