Onion seedlings are the first stage of development for this cool-season crop. The transition from the protected indoor environment to the outdoor garden is sensitive. Successful bulb formation and a generous harvest depend heavily on transplanting the seedlings at the optimal moment, balancing the plant’s physical maturity and the garden’s environmental conditions.
Onions are photoperiodic, meaning the length of daylight hours triggers the switch from leaf growth to bulb formation. Transplanting too late can cause premature bulbing, resulting in small, undeveloped onions. Proper timing ensures the seedlings establish a robust green top, which gathers the energy needed to produce a large, mature bulb.
Indicators That Seedlings Are Ready
Determining the precise moment for transplanting involves assessing the physical size of the seedling and the readiness of the garden soil. Seedlings typically reach the appropriate size for transplanting about six to eight weeks after indoor seeding, allowing them to develop sufficient root structure.
Physically, the stem diameter is one of the most reliable indicators of readiness. An ideal onion seedling should have a stem that is approximately the thickness of a pencil, or about one-quarter to one-third of an inch in diameter. The plant height generally reaches between six and eight inches with three to four true leaves present at this stage. Seedlings that grow much larger, especially those developing five or more leaves, become more prone to bolting.
Environmental readiness centers on temperature, as onions are tolerant of cool conditions but vulnerable to hard freezes. The seedlings can handle light frost, but the soil temperature should reach at least 50°F for optimal root growth. Transplanting typically occurs in early spring, two to four weeks before the last expected heavy frost date for the region. Planting when temperatures are consistently below 45°F can trigger the bolting response.
Acclimating Seedlings to Outdoor Conditions
Before permanently moving the young plants, a process known as “hardening off” is required to prevent transplant shock. Seedlings raised indoors are unprepared for the rigors of the outdoors, including temperature fluctuations and intense sun. This gradual exposure allows plant tissues to thicken, increasing resistance to wind and sun.
The hardening-off period should span approximately seven to fourteen days. Begin by placing the seedlings in a sheltered, shady location outside for only two to three hours on the first day. A covered porch or area next to a building offers protection from direct sun and strong winds.
Each subsequent day, incrementally increase the duration the seedlings spend outdoors by one to two hours. After a few days, introduce them to direct sunlight, starting with gentle morning sun. Continue increasing sun exposure daily, ensuring the plants are well-watered, as sun and wind can quickly dry the soil. Once the seedlings tolerate a full day of sun and remain outside overnight without wilting, they are ready for permanent transplanting.
Step-by-Step Transplanting Process
On the day of transplanting, preparing the seedlings ensures a smooth transition and encourages vigorous growth. If the seedlings are excessively long or “leggy,” trim the green tops back slightly with clean scissors, reducing their length to about five inches. This minimizes water loss and redirects the plant’s energy toward root establishment.
The roots can also be trimmed to about half an inch, especially if they are long or tangled. This minor trimming stimulates the growth of new feeder roots, helping the plant anchor and absorb nutrients. Avoid planting the seedlings too deeply, as this common mistake inhibits proper bulb development.
Create a hole or trench just deep enough to cover the base of the seedling, typically one to one and a half inches deep. Planting deeper restricts the space needed for the bulb to swell and expand properly. Space the seedlings four to six inches apart for large bulb growth, or as close as three inches if harvesting some as green onions early in the season.
After placing the seedlings in the soil, gently firm the surrounding dirt around the roots to eliminate air pockets. Immediately follow transplanting with a thorough, deep watering to settle the soil and reduce shock. For the first few weeks, maintain consistent soil moisture and consider a light application of a balanced fertilizer to support the establishment of the shallow root system.