Growing onions from seed provides access to a wider selection of varieties than purchasing sets or transplants. This method requires precision concerning timing and seed placement. Successfully raising a crop of large, healthy bulbs depends on managing the initial germination phase and understanding the specific requirements of the onion plant.
Understanding Onion Day Length Requirements
Onion bulb formation is dependent on the number of daylight hours, a phenomenon known as photoperiodism. Varieties are classified into three main groups based on the latitude where they are grown. Planting the wrong type for your region will prevent the development of a full-sized bulb.
Short-day onions require 10 to 12 hours of daylight to trigger bulbing and are best suited for southern regions. Intermediate-day, or day-neutral, varieties require 12 to 14 hours of light and perform well in transitional zones. Long-day onions need 14 to 16 hours of daylight to mature and are the choice for northern latitudes, where summer days are longest. Each leaf developed before the bulbing signal translates into one ring in the final bulb, making the timing of this trigger important for maximum size.
Preparing the Soil and Determining Planting Time
Onions are shallow-rooted and require loose, fertile, and well-draining soil to expand properly. The ideal medium is a sandy loam with organic matter, such as compost or manure, which improves drainage and moisture retention. A soil pH between 6.0 and 7.0 is recommended for optimal nutrient availability.
Gardeners choose between starting seeds indoors or direct-sowing outdoors. Starting seeds indoors is the common practice for long-day varieties, allowing plants to develop substantial foliage before bulbing is triggered. Seeds should be sown eight to ten weeks before the last expected spring frost. Direct sowing is possible once the soil is workable and the temperature is above 40°F, usually in early spring.
Precise Seed Depth and Initial Spacing
Onion seeds should be sown shallowly, between one-quarter and one-half inch deep. This shallow depth ensures the small seedlings have enough energy to push through the soil surface, while still providing the darkness and contact needed for germination. Covering the seed lightly with fine soil is important, as the emerging root needs to anchor itself quickly.
Seeds can be sown densely, about one-half inch apart in rows spaced 12 to 18 inches apart. This close spacing maximizes the use of seed-starting trays or garden rows and allows for later thinning, which provides an early harvest of green onions.
Post-Planting Care and Thinning Seedlings
After planting, maintaining consistent moisture is important because the shallowly planted seeds can dry out rapidly. The soil surface must be kept damp until the seedlings emerge, which typically takes seven to fourteen days. A heat mat can hasten germination when starting seeds indoors.
Once the seedlings have reached a few inches in height, they must be thinned to allow the remaining plants enough space to form a bulb. This step is necessary for producing large, mature onions. For large bulbs, the final spacing should be four to six inches between plants. If the goal is to harvest smaller bulbs or green onions, a final spacing of two inches is adequate. Thinning should occur when the seedlings are two to four inches tall, and the removed plants can be used as green onions. Adequate spacing ensures good air circulation, which helps prevent disease and reduces competition for water and nutrients.