The question of how many onions grow from a single seed is a common point of confusion for many new gardeners. The answer depends heavily on the specific type of onion you are cultivating, as not all members of the Allium genus behave the same way. For the standard bulb onion, the primary goal is to grow one large, well-formed bulb suitable for long-term storage, which dictates the growing approach. Understanding the fundamental difference between the single-bulb varieties and the multiplying types is the first step toward a successful harvest.
The Single Onion Rule
For the most common types of onions, such as yellow, white, and red storage varieties (Allium cepa), one seed is designed to produce one single, harvestable bulb. This plant develops from the germination of a single seed into a seedling, which then focuses its energy on developing a single, swollen base.
The process of bulbing is directly tied to the length of daylight, with varieties classified as short-day, intermediate-day, or long-day depending on the required hours of light to initiate bulb formation. Once the necessary photoperiod and temperature conditions are met, the plant redirects energy from leaf growth to the swelling of the underground stem tissue, creating the characteristic large, single bulb. The entire cultivation strategy, including spacing and thinning, is built around maximizing the size and quality of this solitary bulb.
Understanding Bunching and Multiplier Varieties
The idea that one seed can produce multiple onions stems from the existence of certain specialized varieties that naturally divide. These types are exceptions to the single-bulb rule and fall into two main categories: bunching onions and multiplier onions.
Bunching onions, often called scallions or green onions, are typically harvested for their green tops before a large, dry bulb can form. These plants, which can include the species Allium fistulosum (Welsh onion), naturally form tight clusters of multiple stems from a single planting, even though they may be started from a single seed.
Multiplier onions, such as shallots (Allium cepa var. aggregatum) and potato onions, are distinct because they grow by division at the base. When planted, a single clove or small bulb (often called a set, rather than a seed) will split underground to form a cluster of several new bulbs. A single shallot clove can produce a cluster of six to eight new bulbs, though these bulbs are typically smaller than the standard storage onion. Some multiplier onions, like the Egyptian walking onion, even produce small bulbils at the top of the stalk, which can be planted to start new plants. The initial planting material is often a set or transplant rather than a true seed.
Maximizing Bulb Size
The primary focus for growers is ensuring that single bulb reaches its largest possible size. The most important action a gardener can take to maximize the bulb’s potential is proper thinning and spacing. Overcrowding forces plants to compete intensely for water, nutrients, and sunlight, resulting in smaller, underdeveloped bulbs that can sometimes be misshapen.
When starting from seed, seedlings should be thinned to a final spacing of approximately four to six inches between plants within the row, depending on the desired size and specific variety. A spacing of four inches is generally recommended for moderate-sized bulbs, while a greater distance, such as eight inches, encourages the development of truly large, jumbo onions. The timing of this thinning is also important, as it should occur relatively early in the plant’s life to prevent resource competition during the active growth phase.
Consistent moisture is also necessary, as the bulbing process requires a steady supply of water. Furthermore, once the bulb begins to swell, typically indicated by the soil cracking around the base, the application of nitrogen fertilizer should be stopped to encourage the plant to focus its energy entirely on bulb enlargement rather than continued leaf growth.