If you don't have room in the garden to plant onions, consider growing these root vegetables in pots. Onions grow well in large containers where the bulbs have room to develop. The plants need little additional room between the bulbs, so you can grow a decent-sized crop in a small space. Container gardens are not as prone to weeds or soil-born pests, but pot-grown onions do require some additional care if they are to thrive.
Fill a large pot that is at least 12 inches deep and 24 or more inches wide with a sterile potting mixture. Use a soil-less mixture that is rich in organic matter such as peat or compost so that it drains well without drying out.
Mix in a slow-release fertilizer with the soil just prior to planting. Apply a balanced fertilizer at the rate recommended on the label.
Set the pot in an area that receives at least six hours of direct sunlight a day. Morning sun is preferable to afternoon-only sun.
Water the soil from the top of the pot until the excess moisture begins to drain from the bottom. Wait two hours then water in this manner a second time, as this ensures the soil absorbs the maximum amount of moisture prior to planting.
Sow onion seeds 3 inches apart in rows that are also 3 inches apart. Cover the seeds with ¼ inch of potting mixture. Seeds germinate in seven to 10 days.
Water the soil in the pots when the top 1-inch of soil begins to feel dry. Water from the top until the excess moisture begins to drain from the bottom. Pots may need daily watering because they tend to dry out more quickly than garden beds.
Lay a 1- to 2-inch layer of straw mulch over the top of the onions once the leaves are at least 3 inches long. Mulch helps preserve moisture in the pot, but it also protects the tops of the onion bulbs from sunscald.
Harvest the onions when the leaves begin to yellow and fall over. Dump out the container onto a tarp and separate the bulbs from the soil. Add the soil to the compost pile after harvest.