Proper spacing determines the final size and quality of an onion harvest. Gardeners often use small, immature bulbs known as “sets” to get a head start over seeds or young transplants. Sets are miniature onions grown the previous season and kept dormant. Providing the correct space allows the plant to fully develop its root system and foliage, which is necessary for successful bulbing. Without adequate room, mature bulbs remain small and underdeveloped.
Standard Spacing for Maximum Bulb Size
Achieving large, mature storage onions requires a precise balance of space for optimal resource absorption and growth. The in-row spacing, which is the distance between each individual onion set, should be between four and six inches. This ensures each growing bulb has sufficient space to expand laterally without excessive competition for nutrients and water.
Rows should be spaced twelve to eighteen inches apart, measured from the center of one row to the center of the next. This wider gap ensures adequate air circulation, helping to prevent fungal diseases like downy mildew. It also provides the gardener with enough room for cultivation and weeding throughout the growing season. Overcrowding is a common reason for a harvest of small, irregularly shaped bulbs.
Planting Depth and Placement Technique
Beyond horizontal spacing, the vertical placement of the onion set is important for bulb formation. Onion sets should be planted shallowly, typically between a half-inch and one inch deep. Place the small set with the pointed end facing upward, where the leaves will emerge, and the flatter, root end facing down.
Planting the set too deep hinders the final bulb’s expansion because the soil restricts lateral swelling. Planting too shallowly leaves the set vulnerable to being pushed out of the ground by freeze-thaw cycles or irrigation. The goal is to barely cover the set with soil, allowing the developing bulb to sit partially on top of the soil line as it matures. Gently firm the soil around the set after planting, avoiding excessive compaction.
Adjusting Spacing for Different Harvest Goals
Standard spacing can be manipulated to achieve different types of harvests. If the goal is to produce green onions, also known as scallions, a much closer spacing is recommended. Planting onion sets one to two inches apart encourages the production of long, slender, non-bulbing greens.
This close spacing creates competition for resources, preventing the bulb from swelling into its mature shape. The plant focuses its energy on rapid foliage growth, resulting in a tender, immature stalk harvested before the true bulb fully develops. This technique yields a higher number of plants per square foot, providing a continuous supply of green onions.
Managing Density Through Thinning
A common strategy is to plant onion sets closer together initially and then actively manage the density as they grow. Sets can be planted at a tight spacing of two to three inches apart to ensure a high yield of plants. When the foliage starts to compete for sunlight, a process called thinning must be performed.
Thinning involves removing every other plant in the row to give the remaining onions the required four to six inches of space for large bulb development. This is typically done when the onion tops are about six to eight inches tall. The removed plants can be immediately harvested and used as fresh green onions. This practice serves a dual purpose: yielding an early crop of green onions while ensuring the remaining bulbs have the necessary space to grow into mature storage onions.