The onion, a member of the allium family, is related to garlic, chives, leeks and shallots. Most onion varieties exhibit characteristics of biennials, producing foliage during their first growing season before flowering the following year. Onions can be started from seed, sets or transplants. While planting sets, immature onions started from seed the previous year, is the easiest method, most onions may be grown from seed as well. Due to the longer growing season, start your onion seeds indoors before transplanting to your garden after the last frost.
Begin sowing your onion seeds eight to 12 weeks before the last spring frost. Fill your seed trays three-fourths full of a seed starting compost. Moisten the soil until it is as wet as a damp sponge.
Sprinkle three to five onion seeds per cell on the surface of the soil. Cover the seeds with a thin layer of vermiculite, and moisten until the soil is as wet as a damp sponge.
Label each cell with the name of the onion variety sown there, along with the date of planting. This will help you distinguish among the seedlings when you transplant into your garden.
Water your seedlings regularly to maintain the same level of moisture as a damp sponge. As the seedlings begin to emerge, trim the shoots back with sharp scissors or pruning shears to a height of 4 inches every week or two.
Harden off your onion seedlings after the final frost. Place the seedlings outdoors in the warmest part of the afternoon to start, and gradually increase their exposure to the outdoors over a two week period before transplanting into your garden.