Successful onion growth depends on selecting the correct variety for a specific location. The plant’s development is genetically programmed to respond to the length of daylight, a phenomenon known as photoperiodism. This sensitivity means a variety suited for the northern United States will not form a proper bulb when grown in the south. Long-day onions are tailored for regions with extended summer daylight hours.
The Science of Day Length and Bulb Formation
The growth of long-day onions is governed by a precise biological mechanism requiring a minimum number of daylight hours to trigger development. This physiological response dictates when the onion switches from producing foliage to initiating bulb formation. Long-day cultivars require a sustained period of approximately 14 to 16 hours of daylight to receive the internal “bulbing signal,” which halts leaf growth and starts the enlargement of the underground stem tissue into a bulb.
The final size of the onion bulb is directly related to the number of green leaves the plant produces before the bulbing signal is received. Each leaf corresponds to a layer in the mature bulb, so a successful harvest relies on maximizing top growth during the early, cooler part of the season. If the day length never reaches the 14-hour threshold, the plant will continue to produce only green tops without forming a usable bulb.
Geographical Requirements and Common Varieties
Long-day onions are specifically adapted to thrive in northern latitudes where summer days are longest and most intense. These regions generally correspond to areas north of a line drawn roughly from San Francisco, California, to Washington, D.C., encompassing USDA hardiness zones 6 and colder. The ideal growing latitude for these varieties is between 37 and 47 degrees North.
This geographical placement ensures the plants experience the necessary 14 to 16 hours of daylight during peak summer months for proper bulb maturation. Popular examples of successful long-day varieties include Walla Walla Sweet, known for its mild flavor, and Yellow Sweet Spanish, which is often cultivated for its size and storage capabilities. Other common long-day types are White Sweet Spanish and the storage-focused Yellow Globe.
Key Differences from Other Onion Types
The primary distinction between onion types lies in their genetic light requirement for bulbing. Long-day onions are separated from intermediate-day and short-day varieties by several hours of daylight. Short-day onions, best suited for Southern latitudes (Zone 7 and warmer), require only 10 to 12 hours of daylight to form a bulb, allowing them to be planted in the fall and harvested in the spring.
Intermediate, or day-neutral, types fit between the two extremes, needing 12 to 14 hours of light and performing best in central latitudes (USDA Zones 5 and 6). The bulb composition also differs significantly. Long-day onions possess a lower sugar content and a higher sulfur content, which contributes to a more pungent flavor and superior long-term storage life. Short-day varieties are typically sweeter due to higher sugar and water content, making them less suitable for extended storage.
Cultivating Long Day Onions
For optimal yields, long-day onions should be planted very early in the season, typically in late winter or early spring, as soon as the soil is workable. This early start is necessary to allow maximum foliage growth before the summer solstice triggers the bulbing process. Starting with transplants or seeds is recommended over using small bulbs known as “sets,” which are prone to prematurely producing a seed stalk (“bolting”), resulting in a smaller bulb.
The plants perform best in a site that receives full sun and has loose, well-drained soil, such as a sandy loam, with a slightly acidic to neutral pH between 6.0 and 6.8. Consistent moisture is important for the shallow root system, especially once the bulbing phase begins. Harvesting is indicated when the onion tops begin to soften, wilt, and fall over naturally, typically occurring 90 to 120 days after planting.