Onions are cool weather vegetables commonly found in the home garden, but they can also be grown indoors year-round by determined gardeners. They require a container deep enough for bulbs to form, exposure to bright light and soil with excellent drainage. Positioning the chosen container in an area with plentiful sunlight works well during the regular growing season, but artificial lighting is necessary during fall and winter months. Choose an onion variety that produces small heads, as vegetables in containers have limited room to grow.
Fill a 1/2 gallon container with lightweight potting soil. The container needs at least one hole in the bottom so that excess water can escape.
Incorporate 1/4 tbsp. of 10-10-10 timed-release fertilizer with the potting soil.
Plant the onion sets one inch deep. Plant them closely enough to touch one another if you plan to harvest them as green onions. Allow a space of two inches between each set if harvesting the onions when they are fully mature.
Moisten the soil lightly and place the container in a south-facing window.
Monitor the soil closely and water only when the top half-inch feels dry. Avoid letting the soil dry out completely.
Feed every other week with 15-30-15 water-soluble fertilizer, applied at half the recommended strength, after steady growth begins.
Position the container beneath plant growing lights during fall and winter months.
Harvest green onions when their stalks are approximately six inches tall. Harvest dry onion bulbs after the stalks have fallen over.
Make repeated plantings throughout the year. Use fresh potting soil for each planting so that nutrients aren't depleted.