Onion Growing Stages: Uncover the Secrets to Perfect Onions

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If there’s one vegetable that’s earned a place in kitchens around the globe, it’s the humble onion. Belonging to the illustrious Allium family, onions are biennial plants that evolve through a fascinating life cycle. In the first year, they yield leaves and bulbs, while the second sees them producing flowers and seeds. With just 90 days from planting to harvest, there’s no reason not to start your own onion garden today.

So, what exactly happens during these 90 days? Well, the life of an onion can be broken down into six key stages, starting with seed germination and culminating in maturity and harvest. Want to deepen your understanding of these stages? Keep reading!

How Many Growth Phases Do Onions Have?

Opinions can differ when it comes to defining the growth phases of an onion. Some sources mention two phases — the vegetative and the bulbing phase — while others add a third, the flowering phase. In general, though, if you’re looking to harvest onions, you’d want to do so before any flowering or seed production starts.

If you’re interested in saving seeds, you’ll want to track seven growth stages. However, if your main goal is to harvest bulbs, the number of stages depends on whether you started with seeds (four stages) or onion sets (three stages).

Key Phases and Stages in Onion Development

The Allium genus, home to onions, garlic, shallots, and chives, is renowned for its pungent vegetables. Most of these plants are grown for their edible bulbs, although some are also cultivated for their stems and foliage.

Bulb onions, the ones we’re most familiar with, generally take more than 90 days to mature from seed to harvest. Bunching onions, on the other hand, can be harvested in as little as 60 days but are fully mature at around 110 days.

Interestingly, bulb onions are categorized based on the amount of daily sunlight they require to trigger bulb development. These categories include:

  • Short-day onions need 10 to 12 hours of sunlight per day to trigger bulb development. Recommended for gardeners in USDA zones 7 and higher, these onions grow best in the winter and early spring in mild climates.
  • Day-neutral (or intermediate) onions form bulbs when days are 12 to 14 hours long. These onions, known for their mild and sweet taste, can be grown in all but the hottest regions but perform best in zones 5 and 6.
  • Long-day onions require 14 hours or more of daily light for healthy bulb development. These are best suited for zones 6 and colder.

You can start your onion garden in a few different ways. The most popular methods are by sowing seeds or planting onion sets.

Onion seeds, like any other type of seed, must be germinated and nurtured through the seedling stage before they start producing flavorful bulbs or stems.

Onion sets, on the other hand, are immature bulbs that were started the previous year and then stored in dormancy over the winter. They offer a ‘jump start’ to your onion plants in the spring.

The gardening community is divided between the proponents of onion seeds and sets. While seeds offer a wider variety of cultivars, sets are generally considered easier to grow in most climates.

In the end, the choice is yours. Whether you opt for seeds or sets, your homegrown onions will bring a burst of flavor to your meals and a sense of accomplishment to your gardening adventures.

1. The Magic of Seed Germination

If you’ve been flirting with the idea of growing onions, it’s time to turn that dream into a reality. The beauty of these humble bulbs is that they can be started indoors or sown directly in your garden, depending on your local climate and the onion variety you’ve chosen. As a seasoned cool-climate gardener, I’ve had great success starting long-day onions indoors during the tail-end of winter or the dawn of spring.

To get a robust mid-summer harvest, it’s best to kick-off your onion seed journey indoors, 8 to 10 weeks before the last scheduled frost date. If you’re sowing outdoors, start as soon as your soil becomes workable. Plant your seeds just ¼ inch deep in rich, well-draining soil to give them the best chance of survival.

Utah State University highlights that the sweet spot for onion germination is a soil temperature of 75°F. Position indoor seed starts underneath a grow lamp or near a bright window for the best results. If you’re using a grow lamp, don’t forget to set up a daily timer to prevent premature bulb development.

With the right conditions, you should see your onion seeds germinating in 7 to 10 days. Remember that onion seed viability takes a hit after 1 to 2 years, so be cautious about using older seeds left over from previous seasons!

Timing your onion planting correctly is crucial. Starting too early can impact germination and growth, while starting too late may expose your plants to high temperatures, triggering bulb development before they’re ready.

2. Nurturing the Seedling Stage

Onions are monocots, and their first growth, a single proto-leaf, is called a cotyledon. This little sprout may look a lot like a blade of grass, but don’t be fooled. It’s the start of something wonderful! All foliage that appears after the cotyledon is mature foliage.

