Onion Growth Stages

Understanding the life cycle of onions will help you be a better gardener. What growth stages does it go through

Onions may be one of the most used ingredients in cooking and are essential for adding flavor to many dishes. In the garden, onions provide natural, organic pest and disease protection.

The onion plant has a two-year life cycle when grown from seed, which allows it to devote all of its energy to growing in the first season and reproducing the following year. Most gardeners grow onions from bulbs. 

It is possible to grow them from seed, but because it has a two-year life cycle, you must understand how an onion develops and what is happening at each stage.

dried flower with growing small black seeds


The first stage of germination is radicle emergence. The radicle anchors the seed in place and supports the growing seedling. The radicle collects moisture and nutrients to sustain plant growth.

The sprout emerges through the weakened seed covering, eventually breaking the soil’s surface. Soon after, the first real leaves appear, resembling a smaller and fleshier version of the plant’s mature leaves, signifying a shift in the plant’s development as the seedling begins to obtain energy through photosynthesis. 

growing onion sprout in black soil


Before beginning vegetative development, the seedling grows a strong root system, which will continue to develop during the vegetative stage. An onion seedling has a few mature leaves at this stage.

The leaves of an onion develop swiftly during vegetative growth. Your onions, Starting with just a few leaves, grow eight to ten healthy leaves in weeks. The leaf development stops when the onion is ready to produce the bulb.

Bulb Formation

When developing onions have at least four leaves, they are ready to put their energy toward bulb production. The existing leaves continue to grow and elongate, increasing the available surface area for photosynthesis.

Each leaf represents a scale that eventually becomes one of the bulb’s rings. The leaf sheaths swell to form the central storage tissue of the bulbs. After a few weeks, the onion’s bulb emerges from the ground, revealing the onion’s top. 

As bulbs approach maturity, plants have a tops-down physical response; when resources are transported from the top leaves to the scales to ensure that the bulb reaches its optimal size. 

Onions are ready for harvest when the tops become droopy and turn brown.

full grown of onion plant in the garden


If onions are not harvested and left in the ground during the winter, they will re-grow in the spring. The underground bulbs are already fibrous and woody at this point. Because the root system was established the previous year, the plant can focus its entire energy on sending up new shoots to emerge through the soil surface. 

The air and soil temperatures rise as the weather warms, and the plant begins to bolt. Bolting refers to the crop suddenly growing a flowering stalk. When this happens, the plant abruptly stops vegetative growth and prepares for the end of its life cycle. Buds appear once the stalk has fully matured and the top of the plant blooms with a white or purple flower.

Birds, bees, and butterflies pollinate the colorful, round flower heads containing seeds. After the seeds appear, the onion plant’s life cycle is complete. 

blooming small white flower heads
Alaine Connolly
Alaine has been working way too hard in horticulture since 1992, beautifying golf courses, resorts, and hotels. She is a part time landscape designer who works full time caring for a 28,000 square foot public garden. At home, she maintains her own 400 square feet plot. Alaine lives in northern Illinois - zone 5b.
More ArticlesVegetables and Fruits