The Walla Walla onion is a famous variety, renowned for its mildness and exceptional sweetness, traits which make it enjoyable when eaten raw. This distinct flavor profile is partly due to the onion’s lower sulfur content compared to more pungent varieties. Like all bulb onions, its successful cultivation relies on a precise environmental signal to transition from growing green leaves to forming an edible underground bulb. This critical shift is governed by photoperiodism, the plant’s response to the duration of daylight.
Answering the Photoperiod Question
The Walla Walla sweet onion is classified as a Long-Day onion variety. This classification means the plant requires a prolonged period of daylight to signal the beginning of bulb formation. Specifically, this variety needs to experience approximately 14 to 16 hours of continuous daylight to initiate the bulbing process. If the plant does not receive this minimum duration of sunlight, it will continue to dedicate its energy to producing green foliage above ground, instead of forming the large, usable bulb that is characteristic of the variety.
The Science of Onion Day Length
The difference in daylight requirements across onion varieties is a biological process that dictates where they can be grown successfully. This sensitivity to the length of the day, or photoperiodism, acts as a hormonal trigger within the Allium cepa plant. Once the plant senses the required number of daylight hours, it halts the production of new leaves and redirects its stored energy downward to swell the leaf bases into a bulb. Onion varieties are divided into three main groups based on this daylight need.
Onion Day Length Categories
Short-Day onions are best suited for Southern latitudes and begin bulbing when daylight reaches about 10 to 12 hours. These are typically planted in the fall for a spring harvest in mild winter climates. Intermediate-Day onions are adapted to central regions and require a moderate day length of approximately 12 to 14 hours to begin forming bulbs. Long-Day onions, including the Walla Walla, need the longest daylight period, ranging from 14 to 16 hours, making them ideal for Northern regions.
Geographical Limits for Walla Walla Onions
The Long-Day requirement of the Walla Walla onion directly limits the geographical areas where it can be grown effectively. These onions are best suited for planting in Northern latitudes, generally corresponding to areas above 37 to 42 degrees North latitude. This is because only Northern growing regions naturally experience the 14 or more hours of daylight needed during the late spring and early summer to trigger the bulbing mechanism. Gardeners in Southern regions, where the summer days do not achieve this extended photoperiod length, will find that Long-Day onions fail to produce large bulbs.
Planting must occur early in the spring as soon as the soil is workable, which allows the plant sufficient time for vegetative growth. This early start ensures that the plant has developed a large, healthy set of green tops before the summer solstice, when the long daylight hours will initiate the final bulb enlargement.