Saving seeds from your favorite onion variety is a low-cost way to grow them in the vegetable garden each year. Onions are a biennial plant, so they must be left in the ground and protected over winter in order to produce seeds the following year. Gathering and saving the onions seeds is only half the process. They have to be planted properly the following year in order to produce large, tasty onion bulbs for harvest and storage.
Cover onion plants with a 3- to 4-inch layer of straw mulch in fall. This protects the onions over winter so they continue to grow the following year. Remove the mulch once the plants begin actively growing again in spring.
Check the plants daily once flowers begin opening in early to mid-summer. Cut off the flower stalks as soon as the flowers die and begin to dry.
Lay the stalks on paper plates in a warm, dry room for two weeks to finish drying. Rub the dried flower heads between your fingers to release the seeds.
Remove the excess flower petals and other debris from the seeds. Store them in a sealed jar in a dark, cool place until you are ready to replant.
Lay a 2-inch layer of compost over a well-drained garden bed in full sun. Work the compost into the top 8 inches of garden soil with a hoe, spade or power tiller.
Sow the onion seeds as soon as the ground thaws enough to work in spring. Plant each seed 1 inch deep and space them 1 ½ inches apart in rows. Space the rows 1 foot apart.
Water the bed thoroughly after planting until it is evenly moist. Keep the bed moist but not soggy until plants begin growing. Onions require approximately 1 inch of water a week.
Thin the onion plants so they are 3 inches apart in the row once the plants develop five leaves. Pluck out the extra plants and dispose of them.
Lay a 2-inch layer of mulch over the bed to prevent weeds and preserve moisture. Pull out any weeds as soon as they are noticed, as onions do not tolerate competition.
Dig up the onion bulbs once the leaves and stalks die back in fall. Leave some in the ground over winter so you can harvest more seeds next year.