Starting Onions From Seed With a Heat Light

Fill each section of a plant tray with a soil-less peat moss potting mixture, leaving about 1/4 inch of space at the top. Soak the potting mixture with warm water, about 100 degrees Fahrenheit. Most soil-less potting mixtures do not absorb cool water well. Allow the plant tray to cool for about 10 minutes.

Sprinkle three onions seeds in each section of the plant tray. Cover the seeds with 1/4 inch of soil-less potting mixture and fill a spray bottle with warm water. Mist the dry soil-less potting mixture until dampened and place the lid on the planting tray.

Set the plant tray in a warm area and position a heat light over the center, about 12 inches away. Spray the mixture with warm water whenever it begins to dry, or about once every three days. Within two weeks, the onion seeds should begin sprouting.

Remove the plant tray’s lid for one hour a day once the onions begin sprouting. After three days, increase their lidless time to three hours per day; after six days, increase it to six hours per day; after nine days, increase it to 12 hours per day; and after 12 days, completely remove the lid. This process hardens the seedlings to withstand the environment outside of the tray. The seedlings require more frequent watering with the lid removed.

Allow the seedlings to continue growing in the plant tray for about six weeks before planting them in the ground. Onions are hardy in the cold, but seedlings can die quickly if their root structure is not strong enough.