6 Onion Growing Stages From Bulb: Life Cycle & Problems

The globally recognized vegetable is a favorite for many dishes, and growing your own couldn’t be easier.

As part of the Allium family, onions are a biennial plant that produces leaves and bulbs in the first year and flowers and seeds in the second year. You can harvest your own crop of onions after 90 days of planting seeds in the cool season.

Onions go through 6 stages in their life, starting with seed germination and then growing into a seedling. Following the seedling stage, they grow vegetation and form a bulb in the soil. The onion will then mature and be ready for harvest.

Check out this awesome time-lapse video of the onion-growing stages:

Take a look at the onion growing stages in more detail to get ahead on how to get started with your onion crop this year.

What Are The Onion Growing Stages?

Onions can be grown from sets, seeds, and transplants. Onion sets are tiny bulbs that have been harvested early and kept in a cool, dry place.

Growing onions from sets can be quicker than growing from seed; getting a fully grown onion takes less time.

Growing from seed gives you more options when choosing a variety of plants. Seeds take longer to mature but can be started indoors before the planting season.

Growing onions from transplants allow you to skip the germination stage and plant them straight into the ground when they are at the seedling stage.

Depending on the stocks your local garden center holds, they may have a few onion varieties available, so you are limited to what cultivar you are growing.

Seed Planting and Germination

The first stage of the onion growing cycle is when the seed has germinated. This usually happens 6-12 days after planting.

The ideal soil temperature for germination is 70-75°F (21-25°C), and you can achieve this by using a heat mat or starting your onions indoors.

Onions are not 100% successful with germination, so many gardeners prefer to start their onions with small bulbs, known as sets.

To plant onion seeds, you can start 8-10 weeks before your area’s last spring frost date and use a seed tray or egg carton filled with a premium potting mix.

After each cell is filled with potting, mix the soil and plant a few seeds per cell (you can divide them later on). Keep the seeds moist by using a spray bottle of water during this time.

By storing the seed trays indoors at a consistent temperature, you will have the best success rate.

After 6-12 days, you will see the seed shell has cracked, and a small shoot will emerge. If the soil temperature is below 75°F (25°C), the germination will be slower, and you may wait 15-25 days for germination.

You can cover the seed tray with clear plastic, which will imitate a greenhouse and trap the heat and moisture during this time.

If you decide to plant out sets (small bulbs) of onions or transplants, you will skip this stage of the onions growth cycle and save yourself two weeks of seed raising.

The Seedling Stage

After germination, the onion will start to produce a seedling which is in the form of a single green shoot.

You might see the seed’s casing is at the top of the shoot at this point, and you can remove it by clipping it off. This will save the seedling time and energy by trying to remove it itself.

During the seedling stage of the onion’s growth cycle, you will need to provide the small plants with 12 hours of light per day so be sure to position them next to a sunny window.

Because the shoots are skinny at this stage, they can fall over and rot, so it’s best to give them a trim with a clean pair of scissors every other day. By trimming the seedlings, you will promote new thicker growth.

As the seedlings grow, new leaves and roots also develop in the soil.

Onion seedlings generally prefer good root space, and it’s best to keep an eye on the bottom of the seed tray to see if they have started to escape through the drainage holes.

If your onion’s roots begin to outgrow their cells, you can transplant them into pots of the next size up.

Alternatively, when the seedlings have reached 5-6 inches tall, you can plant them into individual cells and grow them further until they are ready to be planted in the ground.

After dividing and transplanting your seedlings, be sure to water them with a half-strength water-soluble fertilizer. This will give the seedlings a boost of nutrients and help them produce larger bulbs later.

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Before planting out your onion seedlings, make sure to harden them off for two weeks; this prepares them for the great outdoors and reduces the stress on the tiny plants.

To harden off seedlings, put them in a sunny spot during the day and return them indoors during the night when the temperatures fall.

Vegetation Grows

As the seedling develops a sturdy root system, the leaves will multiply rapidly, and you will notice the leaves have become thicker and more prominent within a few weeks.

The onion leaves in this stage are true and use the sunlight to photosynthesize and provide the plant with energy to continue growing.

As the spring season approaches, prepare the area where you want to plant your onions. The position of your onion patch should be in full sun, whereas other tall growing plants do not shade it.

It would be best to plant onions in well-drained soil with a pH of 6-6.5. You can test your soil pH levels with a test kit, and they are available online or in garden stores.

Ensure the ground is free from rocks and other solid materials that can obstruct the bulb’s growth. Using a bamboo stick, poke holes into the garden bed ½ inch deep and 4-6 inches apart. And plant in rows 8 inches apart.

When planting onions, don’t compact the soil back onto the roots so heavily, as this will reduce the oxygen that is available to the roots and cause them to rot. Lightly water your seedlings, and the soil will naturally fall into place.

Keep an eye on weeds during the growing stage, as you don’t want them to be competing with the onion roots for nutrients.

