Whether you struggle with a lack of suitable gardening space or you just enjoy the endeavors of container gardening, many vegetables grow energetically in containers instead of in the soil. As long as you select large and deep containers that will accommodate the root growth, almost any vegetable will thrive in a container. Grow garden onions in a container for a harvest of either green or bulb onions.
Time the planting approximately three months before you desire onion bulbs for harvesting or approximately one month before you desire green onions for harvesting. Growing onions in containers gives you the freedom to plant at any time of the year as long as you can keep the growing environment between 55 and 75 degrees F.
Fill the planting container almost to the top with potting soil.
Plant onion seeds over the soil surface (spacing is not important, you will thin the onion seedlings later) and add 1 inch of potting soil over the seeds. Alternatively, plant onion sets in the container so the tops of the onion sets show above the soil level. Plant the sets in rows with 4 inches between each set, spacing the rows 2 inches from the side edges of the container.
Provide a generous watering of the onion seeds or sets immediately after you finish watering them. Keep the soil evenly moist at all times, especially if you sowed seeds, providing at least 1 inch of water each week.
Place the container in a location that receives at least six hours of direct sunlight each day. If you start the onions indoors, move them outdoors when the weather warms above 60 F during the day--onions can tolerate overnight frosts, however. Harden the onions off prior to moving them outdoors completely by keeping them outdoors for part of each day over a one-week period. After the onions grow accustomed to the outdoor environment, keep them outdoors in a sunny location.
Thin onion seedlings (grown from seeds) when the seedlings reach 4 inches in height. Pull out the weakest seedlings to leave the remaining seedlings growing 4 inches apart in the container.
Fertilize the onion plants twice per month during the growing season by mixing the fertilizer with water according to package recommendations for the container size. Pour the fertilizer over the potting soil carefully to saturate the soil.
Harvest green onions when the greens reach 6 inches in height. Harvest bulb onions when the greens begin falling over and turning brown. Dig the onions out of the soil and allow them to dry in a warm location for one week before using them.