The final size of the onion bulb you harvest is directly determined by the amount of physical space you provide it in the soil. Forming a large, dense bulb requires the plant to have unrestricted access to nutrients, water, and sunlight throughout its entire growing season. Understanding the specific distance required between plants and rows is the most important factor for achieving a harvest of large, satisfying onions instead of small, crowded bulbs.
Why Spacing is Crucial for Onion Growth
Proper spacing is required for bulb onions because it manages competition and air circulation. Onions are heavy feeders, and planting them too closely causes root systems to compete aggressively for resources in the soil. This competition for water and nutrients results in stunted growth, leading to smaller, underdeveloped bulbs that may not cure or store well.
Tight spacing creates a microclimate that encourages fungal diseases. When neighboring leaves touch, they trap moisture and block air movement around the base of the plant. This humid environment is ideal for pathogens like neck rot or downy mildew, which can compromise an entire crop. Adequate distance allows sunlight and wind to penetrate the foliage, keeping the leaves and the soil surface dry.
Standard Spacing Measurements for Bulb Onions
For gardeners seeking to grow fully mature, large bulbs suitable for long-term storage, precise spacing guidelines must be followed. The distance between individual onion plants within a row, known as in-row spacing, should be approximately 4 to 6 inches for standard varieties. Increasing this distance to 6 inches is beneficial for growing large, sweet varieties like Walla Walla, which require more room to maximize their bulb size.
The space required between parallel rows, or row spacing, is generally 12 to 18 inches. This wider measurement ensures the gardener has enough room to cultivate, weed, and side-dress fertilizer without damaging the growing bulbs. Onion sets or transplants should be placed shallowly, typically just 1 to 2 inches deep, with the base of the bulb or roots covered to avoid inhibiting the final bulb swelling.
Adjusting Spacing Based on Planting Method
The final spacing required for a mature bulb is constant, but the initial planting strategy varies based on whether you use sets, transplants, or seeds. Onion sets (small, partially grown bulbs) and transplants (pencil-thick seedlings) should be placed directly at their final spacing of 4 to 6 inches apart, as they are intended to grow to full size without being moved.
Starting onions from seed requires a multi-stage approach involving intentional overcrowding followed by thinning. Seeds are initially sown densely, about one-half inch apart, to ensure a good stand of seedlings emerges. Once seedlings are several inches tall, the first thinning reduces competition, leaving plants about 2 inches apart. These removed young onions can be harvested and used as green onions or scallions.
The final thinning must then establish the required 4 to 6 inches of space between each remaining plant. This allows them to fully mature into large storage bulbs. This staged approach maximizes space use by providing a harvest of young green onions while preparing the remaining plants for a successful bulb harvest.