Growing onions in a raised bed is an excellent choice for maximizing yield and controlling the growing environment. Raised beds naturally offer superior drainage and allow for the creation of an ideal, loose soil structure, both of which are highly beneficial for proper bulb formation. This controlled setting allows a gardener to focus on precise horizontal and vertical placement, which are the primary factors determining the size and health of the final harvest. Understanding the specific distance required between plants ensures each onion receives adequate resources to develop fully.
Determining Horizontal Spacing Requirements
The amount of space an onion needs horizontally depends entirely on the desired end product. For full-sized, mature dry bulbs, the standard spacing is three to four inches, measured from the center of one plant to the center of the next. This distance provides sufficient room for the bulb to expand laterally without competing for nutrients or sunlight. To achieve jumbo-sized bulbs, increase the spacing to six or even eight inches for maximum individual growth potential.
If the goal is to harvest green onions or scallions, a much tighter spacing is used. Since these are harvested before the bulb swells significantly, plants can be placed as close as one to two inches apart, maximizing the harvest in a small area.
Planting Material Choices and Initial Placement
Onions can be started from seeds, small transplants, or sets, and the choice affects the initial planting density. Onion sets, which are small, immature bulbs from the previous season, are a popular choice because they are the fastest to mature. These are typically planted at the final spacing of four to six inches, as they are not meant to be thinned later.
Transplants, which are small seedlings sold in bunches, are similarly planted at or very close to the final desired spacing for mature bulbs. If using seeds, however, they must be sown densely, often one-half inch apart, to ensure good germination and a healthy stand. This dense initial placement requires management later in the season.
Intensive Planting and Thinning Techniques
Raised beds are perfectly suited for intensive planting methods like block planting, which maximizes yield by spacing plants equidistantly in a grid rather than in traditional rows. For mature bulbs, a four-inch by four-inch grid is a highly efficient use of the limited space in a raised bed. This approach ensures every plant receives an equal share of the available light and soil resources.
When starting with densely sown seeds, thinning is a mandatory process for successful bulb formation. This technique involves removing the smaller, weaker plants to leave the strongest ones behind, ensuring the final desired spacing is reached. Thinning should occur once the onion tops have established substantial growth. The removed plants are not wasted; they can be immediately harvested and utilized as fresh green onions.
Thinning provides a secondary harvest of scallions and eliminates competition for the remaining plants. By removing the crowded plants, the survivors receive the necessary four to six inches of horizontal space to fully develop their bulbs. Without this thinning, the plants would crowd each other, resulting in numerous small, underdeveloped bulbs instead of a few large ones.
Essential Depth and Soil Preparation
The vertical placement of the onion is just as important as its horizontal spacing. Onions are shallow-rooted, and the bulb forms near the soil surface, not deep underground. Sets and transplants should be planted very shallowly, only covering the roots and the base of the plant. It is crucial that the shoulder of the bulb remains exposed to the air.
Planting too deeply or allowing soil to mound up around the neck of the plant can impede proper bulb enlargement and development. For onion sets, the pointed tip should be left just proud of the soil surface. This shallow placement supports the natural process of the bulb pushing itself upward as it matures.
A loose, well-draining medium is paramount for successful onion growth. The soil should be rich in organic matter to provide consistent nutrition. While onions are heavy feeders, avoid excessive nitrogen, as this promotes leaf growth at the expense of bulb formation. Focus instead on a balanced fertilizer application that supports the growth of the bulb itself.