Onion Plant Growth Stages & Life Cycle (With Pics & Tips)

Knowing about the growth stages and life cycle of the onion plant can help you make gardening decisions and have a successful harvest. While most of their growth is beneath the soil you can use the leaf growth to interpret where your onions are in the growing process, or if they need help. 

Onions are a fantastic edible to have in the garden. They’re healthy and go great in lots of dishes – caramelized, grilled, or raw. There are something like 500 onion varieties that are categorized as white, yellow, or red onions and also by the daytime hours they take to grow, short day vs long day onions.

There are three different starting points for growing onions: from seeds, sets/bulbs, or onion starter plants. Growing from seed takes the longest but gives you the largest selection of onion varieties to grow from and allows you complete control of the plant from seed to harvest.

Growing from sets/bulbs cuts about 4-6 weeks off growing time and onion starter plants 8-12 weeks off of growing time. Onion starter plants can be harvested as quick as 60 days after planting outside. Check out “How Long Do Onions Take To Grow” for more on that.

Onion sets sprouting

Onion Plant Life Cycle

Onion plants are biennials that take two years to complete their life cycle. Unless you want to harvest onion seed to grow the following year you’ll most likely harvest onions as an annual.

You might have experienced onion plants “bolting” or “going to seed” where they go to seed during their first year of growth. The flower and seed production that you see here is usually a result of temperature fluctuations or some other stress on the plant.

After an onion flowers the onion is still edible but is about half the weight of an onion that hasn’t flowered and will not store well.

Onion Seed Germination

Onion seeds germinate optimally with soil temperatures of 75 degrees but will germinate in soil temperatures of 40 degrees or above. They will germinate as fast as 4 days and as slow as 21 days.

Onion seeds, like most seeds, will germinate faster with warmer temperatures and slower with cooler temperatures.

Besides soil temperature, moisture is the other critical element to seed germination. You need to water seeds after planting and again if soil dries out before the plants sprout.

Moisture activates catalysts within the seed that begin the germination process. It also mixes with the proteins or starch present in the seed to form a soluble that provides the nutrients the plant embryo needs before it establishes itself.

The First Root

After the moisture is absorbed and seeds start to grow a node will erupt and eventually puncture shell of the seed, revealing a radicle (the first root of the plant). This root will act as an anchor for the plant allowing it to develop further roots and eventually leaves.

The First Sprout

Once the onion seed has roots forming the next stage is sprouting. The onion will produce green growth that starts pushing upwards towards a light source and developing its first leaves.

Leaf Development

This is about from week 2-8 as new leaves develop and grow. Once onion leaves reach the 4″-6″ mark you can thin them out and use the culled plants as green onions.

Even if you don’t want to cull the onions it can make sense to trim the green onion growth down so the plant grows stocky and study growth above ground.

Up until now the plant has relied on the nutrients supplied by the original seed. At this point it will start relying on energy produced by photosynthesis and other elements pulled from the air by the leaves as well as nutrients and water sourced from the roots.

During photosynthesis, the plant will take in light energy, carbon dioxide, and water to create glucose and oxygen. The glucose will either be used for further leaf growth or will be stored in the root system.

Leek Development

Around week 8-12 the leaves will continue to grow and the section below the leaf but above the roots will expand and grow into a leek. This area is where the onion bulb will grow from.

Bulb Growth

When daylight hours reach the length that the variety of onion being grown needs onion bulb growth will be triggered in the plant.

The current leaves will continue to grow, but the plant will stop developing new ones after it has 8 to 12 leaves. To initiate bulb growth an onion needs at least 4 leaves to gather enough energy for development.

  • Long-day – These need anywhere from 14 to 16 hours of sunlight and typically have the largest bulbs. They need more time for growth though because of their size.
  • Short-day – These require the least amount of time at 10 to 12 hours of sunlight.
  • Day-Neutral – These require 12 to 14 hours of sunlight.

As the bulb continues to bulk up and grow in size it will begin to surface making it seem like it popped up out of the ground. The growth will start to slow down and the leaves will begin to fall over. At this point the onions are ready to be harvested.

Second Year of Growth

Onions not harvested will go dormant in the fall and winter. Top growth will die off and the plant will wait until warmer temperatures return. In regions with cold winters onions need to be winterized by covering them with heavy mulch or bringing them inside.

In the second year when temperatures warm again the bulb will grow leaves and sprout again and a flower stem will grow from the bulb.

The buds (usually purple or white) will bloom and the plant will be pollinated. Seeds will form and the original plant will have very little energy left over. At this point, the onion will enter senescence and the plant has completed it’s life cycle.

On the left are flowering onion plants and on the right are harvested onion seeds.

Are Onions Annuals or Perennials?

Onions are neither annuals or perennials but are biennials! They take two years to complete their life cycle, growing flowers that produce seeds in the second year of their life and then dying off.

Please comment below with any thoughts you have on the onion plants life cycle.

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