The final yield of onions depends entirely on the specific variety of Allium planted. Different types are cultivated for different purposes, which often leads to the misconception that all onions multiply underground like potatoes or garlic. Although the onion plant is naturally a biennial, most gardeners treat it as an annual crop, harvesting the bulb in the first season before it flowers.
Yield from Standard Bulb Onions
Standard bulb onions (including large yellow, red, and white storage varieties) adhere to a strict 1:1 yield rule. Whether planting a seed, a set (small immature bulb), or a young transplant, the result is one single, large, mature onion bulb. The plant’s entire energy is channeled into creating this one fleshy storage unit.
The primary goal when growing these types is to maximize the final size and weight of that single bulb. Proper spacing is crucial, often requiring four to six inches between plants for full expansion. Stressors like inconsistent watering or inadequate nutrients can cause the plant to split into smaller, irregular bulbs, which is undesirable for long-term storage.
Yield from Multiplier and Bunching Onions
The one-to-one rule does not apply to multiplier or bunching varieties, which are cultivated to produce multiple edible parts. Multiplier onions, such as shallots, naturally divide and reproduce clonally underground. A single planted shallot bulb typically yields a cluster of new, smaller bulbs, often producing four to twelve new shallots by harvest time.
The size of the planted bulb influences the final result: smaller bulbs tend to produce fewer but larger harvested bulbs, while larger bulbs result in more numerous but smaller offspring. Bunching onions (scallions or spring onions) offer a different multi-yield harvest. These varieties do not form a large, distinct bulb but instead produce multiple, harvestable stalks or stems. Many bunching onions can be harvested continuously by snipping the tops, allowing the plant to regrow and provide an ongoing supply of greens.
Optimizing Growth for Maximum Harvest
External factors determine the final quality and quantity of the harvest. Onions have a shallow root system, making consistent moisture a major factor in bulb development. A water deficit, especially during the bulbing stage, can severely reduce the final bulb size, potentially cutting the marketable yield in half.
The physical quality of the soil is equally important. Onions require a loose, well-draining medium with a slightly acidic pH between 6.0 and 6.8 to prevent root rot and allow for bulb expansion. Nitrogen supports vigorous leaf growth, which in turn supports the bulb, but applying too much nitrogen late in the season can delay maturity. Finally, after harvest, bulb onions require a curing period of one to two weeks in a well-ventilated, dry area to allow the necks to dry completely, maximizing their storage life.