A Comprehensive Guide to Onion Growth Stages: From Seed to Harvest

Onions are versatile and essential vegetables that add flavor to countless dishes. But do you know the journey these vegetables take from seed to maturation?

Understanding the different stages of onion growth is crucial for successful cultivation. Each growth stage plays a vital role in developing this cool-season crop, from planting the seeds to harvesting mature bulbs.

Keep reading as we explore the onion growth cycle, providing valuable insights and tips for every step of the journey.

7 Onion Growth Stages

The seven stages of onion growth are:

  • Planting
  • Germination
  • Seedling stage
  • Vegetative stage
  • Bulb formation stage
  • Maturation stage
  • Harvesting time


Onions thrive in well-drained soil with a pH level between 6.0 and 7.5. Loamy soil is ideal, but sandy or clay soils can also be suitable with proper amendments.

Choose the onion variety that suits your preferences and growing conditions. Common types include yellow, white, and red onions.

Some onions are better suited for specific USDA hardiness zones, so consider your region’s climate when selecting a variety.

Onions grow from either seeds, sets (small bulbs), or transplants. The timing for planting onions depends on your preferred method.

Seeds: If you’re starting from seeds, sow them indoors 8 to 10 weeks before your area’s last expected frost date. Once the seedlings are around 6 inches tall and the soil has warmed up, you can transplant them outdoors.

Sets: Onion sets are small bulbs grown from seed and dried. They are easier to plant than seeds and provide a head start. Plant sets in early spring, as soon as the soil is workable and temperatures are no longer likely to dip below freezing.

Transplants: Nursery-grown onion transplants can be purchased and planted directly in the garden. Transplant the onions outdoors after the danger of frost has passed.

Dig a trench or individual holes for planting onions. The depth and spacing depend on the onion variety:

Plant seeds around 1/4 to 1/2 inch deep, spacing them about 1 inch apart. Once the seedlings are established, thin them to around 4 to 6 inches apart.

Plant onion sets approximately 1 inch deep and 4-6 inches apart in rows. Space the rows approximately 12 to 18 inches apart.

Dig holes that are deep enough to accommodate the roots of the transplant. Space transplants 4 inches apart in rows, with rows spaced 12 to 18 inches apart.

After planting, ensure the onions receive adequate sunlight, ideally full sun.

Here is a table showing different onion varieties along with their recommended USDA hardiness zones:

Onion VarietyUSDA Hardiness Zones
Yellow SweetZones 3-9
Red BurgundyZones 3-9
White BermudaZones 3-9
Walla WallaZones 5-9
VidaliaZones 8-9
Texas 1015Zones 7-9
CipolliniZones 3-9
ShallotsZones 3-9
ScallionsZones 3-9
LeeksZones 3-9

These are general guidelines, and specific cultivars may have slightly different recommended zones.


After planting, the seed absorbs water from the surrounding soil. This moisture triggers the seed’s metabolic processes and activates the growth cycle.

The onion seed absorbs water, swells, and undergoes internal changes. The outer seed coat may split, allowing the emerging root to penetrate the soil.

Tiny roots begin to grow and extend downward from the seed. These roots anchor the seedling in the soil and absorb water and nutrients necessary for growth.

Also, a shoot emerges from the seed. It grows upward, eventually breaking through the soil surface. This shoot will develop into the stem and leaves of the onion plant.

Here’s a timeline of the germination stage.

  • Day 1: Seed is planted in the soil.
  • Day 2-4: Water absorption causes the seed to swell and undergo internal changes.
  • Day 4-7: Roots start to emerge and grow downward.
  • Day 7-10: The shoot emerges from the soil surface, forming the initial leaves.

Seedling Stage

During this stage, the onion seedlings develop from tiny shoots into young plants with well-established root systems.

After the cotyledons, the onion seedling produces true leaves. True leaves are distinct from cotyledons and have the characteristic shape of onion foliage, typically long and slender. These leaves play a vital role in photosynthesis, converting sunlight into energy for the plant.

It is important to thin out the onion seedlings during the seedling stage if they are overcrowded. Proper spacing allows each seedling to receive sufficient light, nutrients, and space to grow into a mature bulb.

Thinning involves removing excess seedlings, usually by gently pulling them out, leaving adequate space between the remaining plants.

You can transplant the onion seedlings to their final growing location when they have developed a strong root system and several sets of leaves. Transplanting allows the seedlings to continue growing and eventually mature into fully-grown onions.

Vegetative Stage

The vegetative stage is among the most vital onion growth stages. In this stage, the onion plant produces multiple layers of green leaves. Initially, the plant starts with a few small leaves, and as it progresses through this stage, it grows more leaves.

These leaves are essential for capturing sunlight and converting it into energy through photosynthesis.

The onion leaves increase in size and thickness as the vegetative stage advances. The leaves become longer, broader, and more robust, enabling them to capture more sunlight and carry out efficient photosynthesis. The leaf growth contributes to the overall health and vigor of the plant.

During the vegetative stage, the onion plant uses photosynthesis to produce energy. The leaves contain chlorophyll, which absorbs sunlight and converts it into sugars and carbohydrates.

