Onion Growing Stages

I think everyone can agree that onions are definitely a great vegetable to grow in your garden.  If you want to boost your knowledge and successfully grow onions. Then you should without a doubt take the time and understand the onion growing stages first.  

From germination to maturation, the growth stages are broken into individual phases. As each garden plant has their own unique journey during their life-cycle.  Fully knowing and understanding the growth stages of your plant is one of the best ways to be successful.  With this knowledge, you can provide your plants according to their needs as they proceed from one step to another.  

When it comes to vegetable gardening, onions are generally very common.  However, it is very important to understand their growth stages. As much is out of sight and takes place beneath the soil surface.  

How Long To Grow

Compared to other garden vegetables, onions require a much longer growing season.  This is because of the time it takes for the plants to form bulbs.  Depending on the variety and the local growing conditions. It can take 100 to 175 days for the seed to mature.  

Development Stages

Similar to carrots, home gardeners usually grow onions as an annual crop. Waiting to harvest the plants when their bulb is mature and their vegetative tops start to die out.  However, in actuality, Allium cepa are biennial plants. Which means they take two seasons to grow. As they go from a  germinating seed to a plant which can produce its own seeds.  

Although you may have a plan to plant your onions yearly. Let us discuss the individual growth stages into two separate seasons for better understanding.  

Year 1

The very first year of the onion growing stages mainly is focused on growing the tops and developing bulbs.  Most gardeners happen to harvest their bulbs during the first season, so they only know about these stages.    


Depending on your growing zone, onions can be planted during two different times in the year as they are a cool-season vegetable.  If you are a gardener in the northern latitude, then you need to plant them in early spring in order for a late summer crop.  For gardeners in southern latitudes, you need to plant them in the spring or fall.    

  • For those planting in the spring, try to get transplants in the garden beds while the temperature of the soil is above 28℉ consistently.  However, you will need to plant the seeds 8 to 10 weeks beforehand if you are planning to grow them from seed.  
  • If you are planting in the Fall, aim to give them 6 to 8 weeks of time to grow before the temperature begins to fall and get cooler.  Once cool weather begins to settle, they will become dormant and will resume growing again in the spring.  

You must keep the soil moist at all times from the time you plant your seeds.  Within their protective seed coat, seeds that are able to germinate has an embryo and food reserves.  Soil moisture enters the seed and softens the seed coat.  This process is call imbibition.  The moisture is able to trigger cellular respiration once inside of the seed and metabolize the food reserves.  

It takes about 10 days for the seed to germinate.  


The emergence of the radicle through the coat of the seed is the first step in germination.  These privacy root’s job is to anchor the seedlings in the soil and support the plant as the first emerging roots.  Also, In this stages the emerged radicles absorb moisture and nutrients from the soil which helps with the onion growing .  


While the radicle continues to absorb nutrients and moisture from the soil, the first shoot develops and through the softened seed coats, they begin to emerge.   Due to the gravitational forces naturally directing them, the shoot begins to grow upward toward the sun.  The seedings change focus once the shoot finally breaks through the surface of the soil.  At this point, the plants start to put all of their energy into developing leaves.  

First True Leaf Forms

The seedling’s growth is still somewhat slow as the plant still depends on the endosperm for food.  The seedlings proceed to work hard to form leaves as their reserves start to dwindle.  Finally, the first true leaf appears which looks similar to the flat green leaves that are seen on the mature pants, but more smaller.  The seedlings begin producing glucose for food as photosynthesis starts.  

The seedling looks like what is known as green onions at this stage and you can harvest them if you are looking for green onions for any recipe.  

“Leek” Stage

Now that they are able to photosynthesize, the plant will grow at a much faster pace.  The plant growth hormones will work to turn the cells into new leaves once the seedlings develop new and undifferentiated cells.  The rate of photosynthesis is increased by each new set of leaves which drive their growth even faster.  Also, at this stage, you will notice that the neck of the seedling will become thicker.  

During this stage, you will get the first true leaf and then continue on until the plant has around four to seven true leaves.

You can harvest in this stage if you are looking for leeks or prefer small onions.  For Bulbs, allow them to continue growing.  

Bulb Initiation

With light being their main driver, during this stages, the onion that are growing start focusing on the initiation of bulbs.  Regardless of the climate conditions, the plants must have four leaves in order for bulbing to occur.  Each of the leaves that are new and growing forms a scale which develops into a ring of the bulb.  

You will get different varieties depending on the length of the day or how much sunlight the plants need daily in order to “set” bulbs.  

  • 14 to 16 hours of daylight is required for Long-Day varieties and they produce the largest bulbs.  It is because before bulb initiation, they get more time to develop leaves.
  • For Short-Day varieties, they require about 10 to 12 hours of daylight.
  • 12 to 14 hours of daylight is needed for Day-neutral types.

Bulb Development

The onion plant stops producing new leaves once 8 to 12 leaves have developed and completely focuses on their bulbing process.  As the already formed leaves continue to grow, they get bigger in size as it forms the central storage tissue of the bulbs.  

This is the point where the onions begin to pop up out of the ground as they push away the soil.  


A physiological response known as “tops down” is demonstrated as the bulbs get close to maturity.  This is due to the resources moving from the leaves to the scales making the bulb size to get bigger as they swell.  You will also know that it is time to harvest as the depletion of resources from the leaves causes the tops to fall over and dry down.  

Vegetative Senescence

The water within the foliage begins to freeze when autumn temperature drops and frosting begins.  As a result, the fluid leaks out of the cell walls from being ruptured by the sharp edges of the ice crystals.  Without any fluid with the cells and being damaged, the onion tops cannot survive and die back to the surface of the soil.  

Year 2

Many gardeners are not much familiar with the plant’s formation of flowers and seeds since they do not let the onion see the second growing stages.  The onions that are not harvested after the first season and allowed to overwinter in the ground do grow the next spring again where they complete their life-cycle as they set seeds.  

Vegetative Growth

Those onions start to actively grow again once the spring temperature begins to climb up.  The cells start to develop and elongate from the hormones.  It is pretty similar to germination in the previous season.  Since the root system already completely developed the year before, the plant quickly focuses on sending up a shoot in order to break through the surface of the soil.  

Flowering Stalk Emerges

The plants begin to bolt as the air and soil temperature start climbing as the season heats up.  Preparing for the end of its life-cycle, the plant shoots up a flowering stem as the vegtative growth comes to a halt.  The white and purple flowers finally bloom at the top once the stalk is formed and the buds are grown.  

Formation of Seeds

Seeds form from the pollination with the help of birds, bees or even butterflies.  The onion bulbs get all of the nutrients that were reserved in the bulbs.  This is the food reserve that will be used for germination.  The bulbs would be fibrous and woody if you harvest at this point.  


The onion plant’s life cycle comes to a full circle with the formation of the seeds.  With the onions directing most of its resources into developing the seeds, not much is left to grow more plants.  As there is not much resources left and there is no need to keep growing, the genes responsible for senescence are induced by the plant growth hormones and the plant finally dies.  

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