The optimal time to plant onion seedlings, often called transplants, is crucial for a successful harvest of large, mature bulbs. Seedlings are small, immature plants grown from seed, distinct from onion “sets” (small, dormant bulbs) or direct-sown seeds. Starting with seedlings provides a significant head start, allowing the plant to develop a strong root system and ample foliage before bulbing begins. The final size of the mature bulb is directly related to the number of green leaves the plant has when bulbing is triggered.
Understanding Day-Length Requirements
The biological trigger for an onion to stop producing foliage and begin forming a bulb is photoperiodism, governed by the number of hours of daylight the plant receives. Selecting the correct onion type for a specific latitude is necessary, as planting the wrong type results in small bulbs or no bulb formation.
Onion varieties are classified into three main types based on their daylight requirements. Short-day onions initiate bulbing when day length reaches 10 to 12 hours, making them best suited for southern regions. They are typically planted in the fall for a spring harvest, utilizing shorter winter days to develop extensive leaf growth before the bulbing trigger.
Intermediate-day or day-neutral onions require 12 to 14 hours of daylight to start forming bulbs. These varieties perform well in the mid-latitudes, generally between the 35th and 42nd parallels, corresponding to the central belt of the country.
Long-day onions require 14 to 16 hours of daylight, making them the appropriate choice for northern climates. Planting them early in the spring allows the seedlings to experience weeks of growth before the summer solstice triggers bulb formation.
Climate Timing and Hardening Off
The environmental timing focuses on the calendar date relative to local weather conditions. Onion seedlings are cold-tolerant and should be planted as soon as the soil is workable in late winter or early spring. This usually falls four to six weeks before the last expected spring frost date.
While onions can germinate in soil temperatures as low as 40°F, the ideal temperature for active growth and transplanting is closer to 50°F. Planting early allows the seedlings to establish root systems in cool soil, encouraging maximum foliage development before bulbing begins. The plants can tolerate temperatures down to 20°F without significant damage.
Hardening Off Process
Before placing seedlings in the garden, they must undergo “hardening off.” This gradual process acclimates the tender, indoor-grown plants to harsher outdoor elements, including direct sunlight, wind, and fluctuating temperatures. Failing to harden off can cause transplant shock, resulting in leaf burn, stunted growth, or death.
This transition should take place over seven to ten days, slowly increasing the duration and intensity of outdoor exposure.
- The first day involves placing the seedlings in a shaded, sheltered spot for just a few hours.
- Over the following days, incrementally increase the time outside.
- Introduce them to dappled sunlight, eventually leading to full direct sun exposure.
- The process is complete when the seedlings can tolerate a full day and night outside without showing signs of stress.
A risk of planting too early without proper hardening off is bolting, the premature formation of a flower stalk. Bolting is triggered by prolonged cold stress, where temperatures drop below 45°F for several days after active growth has begun. Once an onion bolts, the plant directs its energy into seed production rather than bulb growth, making the resulting onion tough and reducing its storage life.
Transplanting Depth and Immediate Care
The physical act of transplanting must be executed with precision to ensure healthy bulb development. Onion seedlings should be planted very shallowly, with the white base and roots just covered by the soil. Planting them deeper inhibits the natural process of bulb expansion.
Pre-Planting Preparation
Before planting, it is common practice to trim both the roots and the green tops of the seedlings. Trimming the roots to about a half-inch encourages new, vigorous root growth. Trimming the tops back to three to five inches helps reduce moisture loss and prevents the plant from flopping over after transplanting.
For optimal bulb size, space seedlings four to six inches apart in the row, with rows spaced 12 to 18 inches apart. Closer spacing, such as two to three inches, yields a higher quantity of smaller bulbs. Immediately after planting, thoroughly water the soil around the transplants to settle the soil and eliminate air pockets.
Onions are heavy feeders, and the initial stage of growth requires a steady supply of nitrogen to fuel leaf production. A light application of a nitrogen-rich fertilizer immediately after transplanting helps boost this early growth. Subsequent nitrogen-based side dressings should be applied every few weeks until the bulb begins to swell, as continued feeding is necessary for developing the maximum number of layers that will become the final bulb.