The number of onions harvested from a single planted bulb, seed, or transplant depends on the specific variety of Allium cepa (the common onion) being grown and the method of cultivation. Gardeners can expect anywhere from a single, large bulb to a cluster of numerous smaller bulbs or edible green stalks from one initial planting. Understanding these varietal differences is the first step toward a successful onion harvest. The plant’s genetic makeup determines its growth habit, which directly dictates whether it focuses its energy on developing one large storage organ or dividing into a cluster of smaller ones.
The Single Onion Rule for Storage Varieties
For the vast majority of commonly grown storage onions, such as Yellow Globe or Spanish types, the rule is straightforward: one planted item yields one mature, large bulb. This 1:1 ratio is a result of the plant’s biennial life cycle and its primary focus during the first growing season. The onion plant is programmed to store energy in a single, large bulb to survive its first winter, allowing it to flower and produce seeds in its second year.
The mature bulb is a modified stem structure composed of fleshy leaf sheaths that store nutrients. This single bulb is the plant’s energy reserve, and it will typically weigh between 4 and 8 ounces under good conditions. Proper spacing, generally 4 to 6 inches between plants, is necessary to give each individual onion the room to swell into a single, marketable size. If the plants are crowded, the competition for resources can lead to multiple smaller bulbs instead of one large one.
A notable exception that interferes with this single-bulb goal is premature bolting, which is the formation of a flower stalk. Bolting is often triggered by exposure to cold temperatures (vernalization) at a specific stage of development, typically when the plant has seven to ten leaves. Once the plant begins to bolt, it diverts its energy from bulb enlargement to seed production, resulting in a hard, woody center in the bulb that makes it unmarketable and reduces its storage quality. Bolting can significantly reduce the final yield of high-quality, single-bulb onions.
How Multiplier and Bunching Onions Differ
The 1:1 rule does not apply to multiplier and bunching varieties. Multiplier onions, which include shallots and potato onions, are specifically bred to divide and form a cluster of new bulbs from the single planted bulb. A single planted shallot bulb, for instance, can produce an aggregate cluster of three to ten smaller, oval or pear-shaped bulbs underground. These varieties are propagated primarily by division, meaning the harvested bulbs are separated and replanted to continue the cycle.
Potato onions, a type of multiplier, form larger bulbs but with fewer bulbs per cluster than shallots, which tend to be prolific dividers. This growth pattern means that one planted bulb can yield a handful of new bulbs, offering a higher number of individual units, though they are smaller than standard storage onions.
Bunching onions, such as scallions or spring onions, typically do not form a large, dry storage bulb. These are often a different species, Allium fistulosum, which produces multiple shoots or stalks from the base. The yield from a bunching onion is measured in edible stems, which multiply through tillering or basal division, allowing for repeated harvests of green tops.
Understanding the Impact of Planting Source
The material used to start the onion crop—seed, set, or transplant—influences the plant’s development and risk of bolting. Growing from true seed is the most economical method and offers access to the widest variety of cultivars, which is important for matching the onion type to the regional day length. Seeds must be started indoors 10 to 15 weeks before the last frost to ensure a long growing period.
Onion sets are small, immature bulbs from the previous season that provide the quickest path to a harvest. However, sets are more susceptible to bolting because their previous exposure to cold temperatures during storage can prematurely satisfy the vernalization requirement. When planted, these sets may quickly transition to flowering instead of focusing on bulb enlargement, leading to a poorer quality harvest.
Transplants, which are seedlings grown from seed and sold in bundles, strike a balance between the other two methods. They are less prone to bolting than sets because they have not endured the temperature fluctuations of bulk storage. Transplants also shorten the time to harvest compared to direct seeding, providing a reliable start for developing a single, large bulb.
Essential Steps for Maximum Bulb Size
To ensure that a single planted item develops into the largest possible single bulb, environmental factors must be managed throughout the growing season. Onions are shallow-rooted, making consistent water and nutrient availability non-negotiable for swelling the bulb. The soil should be well-drained, ideally a sandy loam, with a slightly acidic to neutral pH between 6.0 and 7.0.
Nutrient management is tuned to the plant’s growth stage, focusing first on vegetative growth before transitioning to bulb formation. Nitrogen is initially required in high amounts to promote the development of numerous, healthy leaves, since a larger top supports a larger bulb. However, nitrogen application must be reduced or stopped once the bulb reaches about two-thirds of its final size to prevent excessive foliage growth that can delay maturity.
Potassium and phosphorus are necessary for maximizing the final size and improving the quality of the harvested bulb. Potassium plays a specific role in regulating water balance within the plant, which enhances the bulb’s size and improves its ability to store well after harvest. Adequate spacing, consistent moisture, and proper fertilization channel the plant’s energy into a single, high-quality, large storage bulb.