Grow onions in containers the fun and easy way! Onions require very little space in which to grow so that makes them a great choice for growing in a container. They also have a short growing season so you will be able to use the same container for two different crops if you time your planting just right. Use these tips and get ready to successfully grow your own onions in containers.
Select a container that is at least 12 inches deep so the onion roots will have plenty of room in which to spread out. A wide container of at least 18 inches is also needed so onions will have enough room to develop large heads. Place a coffee filter over each bottom drainage hole in the container prior to adding the potting soil mix to prevent soil from leaching out during watering.
Use a good quality potting soil mix that contains compost, vermiculite or peat to prevent soil compaction during the growing season. Mix in one-half cup of balanced 10-10-10 or 13-13-13 granulated fertilizer into the potting soil. Water potting soil mix until water runs out of the bottom drainage holes, then allow it to sit 24-48 before planting onions.
Plant onions at the end of March or first week in April. Planting onion sets (also called seedlings) of the desired variety will yield quicker results than starting with seeds. If you do start with seeds, sow seeds into prepared potting soil. Let grow until above-soil shoots reach 4-6 inches tall, then transplant into larger, prepared container.
Make shallow, 1 inch-deep rows in soil 12-18 inches apart. Plant seedlings in the prepared rows, allowing 1 inch of space between green onions and 4 inches of space between slicing onions. Place seedling with the roots facing down, cover with ½ inch of soil and water in well.
Place container in a location that will receive full sun all day. Apply a two inch layer of straw around the onions to help soil retain moisture. Place mulch against stem base to encourage more stem growth on green onions (scallions). Keep soil moist during growing season.
Harvest green onions when the green stems reach about 6-8 in height. Leave all other onion varieties in the container until the tops fall over. Place onions in a single layer on newspaper in a cool, dry place to dry out before storing. Store them hanging up in a mesh bag in a cool, dry location.