Two main types of onions exist: Green onions are grown for their mild flavored green stalks while bulb onions are grown for their mature underground bulbs. Green onions are usually grown from onion sets, which are small onion bulbs that are planted out in the garden. Sets require less work than growing bulb onions from seed, and they are also suitable for growing in pots or in the herb garden. You can also plant some sets for bulb production, though be aware this won't be as successful as planting by seed.
Prepare a garden bed in full sunlight for planting in mid-spring when temperatures are consistently above 48 degrees F. Lay a 2-inch layer of compost over the bed and till it in to a 10-inch depth to improve drainage and soil quality.
Sow each set to a 2-inch depth with the pointed end facing upward. Plant larger sets, or those bigger than ¾ inch in width, close enough that they nearly touch. Plant smaller sets that are between ½ and ¾ inch wide 3 to 4 inches apart if you wish to produce bulbs from them. Space the rows 1 foot apart.
Water once a week as needed to keep the soil moist, providing approximately 1 inch of water to the bed at a single irrigation. No additional water is needed if there is ample rainfall during the week.
Lay a 2-inch layer of organic mulch, such as straw, over the bed once the onions sprout. Mulch preserves soil moisture and prevents weeds in the bed.
Harvest green onions when the stems are at least 5 inches high. Pull them up from the ground, roots included, and use immediately. Harvest bulb onions when most of the leaves have browned and fallen over.