The number of onions harvested from a single plant depends entirely on the variety of Allium cepa. The onion plant is a biennial, typically grown as an annual for its bulb or leaves. Plant genetics dictate whether the energy is channeled into forming one large storage unit or a cluster of smaller, multiplying segments. Understanding this genetic difference is key to predicting the harvest yield.
Yield of Standard Bulbing Onions
For large, globe-shaped onions used for cooking and storage (e.g., Yellow Spanish or Walla Walla), the rule is straightforward: one planted seed, set, or transplant yields one mature bulb. This single-bulb formation results from the plant focusing all energy into a singular, large underground structure. The final bulb size is directly determined by the amount of green foliage the plant develops.
The development of this single bulb is regulated by photoperiodism, the plant’s response to the length of daylight hours. Onions are classified into three types based on the day length required to trigger bulbing.
Short-day varieties initiate bulb formation when daylight reaches 10 to 12 hours, suiting them for southern latitudes. Intermediate or day-neutral onions require 12 to 14 hours of daylight and are adaptable to central regions. Long-day varieties need 14 or more hours of light and are grown successfully in northern regions with long summer days. Planting the wrong type for the region results in poor yield; for example, a short-day onion planted north will bulb too early, while a long-day variety planted south will only produce green tops.
Understanding Multiplier and Bunching Varieties
Not all onions adhere to the single-bulb structure; some variations naturally produce multiple bulbs or are grown solely for their green tops. Multiplier onions, such as shallots (Allium cepa var. aggregatum), are the primary exception to the one-to-one rule. When a single shallot bulb is planted, its energy is distributed into several “daughter bulbs” that form a cluster around the base.
The numerical yield from a single shallot can range from 4 to 12 new bulbs, sometimes reaching 30 or more under ideal conditions. This division occurs because the plant’s basal plate continually splits, creating new growth points. The resulting bulbs are typically smaller than standard storage onions but offer a much higher numerical yield per initial planting.
Bunching onions, commonly known as scallions or green onions, do not produce a large, dry storage bulb. They are grown specifically for their flavorful green leaves and small white bases. These varieties are often planted densely and harvested using a cut-and-come-again method. This technique allows the leaves to be clipped multiple times before the plant is finally pulled up, maximizing the yield of edible foliage.
Essential Factors Influencing Onion Size and Quality
While variety determines the number of bulbs, environmental and cultivation factors dictate the size and quality of the yield. Proper spacing is a primary factor because the shallow root systems of onions compete for nutrients and water. Overcrowding restricts the bulb’s ability to expand, often resulting in numerous small, pearl-sized onions instead of large globes.
For growers targeting large storage onions, a spacing of four to six inches between plants is recommended for sufficient bulb swelling. Consistent moisture is necessary, particularly during the rapid bulb development stage, because the plant’s poor root branching makes it highly susceptible to drought stress.
Onions are heavy feeders, requiring high nitrogen levels, especially early in the season. Nitrogen promotes the extensive leaf growth needed to produce the maximum number of bulb layers, translating directly to a larger final size. However, applying excessive nitrogen too late can delay maturity and negatively affect the bulb’s storage life.
The choice of starting material influences the final harvest size and reliability. Planting from seeds is slow but often leads to the largest, highest-quality bulbs for long-term storage. Using small bulbs, known as sets, is faster but risks “bolting,” or flowering, if the sets are too large. Bolting diverts energy away from bulb expansion, reducing the potential yield size.