Onions are a popular garden crop, but successful cultivation depends less on the familiar USDA temperature hardiness zones and more on a single environmental factor: the amount of daylight received. Unlike other vegetables, the onion plant’s growth cycle is governed by photoperiod. Knowing your geographical location’s maximum daylight hours is the most important information for selecting the right variety. Planting the wrong type will result in a harvest of green tops without the expected storage bulb.
Understanding Photoperiod Requirements
The main factor distinguishing onion growing zones is the photoperiod, the duration of light in a 24-hour cycle. Onions are photoperiodic plants, meaning the length of the day directly controls when the plant shifts from vegetative growth to bulb formation. Bulb formation is triggered by the specific number of daylight hours reaching a threshold.
This mechanism involves the plant shifting energy from growing foliage to creating the underground storage organ. The green tops are directly linked to the size of the final bulb, as each leaf corresponds to a ring in the onion. Once the light threshold is met, the plant stops producing new leaves and begins swelling the base of existing leaves into a bulb.
If the light threshold is met too early, the resulting bulb will be small due to insufficient foliage development. If the day length never reaches the required hours, the plant will only grow green tops without forming a usable bulb. This response to day length makes variety selection specific to latitude.
Long-Day, Short-Day, and Intermediate Onion Types
Onion varieties are classified into three groups based on their specific photoperiod requirement for bulb initiation. This classification ensures the plant develops adequate leaf structure before the bulbing signal is received.
Short-Day Onions
Short-day onions require the least amount of light, needing 10 to 12 hours of daylight to trigger bulbing. These varieties are typically sown in the fall in mild climates, growing through winter and bulbing as days lengthen in late winter or early spring. They are known for being milder and sweeter, but they generally have a shorter storage life.
Long-Day Onions
Long-day onions are adapted for regions with longer summer days, requiring 14 to 16 hours of daylight to begin forming a bulb. These types are planted in early spring, allowing them to develop substantial foliage before the summer solstice signals bulbing. They tend to have a more pungent flavor and are better suited for long-term storage.
Intermediate-Day Onions
Intermediate-day varieties, sometimes called day-neutral, require 12 to 14 hours of light to initiate bulb development. These varieties offer flexibility and perform well in transition zones where day lengths are not extreme. Planting an onion type that does not match the local day length will result in poor or premature bulb development.
Matching Onion Type to Geographical Region
The three onion types are directly mapped to specific geographical latitudes in the Northern Hemisphere. Short-day onions are the appropriate choice for Southern regions, generally located below the 35th parallel north. This area includes the Southern United States, where the mild winter allows for fall planting and the maximum spring daylight hours suit these varieties.
Long-day varieties are suited for Northern regions, typically those above the 42nd parallel, including the Northern United States and Canada. Gardeners in these areas plant in early spring to take advantage of the extended daylight hours of the northern summer.
Intermediate-day varieties thrive best in transition zones, generally between the 35th and 42nd parallels. This central band experiences day lengths that satisfy the intermediate requirement. For best results, long-day onions are planted in the spring for a summer harvest, while short-day onions in the South are planted in the fall for a spring harvest.