Onions reproduce in two main ways: sexually through seeds and asexually through bulb division. Most onions are biennials, meaning they spend their first year growing a bulb and their second year flowering and producing seed. But in gardens and farms alike, onions are also commonly propagated by planting smaller bulbs, bypassing the seed stage entirely.
The Two-Year Life Cycle
An onion plant lives on a two-year schedule. During the first growing season, it focuses all its energy on building leaves and swelling its bulb underground. The plant then goes dormant. In its second season, it shifts gears: instead of making a bigger bulb, it sends up a tall flower stalk. This switch from bulb growth to flowering is called bolting, and it’s triggered by a combination of accumulated warmth and day length. Research has shown that bulbing only begins once temperatures have built up enough heat (around 600 degree-days) and daylight reaches roughly 13.75 hours.
This two-phase life cycle creates a natural tension. The genes that promote bulb growth actively suppress flowering, and vice versa. Modern onion varieties have been bred to maximize bulb size, which means flowering signals are deliberately dampened. That’s great for producing big onions at the grocery store, but it makes seed production more difficult.
How Onions Make Seeds
When an onion does bolt, it produces a round flower head called an umbel at the top of a hollow stalk that can reach two feet or taller. Each umbel contains roughly 480 tiny individual flowers, called florets, all packed into a globe-shaped cluster. These florets are not self-fertile, so pollen has to travel from one plant to another for seeds to form. Wind plays almost no role in this process. Onions depend almost entirely on insects for pollination.
Bees are the primary pollinators, along with various flies. These insects move between flower heads, carrying pollen from plant to plant. Because onions cross-pollinate so readily, seed producers have to carefully isolate different varieties from one another to prevent unwanted mixing. After successful pollination, each fertilized floret develops small black seeds inside a dry capsule. Once the seed heads dry out, the seeds can be harvested.
One important quirk of onion seeds: they lose viability fast. Unlike tomato or bean seeds, which can last for years in storage, onion seeds lose about 50% of their germination ability each year regardless of how well you store them. This applies to all alliums, including shallots, leeks, and chives. If you’re saving onion seeds, plan to use them the following season.
Reproducing Without Seeds
While seeds are the onion’s natural reproductive method, most gardeners and farmers rely on vegetative propagation, which is faster and more predictable. The simplest version of this is planting onion sets, which are small bulbs grown from seed the previous year, harvested early, dried, and sold for replanting. When you buy a mesh bag of tiny onions at the garden center in spring, those are sets. You push them into the soil, and each one grows into a full-sized bulb without ever needing to go through pollination or seed formation.
Transplants work similarly. Onion seedlings are started indoors or in greenhouses, then moved into the field. Both sets and transplants give growers a head start compared to direct seeding, which matters for a crop that needs a long growing season.
Multiplier Onions and Shallots
Some onion varieties take vegetative reproduction a step further. Multiplier onions (also called potato onions) and traditional shallots belong to the Aggregatum group. Instead of forming a single large bulb, each planted bulb divides underground into a cluster of smaller bulbs, sometimes producing six or more from one original. These are harvested, separated, and replanted the next season. In Scandinavian home gardens, where multiplier onions have been grown for generations, flowering is extremely rare and seed propagation is essentially never used. The entire population is maintained through clonal division, year after year.
This purely clonal reproduction means every bulb is genetically identical to its parent. That’s convenient for maintaining consistent flavor and size, but it leaves the population vulnerable to disease since there’s no genetic shuffling happening between generations.
Egyptian Walking Onions
The most unusual reproductive strategy belongs to the Egyptian walking onion. Instead of producing flowers at the top of its stalk, this variety grows a cluster of tiny bulbils, miniature bulbs that form right in the air. These bulbils start out small and green, then grow larger and develop reddish-brown skins. As they get heavier, the stalk bends under their weight and eventually flops over, pressing the bulbils into the ground where they take root. The plant literally “walks” across the garden over successive seasons, each generation landing a stalk’s length away from the parent.
Some of these aerial bulbils even sprout small green shoots while still attached to the parent plant, and occasionally a second stalk with another cluster of bulbils grows from the first, creating a cascading chain. Propagation is as simple as snapping off the bulbils and pressing them into soil wherever you want new plants.
Why Most Growers Skip the Seed Stage
Growing onions from seed to harvest takes patience. You need to either start seeds indoors 8 to 10 weeks before transplanting or commit to a two-year timeline if you want the plant to complete its full reproductive cycle. Seed germination rates drop quickly in storage. And because modern varieties have been bred to resist bolting (to keep bulbs from going woody and hollow), coaxing them into flowering for seed production takes careful management of temperature and day length.
For all these reasons, vegetative methods dominate. Home gardeners plant sets or transplants for a reliable harvest in a single season. Shallot growers simply replant a portion of each year’s crop. And Egyptian walking onions handle the whole process themselves, no human intervention required. Sexual reproduction through seeds remains essential for breeding new varieties and maintaining genetic diversity, but for putting onions on the dinner table, bulb division is the practical choice.