When onions are vegetative, the leaves look harvestable but without a bulb form. You can harvest now if you want a vegetable like leeks, chives, or small onions.

Otherwise, if you are growing a whole onion bulb, you will need to wait a little longer until the leaves drop.

During spring, apply a small amount of nitrogen-rich fertilizer, such as rotted manure, to give the onions a boost of nutrients. Be careful not to overfeed, as this will cause the onion plant to concentrate on growing foliage rather than bulb growth.

If you see any flower stems start to shoot at this stage, snip them off with a pair of sharp scissors to help the plant focus on bulb production.

The Bulb Begins to Form

This is the exciting stage of the onion’s life cycle, where it starts to form a bulb. The onion seedling needs at least four true leaves to develop a bulb. Each new leaf that grows contributes to the layers within the bulb.

When you cut open an onion, you see the layers form, and these are called scales.

During this time of the onions’ life, the amount of sunlight will determine how fast this process goes and how many leaves grow.

Some onion varieties require different amounts of sunlight to form their bulbs, and the following names categorize them:

  • Long day – Require 14-16 hours of daylight, making them ideal for zones 6 to 3. Cultivars such as Walla Walla, Copra, and Yellow/White Spanish.
  • Short day – Require 10-12 hours of daylight, making them ideal for zones seven and warmer. Cultivars such as Red Creole, Texas Super Sweet, and Texas Early White.
  • Day-neutral – Require 12-14 hours of daylight, making them ideal for zones 5 and 6. Cultivars such as Candy Onion, and Sweet Red Cimarron.

After 8 to 12 leaves have grown on your onion plant, the leaves will stop growing, and the plant will concentrate on bulb growth.

The leaves will grow tall and reach 10-30 inches (25-76cm) and 4-24 inches (2.5-15cm) wide. Depending on the variety and conditions, the bulbs will grow 1-6 inches (2.5-15 cm) in diameter.

When the bulb has reached its maximum size, it will begin to push out to the top of the soil and will begin the maturation stage of its life cycle.

Onions Reach Maturity

This stage of the onion’s growth cycle is when the tops that sprout from the neck of the onion start to fall down. Shortly after the tops have fallen down, the bulb will surface to the top of the soil.

The bulb has reached its mature size after 90 days of sowing the seeds, and the leaves are no longer needed to photosynthesize and provide energy to the bulb.

You will see that the leaves will dry out during this time, going from a lush green color to a pale yellow or white. As this happens, the bulb would have swelled in size too.

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When 20-50% of the leaves have fallen and dried off, the onions will be ready for harvesting.

If you are growing onions from sets (small bulbs), you can expect the maturation stage to occur after 80 days and will show the same signs of maturity.

Time to Harvest

You can harvest green onion tops once the leaves have reached 6 inches (15 cm) in height, and they are great to use in a wide range of dishes.

After the onion has reached maturity and the bulb is fully formed (usually after 100 days of planting seeds), you can harvest the bulbs.

To harvest the bulbs, use a garden fork and gently dig into the soil around the edges of the bulb. Don’t dig too close to the bulb, as you don’t want to pierce it with the fork, which can cause rotting and bacterial infections later on.

As you loosen up the soil around the onion, you can pull it, and it will surface from the ground. Shake the excess soil off the bulb and roots and avoid using water to wash them at this stage.

After harvesting your onions, you will need to leave them to dry out for 1-2 weeks before use. To dry your onions, place them on a tray in a well-ventilated area, or some gardeners prefer to hang them up using their dried leaves.

After the onions have been through the drying stage, the tops will be completely dry, and the outer layer will become brown and crisp like paper. Cut the dried leaves off the tops of the onion, leaving 2 inches (5 cm).

Store your onions in a wire basket or bag in temperatures between 32-40°F (0-4°C) to help them keep for up to three months. Avoid storing onions in humid areas to prevent rotting.

It’s a great idea to leave a few onions in the ground to continue their lifecycle until the following year.

As onions are biennial, they will begin to flower in the second year and produce a batch of seeds you can save for the next season.

Saving seeds this way saves you money and gives you a variety of onions that have adapted to your garden’s conditions.

After the onions have reached maturity and the leaves have entirely died off, leave the bulbs in the ground over the winter. You can apply light mulch to them at this stage to protect them from frost or insects.

As the temperatures warm up during spring, they will reshoot leaves and grow a flower stalk. After flowering, they will produce small seed heads that you can collect and store.

Common Onion Growing Problems

Although onions make an excellent pest deterrent to other plants, they can experience their own share of problems during the growth cycle.

Some of the problems can be caused by bacterial or fungal infections such as soft rot, downy mildew, or blight. There are also a few common pests to keep an eye out for when growing onions too.

Take a look at these infections in more detail to give yourself a better understanding of what to look out for:

Soft Rot

Soft rot is a bacterial problem that occurs on mature bulbs that can result from heavy rains, overhead watering, and insects. You can identify soft rot on your bulbs by seeing the tops of the onions turn pale in color and look water-soaked and mushy.

