Onions are a cool-season vegetable. The type of onion you plant depends on where you live. In Southern gardens that have short summer days of less than 12 hours, short-day varieties are plant. In the North, where days can be 14 hours or longer, plant long-day types. Starting onions from seeds is the surest way to get large bulbs, regardless of the onion variety planted, according to Utah State University Extension. Start seeds indoors six to eight weeks before you transplant them out into the garden in late spring to ensure they have time to reach maturity.
Fill a seed-starting flat with a fine-textured potting mixture. Fill a tray with 1 inch of water and set the flat in the tray. Drain the excess water from the tray once the surface of the soil mix in the flat is moist.
Sow seeds on the soil surface, sowing approximately 2 seeds per inch in rows that are 2 inches apart. Cover the seeds with a 1/4-inch layer of soil, then place the flat inside a plastic bag. The bag helps retain moisture in the potting mix during germination.
Set the flat in a warm room to germinate. Remove the bag once sprouts appear, usually within five to seven days of planting.
Place the tray in a sunny windowsill, such as a south-facing window. Water the onions when the surface begins to feel dry. Fill the tray with water and set the flat in it until the soil surface becomes moist, then empty the tray.
Trim the onion leaves once they reach 5 to 6 inches tall. Cut them back to a 3 to 4 inch height with a pair of shears. Trimming prevents the onions from going to seed early.