Onions come in two main varieties: long-day and short-day. Gardeners living north of 35 degrees latitude should plant long-day onions while those living south of 35 degrees latitude should plant short-day onions. Most gardeners grow onions from sets, small seed onions sold by the bunch. Popular varieties include White Sweet Spanish, Walla Walla Sweet and scallions.
Cool temperatures aid in onion growth, so plant the sets in early spring. Green onions can be spaced as close together as 2", while larger storage onions should be spaced 6" to 8" apart. Poke a hole in the soil with a finger and place the sets 1 1/2" deep. Gently press the dirt around them and spray the rows with a misting of water.
Seedlings need to be watered everyday until they get established. An even amount of moisture is best, so check the soil each time before watering. Keep the onion beds free of weeds. Add a layer of mulch around the onion plants to help keep weeds down and moisture in.
To develop tender white stalks on green onions, mound soil up and around the stem when plants reach about 4".
Onions develop in two stages. First the tops of the plants fill out. Then the plant uses the energy from the leaves to mature the bulb. Green onions can be pulled and eaten at almost any stage. Larger onions are ready for harvest when the tops bend over and the plant begins to look less vibrant. Harvest onions by hand so as to not damage the bulb. Cure onions for storage by laying them outside in a dry shady place.