Onion sets are small, immature onion bulbs stored for replanting, giving gardeners a head start compared to planting seeds. Correct spacing is the primary factor determining the final size and health of the mature onion bulb. The space an onion set receives dictates how much light, water, and nutrients it can access. Ignoring proper spacing leads to smaller, underdeveloped bulbs due to competition for limited resources. The specific distance chosen depends entirely on your desired harvest.
Spacing for Specific Outcomes
The primary decision a gardener faces is whether they want to grow large, dry storage bulbs or smaller, immature green onions, sometimes called scallions. These two goals require vastly different planting densities. For large, storage-grade bulbs, the goal is to maximize resource availability for each individual plant. This requires a generous amount of space for the bulb to expand laterally in the soil.
To achieve large, mature onions suitable for long-term storage, aim for 4 to 6 inches between each set in the row. This distance ensures each bulb has adequate soil volume to draw nutrients (nitrogen, phosphorus, and potassium). The wider spacing also promotes better air circulation around the foliage, preventing common fungal diseases. This spacing allows the plant to develop a robust root system and larger leaves, contributing to the final bulb size.
In contrast, if the goal is to harvest green onions or scallions, a much tighter density is acceptable. Plant the onion sets 1 to 2 inches apart. This close spacing works because the plants are harvested before they reach their full bulbing stage. They are pulled while the leaves are still green and the bulb is only slightly swollen, meaning they do not require the expansive space needed for a full-sized bulb.
Row Layout and Planting Depth
While in-row spacing is critical for bulb development, the distance between rows is equally important for garden management and plant health. Rows should be spaced 12 to 18 inches apart. This separation allows easy access for necessary tasks like weeding, which is vital since onions are poor competitors against weeds.
Row spacing also plays a significant role in disease prevention by ensuring adequate airflow through the foliage. Good air circulation helps dry moisture from the leaves quickly after rain or irrigation, minimizing conditions favorable for fungal pathogens. Placing rows too close together creates a humid microclimate, increasing the risk of diseases like downy mildew.
Proper vertical placement of the set is also a factor in successful bulbing. Onion sets should be planted shallowly. Press the set into the soil so that only the bottom half to two-thirds of the bulb is covered. Leaving the pointed tip slightly exposed above the soil line is the correct technique. Planting the set too deeply hinders the lateral expansion of the bulb, forcing it to grow a long neck rather than a round, mature bulb.
Managing Density After Planting
Even when growing large bulbs, it is beneficial to start with a slightly higher density and reduce it later. Gardeners initially plant sets 2 to 3 inches apart to ensure a full stand, knowing they will remove some plants. This intentional overplanting guarantees a sufficient number of healthy plants for the final harvest.
The process of selectively removing plants to achieve the final desired spacing is known as thinning, and it is necessary for large bulb production. Thinning should occur once the onion tops are about 6 to 8 inches tall and the plants begin to crowd. Selectively remove the smaller or weaker plants until the remaining ones are spaced 4 to 6 inches apart.
The removed young onions are perfectly edible and should be used immediately as green onions. This converts a maintenance task into an additional, early harvest of immature scallions. Thinning ensures that the strongest plants remain to mature into large bulbs, receiving the full benefit of available resources.