Onions are cultivated annually for their edible bulb. The goal of planting is to encourage maximum bulb expansion. This requires providing each plant with adequate space to grow and utilize resources without competition. Insufficient distance restricts the bulb’s ability to swell, resulting in small, underdeveloped onions. Proper initial planting distance is the most influential factor in securing a successful yield of full-sized bulbs.
Spacing Based on Planting Material
Onion Sets
Onion sets, which are small, dormant bulbs, require two to four inches of spacing between each set in the row. To achieve the largest possible bulbs, increase this in-row distance to five or six inches to reduce competition. Maintain twelve to eighteen inches between adjacent rows for proper cultivation, weeding, and air circulation.
Transplants
Transplants, or small seedlings, require a similar distance for optimal bulb development. Place them individually, with four to six inches between each plant within the row for medium-to-large bulbs. Planting closer than four inches is acceptable if the intention is to harvest some plants early as green onions, thinning the rest to the final spacing.
Direct Sowing
Direct sowing of seeds requires a different initial strategy focused on germination, not final spacing. Seeds are sown densely, about one-half inch apart, or scattered thickly within the row. This high initial density guarantees a good stand but requires management soon after germination. Rows for direct-sown seeds should still be spaced twelve to eighteen inches apart.
Optimizing Bulb Size Through Thinning
Thinning is necessary after dense sowing or when plants are placed too closely. This process corrects the initial layout, ensuring remaining plants have sufficient room for full bulb expansion. Thinning should occur when seedlings are small, typically four to six inches tall. Carefully remove the weaker seedlings, leaving the healthiest plants to mature, which immediately reduces competition for resources.
The final desired spacing after thinning should be a minimum of four inches between remaining plants. For very large onions, increase the final spacing to six or eight inches. Seedlings removed during this process can be used immediately as tender green onions. Timely thinning is important because delaying removal allows competing plants to draw resources away from the mature bulbs.
Planting Depth and Soil Requirements
Planting depth is a distinct factor from horizontal spacing and directly impacts bulb formation. Onion sets and transplants should be planted very shallowly, only about one inch deep, or just enough to cover the root mass or base of the set. The top tip of the set or the root-to-shoot transition point of the transplant should remain close to the soil surface. Planting too deeply inhibits the bulb’s ability to swell laterally, leading to poorly shaped bulbs.
The immediate soil environment is equally important, as onions are shallow-rooted and require consistent conditions. They thrive best in loose, well-draining soil, ideally a sandy loam, amended with organic matter. This texture prevents compaction that restricts bulb growth and ensures roots access moisture and nutrients.