The number of new onions grown from a single initial onion depends entirely on the cultivation method chosen, ranging from a handful of greens to hundreds of new bulbs. The common bulb onion, Allium cepa, is a biennial plant, completing its life cycle over two seasons. Yield potential is determined by whether the initial onion is used to produce seeds, divide into new bulbs, or regrow its green tops.
Yielding Hundreds: Starting with Seed Production
The highest potential yield comes from allowing a mature storage onion to complete its full, two-year life cycle and produce seeds. For this method, a healthy bulb must be planted and allowed to overwinter. This exposes the bulb to the necessary cold temperatures for vernalization, which triggers the plant’s reproductive stage the following spring.
Once the weather warms, the onion will “bolt,” sending up a thick, hollow flower stalk called a scape. This stalk terminates in a large, spherical cluster of tiny flowers known as an umbel. A single, well-developed umbel can contain hundreds of individual flowers, which must be pollinated to develop a seed capsule.
Each capsule within the umbel typically holds two to three small, black onion seeds. One healthy flower head can yield several hundred viable seeds, and a single bulb sometimes produces multiple scapes. Planting one onion bulb can therefore lead to a harvest of hundreds of seeds, which have the potential to grow into hundreds of new onions. This process requires two full seasons. The size of the initial bulb significantly influences the final seed yield, with larger bulbs producing a greater quantity of seeds.
Yielding Multiples: Dividing Mature Bulbs
For a faster, single-season increase in bulb count, specific varieties known as multiplier onions offer a medium-yield approach. Unlike standard storage onions that focus on a single large bulb, multiplier types such as shallots or potato onions naturally split and produce a cluster of smaller bulbs underground. Planting a single bulb from a multiplier variety results in a clump of typically four to twelve new bulbs at harvest time.
This multiplication occurs because these varieties have a genetic predisposition to form multiple growing points from the original planted bulb. The resulting bulbs are smaller than a traditional storage onion but offer a direct, single-season return on the planting investment. The new bulbs can then be replanted the following season to continue the cycle.
A similar, though less reliable, technique involves intentionally cutting a large, non-multiplier onion into sections for replanting. This requires slicing the bulb in half or quarters, ensuring each piece retains a portion of the basal plate—the flat area where the roots emerge. While this can stimulate a few new growing points, it yields fewer, smaller, and less uniform bulbs compared to naturally multiplying varieties.
Yielding Greens: Regrowing the Base
The lowest yield method involves regrowing the base of a store-bought onion, often done with kitchen scraps. This is achieved by cutting off the root end, including the basal plate and about an inch of the bulb tissue, and placing it in water or soil. This process provides a quick and continuous harvest of green tops, similar to scallions.
The existing nutrient reserves stored within the base of the bulb fuel the rapid growth of these green shoots. However, this method rarely regenerates into a full-sized, mature storage onion. The remaining root plate lacks the necessary energy reserves and time to rebuild a large bulb structure.
The primary result of regrowing the base is a small, ongoing yield of fresh greens for immediate consumption. This process is best viewed as maximizing a food scrap rather than as a method for bulk onion production. The base will continue to produce greens for a few weeks before its energy is depleted.