Ever wondered how onions grow? From germination to harvest and more, this guide will take you on a journey through the different stages of onion growth.
Along the way, you’ll learn about the science behind onion growth, as well as some fun facts.
So whether you’re a curious cook, a budding gardener, or just someone who loves learning new things, this guide is for you!
Onion Growing Stages
Onions come in many varieties, including red, yellow, white, and sweet onions. They also come in various shapes and sizes, from small pearl onions to large Spanish onions.
An onion plant is a biennial plant, meaning it takes two years for the plant to complete its lifecycle. In the first year, the plant produces bulbs, and in the second year, the plant produces seeds.
The onion plant takes approximately 90 to 120 days to reach full maturity from planting to harvesting. However, if you are growing onions for seeds, the process can extend up to 16 months.
There are 13 onion growth stages, each with its own unique characteristics. Let’s take a closer look at each stage of onion growth, starting with planting.
Stage 1. Planting
The first stage of onion growth is planting. There are two main ways to plant onions. You can plant onion seeds or bulbs.
Planting Onion Seeds
Onion seeds are relatively inexpensive and easy to find. You can start your onions early in the season. However, onion seeds can take a long time to form a bulb, and they are more susceptible to pests such as slugs attacks at the early stages of growth compared to onion bulbs.
To plant onion seeds, loosen the soil and remove any debris like sticks and rocks. Sow the seeds in rows that are 1/2 inch (1.3 cm) deep. After sowing, lightly cover the seeds with soil and water them gently, avoiding washing them out.
Pro Tip: When planting onion seeds, you can use the overseeding method to increase the chances of successful germination and maximize yield. With the overseeding method, you scatter the seeds in a dense manner. This ensures that more seeds will germinate.
Planting Onion Bulbs
Onion bulbs are more expensive than onion seeds, but they grow more quickly and are initially less susceptible to pests. However, onion bulbs have a shorter shelf life than onion seeds, so you need to plant them in the same season you bought them.
To plant onion bulbs, dig small holes in the prepared garden soil, ensuring they are just deep enough to cover the bulb. Place each bulb in a hole 4 to 6 inches (10 to 15 cm) apart, root side down, and cover them with soil. Plant the bulbs so that the top of the bulb is just barely covered with soil.
|Planting with seeds||Inexpensive, easy to find, can start onions early in the season.||It can take a long time to form a bulb, more susceptible to pest attacks at early stages of growth.|
|Planting with bulbs||It can take a long time to form a bulb, more susceptible to pest attacks at the early stages of growth.||They are more expensive, have a shorter shelf life, and need to be planted in the same season you bought them.|
Regardless of whether you choose seeds or bulbs, selecting an area with loose, well-draining soil and full sun for successful onion growth is a must.
Pro Tip: Planting onions in compact soil can hinder bulb expansion, potentially limiting their full size. To ensure larger and healthier onion bulbs, plant them in loose, well-draining soil that gives more room for growth and development.
Stage 2. Germination
The second stage of onion growth is germination, which is when the seed or bulb sprouts tiny roots and a shoot. This usually takes about 7 to 10 days after planting, depending on the variety of onions, the temperature, and the soil’s moisture level.
Once the onion seed or bulb has germinated, the first leaves will emerge from the soil and will continue to grow.
The Conditions For Germination
Onion seeds need warm soil to germinate. The ideal soil temperature for germination is 65-75 oF (18-24 oC). If the soil is too cold, germination will be delayed.
Onion seeds also need moisture to germinate. The soil should be moist but not soggy. If the soil is too dry, the seeds will not germinate.
Pro Tip: If you overseeded, don’t rush into thinning the seedlings. At this stage, onions are susceptible to slug damage. Wait till the onions grow a bit bigger, and then thin out the damaged and the weak ones keeping 4 to 6 inches (10 to 15 cm) apart.
Stage 3. Vegetative Growth
The third stage of onion growth is vegetative growth, which is when the onion plant grows leaves and a root system. This stage usually lasts for about 60 days.
During vegetative growth, the onion plant will produce a cluster of leaves called a scallion. The scallion will continue to grow and eventually form a bulb at the base.
Healthy onion plants will have a deep green color and a strong root system. The leaves should also be upright.
As the onion plant grows, its leaves will become heavier, causing them to bend and touch the ground. Strong winds can also lead to the bending of onion leaves. This is a normal occurrence and does not indicate any problem with the plant.
Also, as the plant grows, it redirects its energy to the bulb and the formation of new leaves. This will make the older leaves start to yellow and wilt. While this is a part of the normal onion growth process, you can remove the older leaves for aesthetic reasons or just leave them be.
Pro Tip: Mulching around your onion plants will help keep the soil moist and suppress weeds.
Stage 4. Bulb Development
After the vegetative growth stage, the onion plant enters a phase known as bulb development. During this stage, the plant initiates the formation of its bulb, a part of the onion we use in the kitchen.
As the plant continues to grow leaves, the bulb also starts to take shape. The leaves and bulb grow in tandem, complementing each other’s progress. Over time, some of the older leaves may naturally wilt, which is a normal part of the plant’s growth process.
