New gardeners often expect onion seeds to behave like potato pieces or garlic cloves, yielding multiple bulbs from a single planted unit. However, the reproductive biology of the onion plant is different from these tuber or clove vegetables. This article clarifies the fundamental botanical rule for standard bulb onions. It also explains why certain varieties appear to defy this principle.
The Fundamental Rule: One Seed, One Bulb
The definitive answer for nearly all standard, storage-quality onions (yellow, red, or white varieties) is simple: one seed produces one bulb. This outcome is rooted in the plant’s basic botany and genetic programming. The onion (Allium cepa) is a monocot, meaning its growth pattern is designed to produce a single, large, subterranean storage structure.
The true seed holds the genetic information required to grow one individual plant. As the seedling matures, energy captured through photosynthesis is directed toward thickening the base of the leaves. These leaves swell to form the single, desirable bulb. This dedicated growth is what commercial growers and home gardeners aim for when cultivating large onions meant for long-term storage.
The genetic makeup of standard onions prioritizes the development of a large, singular structure rather than lateral division. This process, known as bulbing, is triggered by day length and temperature. Bulbing results in the expansion of the leaf bases above the root plate. This singular focus helps standard onions develop the thick, papery outer skin necessary for long-term curing and storage.
Understanding Onion Types That Appear Different
The perception that some onions yield more than one bulb per seed stems from confusion between different members of the Allium family. Bunching onions, often called scallions or green onions, are a primary source of this misunderstanding. While they can be grown from seed, they are harvested much earlier than bulb onions, typically before significant bulb formation occurs.
Their appeal lies in their green tops and small white bases. They naturally produce offsets, which are small side shoots that grow adjacent to the main stem. These side shoots create a “bunch” of stalks from a single initial planting. This makes it appear as if the single seed produced multiple plants, though it is technically a form of asexual reproduction.
Multiplier onions, including shallots and potato onions, also confuse the one-seed rule. These varieties are almost exclusively grown from “sets,” which are small, immature bulbs from the previous season, not seeds. When a multiplier set is planted, the single small bulb divides underground over the growing season. This results in a cluster of several mature bulbs through vegetative propagation, not seed germination.
Practical Planting Density and Thinning
Because a standard onion seed yields only one bulb, successful cultivation requires specific techniques to manage planting density and ensure optimal growth. When direct sowing, gardeners often plant two or three seeds together in the same spot. This practice accounts for the natural variability in seed viability and germination rates, which are rarely 100%. This initial over-seeding provides insurance against gaps in the row, ensuring a consistent stand of plants.
Once the seeds have successfully germinated and the seedlings reach a few inches in height, the process of “thinning” becomes necessary. Thinning involves carefully removing the weaker or excess seedlings. This leaves only one healthy plant to grow at each desired location. Failing to thin means multiple plants will compete fiercely for water, light, and soil nutrients.
This competition prevents the necessary energy accumulation required for the bulb to swell. Overcrowding typically results in a harvest of many small, stunted, or “pencil-necked” onions. These onions never properly form a large, firm storage bulb. The plants prioritize leaf growth and stem development over bulb expansion when resources are limited.
To achieve mature, storage-size bulbs, the remaining single plants must be spaced appropriately. This typically requires a final spacing of four to six inches between the centers of each plant. This distance allows the underground bulb to expand laterally without physical obstruction. The young seedlings removed during thinning can often be used immediately as green onions, providing a secondary, early harvest.