White onions are a rewarding and flavorful crop, but growing large, firm bulbs depends almost entirely on precise timing. The onion plant’s development is programmed to respond to the changing length of the day, making the planting date a determining factor for a successful harvest. Planting too early or too late for your region results in small bulbs or prevents bulbing altogether. This guide aligns your planting schedule with the onion’s unique biological clock.
Determining the Right Planting Season
The mechanism that dictates when an onion transitions from growing green tops to forming a bulb is called photoperiodism—the plant’s response to daylight duration. Onion varieties are classified into three types based on the daylight hours required to trigger bulbing. Understanding this classification is the first step in selecting the correct planting season for your location.
Short-day onions, best suited for latitudes south of the 36th parallel, begin forming bulbs when daylight reaches 10 to 12 hours. They are typically planted in late fall or early winter, allowing foliage to establish during cooler months before the short-day trigger arrives in late winter or early spring. Intermediate-day onions (day-neutral) require 12 to 14 hours of daylight, making them adaptable for growers between the 35th and 42nd parallels.
Long-day onions are preferred for northern latitudes, generally above the 42nd parallel, requiring 14 to 16 hours of light to initiate bulbing. These varieties are planted in early spring, allowing them to develop substantial green tops before the long summer days trigger bulb formation. Planting the wrong type for your latitude can cause premature bulbing while the plant is small, or prevent bulbing entirely if the light duration never reaches the required threshold.
Scheduling Based on Planting Material
The precise calendar date for planting depends heavily on the material chosen: seeds, sets (small immature bulbs), or young transplants. Each method has a distinct schedule to ensure the plants are ready to respond to the correct day length in your region.
Starting from Seeds
Starting seeds is the most economical approach and offers the widest selection of varieties, but requires the earliest start indoors. Seeds should be sown 10 to 12 weeks before the intended outdoor planting date, generally in late winter (January or early February). This indoor head start allows seedlings to reach a transplantable size, roughly the thickness of a pencil, necessary for a successful transition.
Using Onion Sets
Onion sets are the easiest way to establish onions, as they are small, dormant bulbs from the previous season’s growth. They are planted directly into the garden in early spring, as soon as the soil can be worked, often four to six weeks before the last expected spring frost. While convenient, sets are prone to “bolting” (sending up a flower stalk) if exposed to prolonged cold, which diverts energy from bulb development.
Planting Transplants
Transplants, bundles of young, actively growing plants typically ordered from a supplier, offer the best balance of ease and quality bulb production. These are planted slightly later than sets, usually around four weeks before the last expected frost, or once the seedlings have been properly hardened off. The plant must be established and actively growing before the critical day-length trigger occurs for the chosen variety.
Site Preparation and Initial Growth Care
Proper site preparation provides the optimal environment for initial root establishment. White onions perform best in full sun, requiring a minimum of six to eight hours of direct sunlight daily.
The soil should be loose, fertile, and exceptionally well-draining, ideally a sandy-loam texture, to prevent waterlogging and bulb rot. Onions thrive in a soil pH range between 6.0 and 6.8. Before planting, amend the bed with aged compost or a balanced fertilizer, as onions are heavy feeders and require consistent nutrition to build large bulbs.
When planting sets or transplants, set them shallowly, with the bulb base no more than one inch deep, ensuring the top remains close to the soil surface. This shallow placement allows the bulb to expand easily as it matures. After planting, consistent watering is essential to settle the soil and support the rapid growth of the green tops.
Harvesting and Curing White Onions
Harvesting requires careful observation, signaled by the plant itself. Onions are ready to be lifted when the green tops naturally begin to yellow, soften, and fall over, typically after about two-thirds of the foliage has collapsed. This indicates the plant has ceased vegetative growth and is redirecting energy into the bulb.
Once the tops have fallen, gently lift the bulbs from the ground, avoiding bruising, which can create entry points for pathogens during storage. After harvesting, the bulbs must undergo curing, which is necessary for long-term storage and involves drying the outer layers and the neck.
Curing is best achieved by spreading the onions in a single layer in a warm, dry, and well-ventilated area, such as a garage or covered porch, out of direct sunlight. The goal is to dry the neck and outer skin until they become papery and rustle when touched, a process that usually takes two to four weeks. Once fully cured, trim the tops to about one inch above the bulb, and store the onions in a cool, dry location.