Onion seeds offer the widest selection of varieties for growing large, flavorful bulbs. Successful bulb development depends heavily on managing initial planting density and subsequent spacing. Gardeners commonly plant more seeds than the number of mature onions desired, which insures against poor germination or early seedling loss. This initial density requires careful management afterward to ensure each plant has enough space to grow a large, healthy bulb.
Preparing the Seedbed and Timing
Onions thrive in loose, well-draining soil with high organic matter content. Before planting, cultivate the seedbed to a depth of at least six inches to prevent root restriction, which limits bulb size. Mixing in aged compost or a balanced fertilizer provides the high nutrients these “heavy feeders” require for leaf growth, as each leaf layer corresponds to a ring in the final bulb.
The timing of planting is crucial because onion bulb formation is triggered by day length, a process known as photoperiodism. Gardeners must select a variety—long-day, short-day, or intermediate-day—that matches their geographical latitude. Long-day varieties require 14 to 16 hours of daylight and are suitable for northern regions, while short-day varieties need 10 to 12 hours for bulbing and are grown in the South. Planting should occur early in the spring, typically in late March or early April, allowing the plant time to grow substantial foliage before bulbing begins.
The Initial Seeding Density
To maximize the chance of a successful stand, sow multiple seeds for every intended final plant location. For a direct-sown row, plant seeds thickly, roughly one seed every half-inch in the furrow. Alternatively, plant a cluster of two to four seeds if using a specific “hole” or cell.
This high initial density acts as a safeguard because onion seeds have a short shelf life and unpredictable germination rates. Planting a small cluster ensures at least one viable seedling emerges at the desired spot. Seeds should be planted shallowly, at a depth of only one-quarter to one-half inch, and covered lightly with fine soil, as planting too deep negatively affects bulb development.
The Critical Step of Thinning
Thinning is a non-negotiable practice for gardeners aiming to produce large, mature storage onions. Competition among crowded seedlings for water, nutrients, and sunlight results in small, stunted, or misshapen bulbs. Each onion plant needs dedicated space to maximize leaf production, which directly determines the size of the final bulb.
Thinning should commence once seedlings reach two to four inches in height and have developed their first true leaves. The goal is to reduce density to the final spacing of four to six inches between individual plants. When removing seedlings, cut them off at the soil line with scissors or shears rather than pulling them out. Pulling a seedling can easily disturb the shallow root systems of the adjacent plants intended for keeping.
Adjusting Seeding for Desired Outcome
The initial seeding density should be contextualized by the gardener’s ultimate harvest goal. If the objective is to grow large, long-storage bulbs, strict adherence to the four to six-inch final spacing is necessary. This requires diligent thinning to ensure each plant receives maximum resources and forms a substantial bulb.
If the primary purpose is to harvest green onions or scallions, the seeding strategy changes significantly. Green onions are harvested before they form a large bulb, allowing them to be grown much more densely. Seeds can be planted as little as one to two inches apart and harvested continuously without strict thinning, yielding more usable greens from a smaller area.