Sometimes you can squeeze a foul-smelling liquid from the neck of the onion. Soft rot can occur after harvesting, too, so be sure to store them at the correct temperatures to avoid any risks.

To control and prevent the risk of soft rot forming, avoid overhead irrigation and use pesticides to prevent insects from carrying bacteria around your plants.

Downy Mildew

This fungal infection can be seen where lesions on the older leaves start as a pale green color turning yellow later on.

As it is a fungal infection, the spores are carried through the air or in rain splashes. Downy mildew can cause the onion to lose all of its leaves, potentially destroying the bulb formation stage of its lifecycle.

You may see this during spring when the climate is transitioning from cooler wet weather to warmer dry days.

To control and prevent the risk of downy mildew, you can use a fungicidal spray to stop the spores from spreading. Wash your seeds before planting, and keep a clean area around your plants free from leaf litter and rotting debris.


Leaf blight is a fungal infection that occurs on onion leaves. It can be seen as small yellow to white flecks 1-5mm in diameter.

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The flecks will sink into the leaves and eventually cause the leaf to die 5-12 days after the first few flecks have appeared.

The loss of foliage to the onion plant causes the irregular formation of the bulb and will prevent the onion from reaching its mature size.

As a fungal infection, it spreads through spores and favors heavy rainfall areas and climates with high humidity. To control blight on your onions, use a fungicidal spray when you first see the signs to prevent the spores from spreading.

Maintain the planted area and keep it free from rotting debris and leaf litter.

Onion Maggots & Flies

The most common pests when growing onions can be identified by tiny white-bodied maggots that resemble a grain of wheat.

They live in the soil when the plant is dormant during winter, and when the weather warms up in spring, they turn into an adult and attack your plants.

Onion maggots and flies are not fussy about the size of the onion plant or its level of maturity and have been known to attack and destroy seedling crops too.

To control onion maggots or onion flies, use a pesticide pellet or spray and ensure that you harvest every onion from the area once they have reached maturity. Any onions left in the ground will cause you the same problem the following year.


Adult thrips are usually thin and pale yellow to brown in color. They are so tiny and 1/12 of an inch long, so spotting them can be difficult.

Thrips thrive in the tightly packed spaces of the foliage and feed off the plant. You will see tiny holes or streaks made where they have been.

As the thrips continue to feed, the foliage will turn brown and yellow and eventually die off, interrupting the growth of your onion bulbs.

To control onion thrips, ensure good garden hygiene by removing any rotting debris from around the plants and using insecticide spray on the foliage where it can be penetrated.

While you can treat most of these problems with a chemical fungicide or pesticide, I recommend trying an organic option on the first attempt.

Opt for making a neem oil spray to deter pests and reduce the spread of fungal spores on your plants.

Neem oil is available online and can be used on the soil to stop the cycle of Lavea forming. You can also use neem oil as a foliar spray to treat thrips and fungal infections too.

Another organic option to reduce the pests on your onion plants is using pyrethrin spray, paralyzing bugs and insects.

Using an organic treatment method for these pests ensures that your vegetable crops are free from harmful chemicals.

FAQ About The 6 Onion Growing Stages From The Bulb

How do you know when an onion is grown?

You will know your onions are ready to harvest by the number of leaves they have and when the leaves have dropped.

Thirteen leaves are classed as the perfect onion, and you can check the size of the bulb at this stage.

What conditions do onions need to grow?

Onions prefer cool conditions, so plant the onion sets when the ground is workable and the temperatures are not likely to go below 28°F (-2°C).

If transplanting indoor-grown seedlings, wait until the temperatures are 50°F (10°C) to transplant outside.

What is the average height of a grown onion?

The average height of a grown onion is 6-8 inches (15-25 cm) when grown in suitable conditions.

To End

It’s a fact onions are widely used in the kitchen to add flavor to most meals, and if you have enough space in your garden, you can grow a bumper crop to supply your household for a good portion of the year.

Most gardeners will work on 16-20 plants per square meter, and you will receive one bulb per plant; this makes them an excellent winter crop to grow when all other plants have slowed down. Onions are carefree once they are planted, and if you start the seeds off indoors, you will allow yourself to grow different varieties.

Onions are great to grow as a beginner gardener, and after reading this guide, you will better understand what to expect while your onions grow.

Lucy Young

Meet Lucy, a seasoned gardener with a green thumb and a wealth of experience cultivated over 10 years in her own backyard oasis. Now, she channels her passion into writing, sharing invaluable gardening knowledge on her website. From nurturing plants to expert pruning techniques, Lucy's articles are a treasure trove for both seasoned enthusiasts and budding gardeners. Join her on this leafy journey as she sprinkles insights, tips, and tricks to help you create your own flourishing paradise. Get ready to dig into her gardening wisdom and unlock the secrets of a thriving garden!

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