Starting onions from seed allows you access to a greater variety of onions, as onion sets are sold in a limited variety. Seed-grown onions also are less likely to flower, which results in larger bulbs and longer storage life. Onion seeds are planted and germinated directly in the garden bed, so it is vital to plant at the correct time and provide the proper conditions for germination. The good news is that onions are forgiving and usually germinate as long as the basic requirements are followed.
Prepare the garden in the fall before planting. Lay a 1- to 2-inch layer of mature compost over the entire bed. Mix it into the top 8 inches of soil with a power tiller or hoe.
Sow seeds in early spring once the ground has thawed and after most danger of hard frost is past. Sow seeds 1 inches in rows that are 10 inches apart, planting them on the soil surface. Sprinkle ¼ of an inch of soil over the seeded area to cover the seed.
Mist the soil surface gently with water if the soil is dry. Alternately, water the bed before planting so it is moist but not soggy. Take care not to wash away the onion seeds.
Thin the onions once they have germinated, usually within seven to 14 days of planting. Pluck out the excess onions in each row so the remaining plants are spaced approximately 3 to 4 inches apart.
Place a 2-inch layer of mulch around the onion plants once they are 4 inches tall to protect the bulbs and guard against late-season freezes.