The bulb steadily expands and matures throughout the growth cycle, a process that typically spans 60 days, with variations depending on the onion variety.
Stage 5. Harvesting Leaves
Once the onion bulb has grown slightly and the leaves have matured, you can begin leaf harvesting. To do this, gently snip off 2-3 leaves from each onion at the base of the plant. These leaves can be used fresh or stored in the refrigerator.
The best time to harvest the leaves is when they are vibrant green, as they are most tender and flavorful at this stage. You can perform leaf harvesting daily, allowing the plant to continue growing and producing new leaves.
Leaf harvesting offers a delightful way to savor fresh onions throughout the growing season.
Pro Tip 1: Do not cut all the leaves! While the onion will survive and regrow the leaves from the cut base, it will stun the plants growth and reduce the overall bulb size.
Pro Tip 2: If you have easy access to fresh onion leaves, there’s no need to harvest them for storage, as they will lose their crispness on the same day.
Stage 6. Maturation
After the bulb develops, the onion plant will enter the maturation stage. This is a period when the onion’s flavor, texture, and overall quality reach their peak.
As the onion bulb matures, the bottom leaves will gradually wilt, and the bulb will become firm. The neck of the bulb will toughen and may eventually fall over. Once the bulb reaches full maturity, it will be ready for harvest.
Stage 7. Onion Harvest
At this stage, you have to make an important decision. You can either harvest the bulb or leave it in the ground for next year. Harvesting the onion bulb gives you immediate access to fresh onions, while leaving it in the ground offers the opportunity to enjoy a more extended onion growing experience with the potential for future leaf harvests and seeds.
The decision of whether to harvest the bulb or leave it in the ground ultimately depends on your preference and needs.
If you choose to harvest the bulb, that marks the end of the onion growing stages for this harvested plant.
To determine if your onions are ready for harvesting, look for these signs:
- Yellowing Tops: When the foliage of the onion plant turns yellow and starts to droop, it indicates that the plant is diverting its energy from the leaves to the bulb, which is a sign of maturity.
- Bulb Size: Check the size of the bulb by gently digging around the base of the plant to expose the bulb. Mature onions should have reached a suitable size depending on the variety grown.
To harvest the bulb, use a garden knife or a garden fork to loosen the soil around the onion. Then, gently pull it up, taking care not to damage the bulb. Once the plant is out of the ground, trim the leaves to about 2 inches long.
Pro Tip: Onion bulbs can be kept in the ground and harvested as needed. This is a great way to have fresh onions on hand throughout the colder months.
However, should you decide to leave the bulb in the ground throughout the winter, the onion growing stages will continue into the following year. The plant will produce more leaves for you to harvest. Eventually, it will form onion flowers and seeds, allowing you to continue the onion’s life cycle in your garden.
So, the following onion growing stages are applicable if the plants were not harvested and were left to mature further in the ground. This decision opens up the potential for an extended gardening experience.
Stage 8. Vegetative Senescence
If the onion plant is not harvested, it undergoes visible changes as it prepares for the winter. As the plant ages, the foliage begins to show signs of senescence, which refers to the aging and deterioration of plant tissues.
The once vibrant green leaves gradually turn yellow and may start to wither. This process is entirely normal and essential for the onion plant to progress in its life cycle.
There are a few reasons why vegetative senescence occurs. First, the plant can no longer photosynthesize as efficiently as it once did due to reduced daylight. Second, the plant is starting to store energy for the next growing season. Third, the plant is preparing to enter a period of dormancy.
Vegetative senescence is a natural process that all onion plants go through. To help the onion plant through this stage, cut all the leaves at the base of the plant to prevent the spread of disease and leave the plant undisturbed throughout the cold months.
Stage 9. Year 2 Vegetative Growth
After the onion plant has gone dormant for the winter, it will begin to grow again in the spring. This is known as year two vegetative growth.
During this stage, the onion plant will produce new leaves. The leaves will be similar to those that were produced in the first year, but they will be slightly larger. The onion plant will continue to grow in this stage until it reaches maturity.
Pro Tip 1: You can harvest onion leaves at this stage the same way as described in “Stage 5: Harvesting Onion Leaves.”
Pro Tip 2: At this stage, some onion varieties may exhibit bulb splitting, giving rise to multiple bulbs and subsequently producing more leaves and, eventually, flowers.
Stage 10. Flowering
After the onion plant has completed its year two vegetative growth, it will begin to bolt. Bolting is a natural process that occurs when the onion plant senses that the days are getting longer and the temperatures are getting warmer. When this happens, the plant will stop producing leaves and start to produce a flowering stem.
The flowering stem emerges from the center of the onion bulb and grows rapidly, reaching a height of several feet. At the top of the stem, a flower head develops, containing a spherical umbel of flowers.
The flowers will typically be white or purple. Still, they can also be yellow, pink, or red, depending on their variety. The flowers will bloom for a few weeks until they are pollinated.
