The easiest way to grow onions--both large dry onions and small green onions--in the home garden is from small onion bulbs, which in the gardening world are better known as "onion sets." They require little to grow and thrive and are available for harvest the first year, unlike onions planted from seeds, which take two years to harvest. Plant onion bulbs in the spring when you can finally work the soil and temperatures stay above 48 degrees F.
Choose onion bulbs labeled for planting at your local nursery or from an online or catalog nursery. They should be less than 1 inch in diameter, about the size of a marble. Onion bulbs should be firm and free from sprouts.
Turn over the garden bed about 6 to 8 inches deep with a hoe or tiller. Mix in a couple of inches of organic matter to improve the soil's fertility and water drainage. About 2 or 3 inches of compost, peat moss or another organic matter will suffice.
Plant onion bulbs 1 1/2 to 2 inches deep in rows that are 12 to 18 inches apart. Space bulbs (use the largest ones) that you want to grow into large dry onions 3 to 4 inches apart to allow sufficient room for growth. Space the smaller bulbs you want to grow as green onions so close they are almost touching. Water the area with 1 inch of water.