The onion plant actively takes up nutrients from the soil during the vegetative stage to support leaf growth. The roots absorb essential minerals, such as nitrogen, phosphorus, potassium, and micronutrients necessary for healthy plant development. Adequate nutrient availability is crucial during this stage to ensure optimal leaf growth.

On average, the vegetative stage can last for several weeks to a couple of months, during which the plant focuses on establishing a robust leaf canopy.

The duration of the vegetative stage can vary depending on several factors, including onion variety, growing conditions, and environmental factors.

Bulb Formation Stage

After the vegetative stage, the onion plant transitions to bulb formation. This transition is primarily triggered by environmental cues, particularly changes in day length and temperature.

During bulb formation, the onion plant redirects its energy and nutrients from leaf growth toward the development of the bulb.

The plant allocates resources to the underground part of the stem, which thickens and forms the basis for the bulb. This resource allocation shift is crucial for developing a healthy and sizable bulb.

As the bulb formation progresses, the stem of the onion plant thickens. This thickening occurs at the base of the stem, where it transitions into the bulb. The thickened stem provides support and structure to the developing bulb.

Within the bulb formation stage is a specific phase called bulb initiation. It is during this phase that the actual formation of the bulb begins. The plant triggers the growth of modified leaves, which will eventually develop into the layers of fleshy scales that compose the onion bulb.

Onions are classified as long-day or short-day varieties, depending on their response to day length. Long-day onions form bulbs when exposed to longer daylight hours, while short-day onions require shorter daylight hours to initiate bulb formation.

Temperature also affects the rate of bulb development, with optimal ranges varying for different onion varieties.

Typically, it takes several weeks for the bulb to form and mature. The specific time can range from around 8 to 12 weeks or more, depending on the variety and environmental conditions.

Maturation Stage

During the maturation stage, the onion bulb continues to grow in size as it accumulates nutrients and moisture.

The layers of fleshy scales within the bulb expand, resulting in a larger and more developed onion. This expansion is essential for achieving a mature bulb suitable for harvest and storage.

As the onion bulb matures, the leaves above the bulb start to dry and wither. This natural process indicates that the plant is approaching harvest time.

The withering leaves signal that the onion has allocated most of its resources to the bulb and is diverting energy from the foliage. The leaf’s color might turn yellow or brown.

The outer layers of the bulb become more tightly packed, protecting the inner layers. The firmness of the bulb is an indicator of its readiness for harvest. A mature bulb will have a firm texture and resistance when squeezed.

Please provide optimal growing conditions to ensure the healthy development of the onion bulb. This includes providing adequate water, avoiding overwatering to prevent rotting and ensuring sufficient sunlight exposure.

Monitoring the onion plants closely during maturation is essential to identify and address any signs of disease or pest infestation.

Common onion pests include onion maggots, thrips, and various fungal diseases. Preventive measures and promptly treating any issues can help protect the crop and ensure a successful harvest.

Harvesting Time

Green onions or scallions are harvested for their tender, young shoots and are commonly used in salads, stir-fries, or garnish. These onions can be harvested at any stage when the leaves have reached the desired length. Typically, this is done when the leaves are about 6-8 inches tall.

To harvest green onions, you can use scissors to cut the leaves above the soil level, leaving the root and the base intact. This technique allows the remaining plant to regrow and produce additional shoots for future harvests.

The harvesting process is different if you’re growing onions specifically for mature bulbs. Onions meant for bulb production require a longer growing period to reach full size and flavor potential. Harvest timing depends on several factors, including the onion variety and local growing conditions.

When the leaves of the onion plant have entirely dried and fallen over, it indicates that the bulbs have reached maturity and are ready for harvest. This occurs in late summer or early fall, depending on the planting time.

Here’s how to harvest mature onion bulbs:

Loosen the Soil: Gently loosen the soil around the bulbs using a garden fork or shovel, being careful not to damage the bulbs.

Lift the Bulbs: Lift the bulbs from the soil by grasping the foliage close to the bulb. Avoid pulling or yanking the foliage, as it may separate from the bulb.

Remove Excess Dirt: Gently shake off or brush off any excess dirt from the bulbs. Be careful not to remove the papery outer layers of the bulb.

Curing: After harvesting, it’s important to cure the onions before storage. Curing allows the outer layers of the bulb to be dry and form a protective layer, which extends their storage life.

Place the harvested onions in a single layer or hang them in a well-ventilated, dry area away from direct sunlight. Allow them to cure for a few weeks until the necks (where the foliage meets the bulb) become dry and the outer skin becomes papery.

Storage: Trim the foliage to about an inch above the bulb once the onions are properly cured. Store the onions in a dry, cool, and well-ventilated location, such as a basement or pantry. Properly cured and stored onions can last for several months.

Specific timing for harvesting onions may vary depending on the variety, local climate, and personal preference.

Some growers prefer to harvest onions while the foliage is still partially green for a milder flavor. Others prefer to wait until the foliage has completely dried for a stronger flavor and better storage potential.

onion growth stages

Wrapping Up

Growing onions can be a rewarding experience, and understanding the different onion growth stages is vital to a successful harvest. Providing optimal growing conditions, timely care, and monitoring allows you to cultivate healthy onions. Enjoy the fruits of your labor in the kitchen.