Onion is a cool-season vegetable that grows into bulbs from seeds. Onion seeds germinate at a temperature between 45 and 81 degrees Fahrenheit, says North Dakota State University. Most homes provide the right kind of heat for the germination of onion seeds indoors. Transplanting onion seeds outdoors is a cheaper option than buying onion plants each year. Choose long- or short-day varieties depending on the sunlight available in your home, recommends the University of Minnesota.
Fill your seed trays with sterile vermiculite.
Plant onion seeds between 1/4 and 1/2 inch deep in the sterile vermiculite. Onion seeds are best planted three per cell or in dense rows when planted in 4- to 6-inch deep containers, says the University of Minnesota Extension.
Water the soil so that it is moist, and keep the soil moist while the seeds are germinating.
Place the tray by the window to raise the soil temperature and germinate the seeds. Keep the onion tops trimmed to 3 or 4 inches tall.
Harden the onion plants by setting them outdoors, after the threat of frost has passed, for longer periods each day before planting, says the University of Minnesota. This prevents cold damage.
Plant the seedlings in the early spring, spaced 4 to 5 inches apart, with 12 to 18 inches between rows, says the University of Illinois.