Onions (Allium cepa) thrive in cool, frost-free weather, growing as a spring or fall crop in most climates, or a winter crop in mild areas. Growing the onions from sets, which are small onions bulbs, allows you to raise onions quickly without fear of a late spring frost damaging the plants. Sets also ensure the onions have time to reach maturity before summer heat damages the bulbs. Onion sets grow exceptionally well in a raised bed because of the improved soil and drainage. Sowing the sets just a few inches apart lets you harvese a large quantity of onions from a small space.
Mix a 2-inch-thick layer of compost and 1/4 pound of all-purpose 16-16-8 fertilizer into every 25 square feet of soil in a raised garden bed. The compost and fertilizer revitalizes the soil with nutrients and organic matter, which results in better onion growth.
Arrange the onion sets on the soil surface with their pointed ends up. Space the sets 4 inches apart in a row, setting the rows 4 inches apart. Plant in long rows as you would in a traditional garden bed, or plant the onions in 12- by12-inch blocks with each row no more than 12 inches long. You can grow nine onions per square foot with block planting.
Push the onion set into the soil so only the tip of the bulb is visible. Onions develop near the the soil surface, with the bulb visible as it begins to mature.
Water the bed immediately after planting to settle the soil and provide some moisture for the newly planted onions. Supply about 1 inch of water, or enough to moisten the soil in the raised bed to its full depth.
Spread a 2-inch layer of straw mulch over the bed once the onions put out foliage. The mulch conserves moisture and protects the developing onions from heat damage.