Pro Tip: You can harvest onions that started to flower. However, the middle part of the bulb will be hollow and not as tasty as during the first year of harvest. This is because the plant is diverting its energy from the bulbs and towards the flowering process. The bulbs will become tough and fibrous, and the flavor will become sharp and unpleasant.
Stage 11. Pollination
Once the onion flowers have bloomed, they must be pollinated to produce seeds. Pollination is the transfer of pollen from the male anthers of one flower to the female stigma of another flower. Insects like bees, bumblebees, butterflies, and hoverflies usually do this. However, pollination can also be done by wind, gravity, or by hand pollination.
The flowers will remain in bloom for several weeks while they await pollination. The ultimate success of this stage depends on the presence of pollinators in the garden, making it essential to create a pollinator-friendly environment to support onion seed production.
Once the flowers are successfully pollinated, they begin the process of seed development. 
Stage 12. Seed Formation
Once the flowers are successfully pollinated, the seeds begin to develop. They will continue to grow over the next few weeks, increasing in size and weight. Eventually, they will develop a hard outer shell and fall to the ground, where they will germinate the following spring.
The seeds will be ready to harvest when they are fully mature and dry. You can tell when the seeds are mature by looking at their color and texture. The seeds should be black and hard.
To harvest onion seeds, follow these steps:
- Wait until the seeds have turned black and are fully dry.
- Then, gently shake the flower heads to release the seeds.
- Collect the seeds in a paper bag or container.
- Store them in a cool and dry place until you are ready to plant them.
Pro tip: If you leave the seeds on the plant for too long, their weight will eventually bend the stem and cause it to drop on the ground, scattering seeds around it. This can result in unwanted volunteer plants or seed loss. To avoid this, harvest the seeds as soon as they are dry.
Stage 13. Plant Senescence
Senescence is the final stage of an onion plant’s life cycle. It occurs after the seeds have formed and matured and the plant has directed most of its resources toward seed production. As a result, there are few resources left for further plant growth.
The senescence process is triggered by a combination of factors including the completion of the reproductive stage, the depletion of nutrients, and the onset of environmental stressors such as cold weather.
As the plant enters senescence, its stem will begin to yellow and brown, eventually withering and collapsing. At this stage, seeds left on the plant will disperse through wind, animal activity, or human intervention.
Senescence is a natural part of the onion plant’s life cycle. It allows the plant to clear the way for new growth as the old plant dies and decomposes.
And there you have it! All the stages of onion growth explained in a nutshell.
Onion Growing Stages Frequently Asked Questions (FAQs)
How Long Do Onions Take To Grow?
From the time of planting to harvesting, it typically takes the onion plant around 90 to 120 days to achieve full maturity. This duration may vary depending on the onion variety, local climate, and growing conditions.
However, if you are growing onions specifically for seeds, the process can extend up to 16 months, requiring more patience and long-term commitment.
How Do You Know When Onions Are Ready To Dig Up?
Harvest onions when their tops turn yellow and fall over, indicating their maturity. Additionally, check the size of the bulbs by gently digging around the base of the plant to ensure they have reached a suitable size for the variety you are growing. The bulbs should be firm and well-formed.
Once you’ve determined they are ready for harvest, gently lift the onions from the soil using a garden fork, garden knife, or shovel, being careful not to damage the bulbs during the process.
How Long Does It Take An Onion To Complete Its Life Cycle?
An onion follows a biennial life cycle, which means it takes two years to complete its growth stages. In the first year, the plant produces bulbs, which are the underground storage structures that we commonly eat.
Onions are typically harvested during this first year, as they are in their prime for culinary use and have not yet transitioned to the seed production stage. If left in the ground and allowed to grow through the second year, the onion will focus on seed production. During this time, it sends up a flowering stem and produces seeds in spherical umbels atop the stems.
Since most onions are harvested in their first year to be consumed as bulbs, the majority of gardeners do not witness the seed production stage in their home gardens.
How Many Onions Do You Get From One Plant?
You can typically harvest one large, mature onion bulb from each onion plant. However, certain onion varieties, known as multiplier onions, have the ability to produce multiple bulbs in a cluster. These varieties can yield several smaller onions per plant, providing a convenient option for continuous harvesting.
Do Onion Sets Multiply?
Yes, onion sets have the ability to multiply. When an onion set is planted, it develops a single bulb in the first year. However, some varieties also generate a cluster of small bulbs, referred to as “sets,” around their base. These sets can be replanted the following year to produce a greater yield of onions.
Onion growth is a complex process that involves a number of different stages. In this article, we have explored the 13 stages of onion growth, from germination to senescence. We have also discussed the factors that influence each stage of growth, as well as some tips for growing healthy onions.
We hope you enjoyed this overview of the onion growth stages. With a bit of care and attention, you can enjoy fresh, homegrown onions all season long. If you have any questions, just drop us an email.
If you enjoyed reading this article, check out our other similar ones in the Gardening How To section.
Want to share your thoughts, or have you read something you disagree with? Send us an email! We would love to discuss it 🙂