Onion sets are small, dormant onion bulbs that are ready to be planted outdoors. Their best use is for green onions. Harvesting the green onions is a good way of thinning the row so the remaining onions are 3 to 4 inches apart. Onion transplants can be purchased at garden centers and through mail order catalogs. Plant transplants 3 to 4 inches apart and about 1 inch deep.
Onions are shallow rooted.
Cultivate carefully to control weeds and avoid damaging the root system. Onions, like other garden vegetables, require an inch of water a week during the growing season. The size of the onion bulb is dependent on the green growing portion of the plant. For each green leaf there will be a ring in the bulb. The larger the leaf above the soil, the larger the ring in the bulb.