The onion, a staple in global cuisine, is a fascinating plant whose cultivation yield varies widely depending on its specific type and the stage of its life cycle when planted. The simple question of how many onions grow from one is answered not with a single number, but with an explanation of different growth habits across the Allium genus. A single planting unit—whether a seed or a small bulb—may result in one large bulb, a tight cluster of smaller bulbs, or a continuous supply of green stalks. The outcome is determined by the variety chosen and the starting material.
Yield from Planting Seeds and Sets
For most gardeners, the goal is to produce a single, large, dry-storage bulb, which is the standard outcome for the common onion species, Allium cepa. This single-bulb yield is consistent whether the gardener starts with a seed or an onion set.
Seeds
A seed is a product of sexual reproduction, and when sown, the resulting plant focuses all its energy on developing one bulb during its first growing season. This method provides the widest selection of varieties. Seeds are less likely to prematurely bolt in their first year, often producing higher-quality storage onions.
Onion Sets
Onion sets are small, immature bulbs grown from seed the previous year. They are favored for convenience and speed, offering a significant head start and maturing faster than plants grown from seed. A single set will still develop into only one harvestable, full-sized onion bulb. Sets are generally recommended for novice gardeners because they are less prone to failure and more tolerant of varying soil conditions. A drawback of planting sets is their tendency to “bolt,” or prematurely send up a flower stalk, especially if exposed to fluctuating cold and warm temperatures.
Understanding Multiplier and Bunching Onions
The expectation of getting multiple onions from a single planting is met by multiplier onions. These plants, which include shallots and potato onions (Allium cepa var. aggregatum), are characterized by natural division.
Multiplier Onions
When a single multiplier bulb is planted, it does not grow larger as a single unit but instead splits underground to form a tight cluster of new, smaller bulbs. This multiplication process is prolific, with a single planted bulb typically yielding a cluster of between five and fifteen new bulbs by the end of the growing season. The smaller bulbs are saved and replanted to continue the cycle, while the larger ones are consumed. Multiplier onions are sometimes referred to as perennial onions because they can be left in the ground to overwinter and continue dividing year after year.
Bunching Onions
Bunching onions, often called scallions or green onions, represent another deviation from the single-bulb rule, providing an extended harvest of green foliage. These varieties, such as the Welsh onion (Allium fistulosum), never form a large, dry storage bulb. Instead, they produce multiple tubular leaves and small, elongated basal bulbs. A single planting unit develops multiple stems from its base, forming a clump that can be harvested continuously. Gardeners can repeatedly cut the green tops for use while leaving the root base in the ground to resprout, allowing for an ongoing yield of fresh green onions.
The Cycle of Seed Production from a Mature Onion
When a mature, dry onion bulb is replanted, the plant does not produce new, edible bulbs. Instead, the plant shifts its biological focus from vegetative growth to reproduction, beginning its second-year life cycle. The common onion is a biennial plant, requiring two full seasons to complete its life cycle from seed to seed. The first season forms the edible storage bulb, while the second is dedicated to flowering and seed production.
Replanting a mature bulb forces the plant into this second phase, marked by bolting. The onion uses the stored energy to send up a tall, rigid, hollow flower stalk, known as a scape. This stalk culminates in a spherical flower head that produces numerous small black seeds.
Once an onion bolts, the quality of the underground bulb rapidly declines because the plant diverts all nutritional resources to the developing seeds. The bulb stops enlarging, often develops a tough, woody core, and loses its ability to store long-term. Replanting a mature onion is a strategy employed only by those seeking to harvest seeds, not by those looking to increase their yield of edible bulbs.