While onion seedlings might not be as hungry as their full-grown counterparts, they still need a steady source of nitrogen. Pennsylvania State University research reveals that most potting media will support seedlings for about 2 to 3 weeks before additional fertilizer is necessary. For best results, wait to apply fertilizer until after the onions’ first adult leaves have emerged.

You may have heard that trimming onion plants at this stage could encourage stronger, more compact growth. While I don’t feel strongly about this practice, some gardeners swear by it.

If you decide to trim back your seedlings, always use a sharp, sanitized blade. Iowa State University suggests cutting back onions to 4 inches tall once the seedlings surpass 5 inches.

This is also the perfect time to thin out your seedlings if they’ve grown too crowded. Since onion seed germination rates can be unpredictable, it’s common to sow heavily. However, thinning seedlings to a space of at least 2 inches apart (or over 4 inches apart for larger cultivars) is crucial for proper bulb development.

3. The Vegetative Growth Phase of Onions

Spinach and onion plants  on a vegetable garden ground with other vegetables in the background

As your seedling develops a robust root system, the leaves will begin to multiply at a rapid pace. Within a few weeks, you’ll notice the leaves have grown thicker and more prominent. These leaves are now “true” leaves and will use sunlight to photosynthesize, providing the plant with the energy it needs to continue growing and thriving.

As spring arrives, it’s time to prepare the area where you plan to plant your onions. Your onion patch ideally should be located in a space that gets full sun, ensuring it isn’t shaded by taller plants.

When it comes to soil, onions prefer well-drained soil with a pH of 6-6.5. You can easily test your soil’s pH levels using a test kit, which is readily available online or at your local garden store.

Before planting, ensure the ground is free from rocks and other solid materials that could obstruct the growth of the onion bulb. You can create planting holes using a bamboo stick—these should be ½ inch deep and spaced 4-6 inches apart—with rows 8 inches apart.

When you plant the onions, avoid compacting the soil too heavily onto the roots, as this can limit the oxygen available to the roots and potentially cause them to rot. Lightly water your seedlings, and the soil will naturally settle into place.

During this growth phase, stay vigilant about weeds. They can compete with the onion roots for nutrients, which can hinder your onions’ growth.

At this stage, the onion leaves may appear ready for harvest, even though the bulb has not yet formed. If you’re eager, you can harvest now for a product similar to leeks, chives, or small onions. However, if you’re aiming for a full onion bulb, you’ll need to be patient and wait until the leaves drop.

Give your onions a nutrient boost during spring by applying a small amount of nitrogen-rich fertilizer, such as rotted manure. But be cautious—overfeeding can cause the onion plant to focus on foliage growth instead of bulb development.

If you notice any flower stems starting to shoot up at this stage, snip them off with a pair of sharp scissors. This will redirect the plant’s energy to bulb production rather than flower growth.

4. Bulb Formation: The Onion’s Grand Performance

Here’s the part of the onion’s life cycle that packs a punch – the formation of the bulb. For this to occur, your onion seedling needs at least four true leaves. Each new leaf contributes to the layers within the bulb.

When you slice open an onion, the layers or ‘scales’ become evident. The speed at which these scales form, and the number of leaves that grow, is directly influenced by the amount of sunlight the plant receives.

Interestingly, different onion varieties have varying sunlight requirements to form their bulbs. They fall into three categories:

  • Long day – Budding onion farmers in zones 6 to 3 should opt for these. Requiring 14-16 hours of daylight, cultivars like Walla Walla, Copra, and Yellow/White Spanish are ideal.
  • Short day – If you’re in zones seven and warmer, these are your go-to onions. They need 10-12 hours of daylight. Cultivars include Red Creole, Texas Super Sweet, and Texas Early White.
  • Day-neutral – Perfect for zones 5 and 6, these require 12-14 hours of daylight. Cultivars to consider are Candy Onion and Sweet Red Cimarron.

Once your onion plant sprouts 8 to 12 leaves, the plant shifts focus to bulb growth, halting leaf production. The leaves grow tall, reaching 10-30 inches (25-76cm) and 4-24 inches (2.5-15cm) wide. The bulbs, depending on variety and conditions, grow 1-6 inches (2.5-15 cm) in diameter.

4.1 Onions Reach Maturity: The Grand Finale

This stage of the onion’s growth cycle is like the grand finale of a fireworks display. The tops sprouting from the onion’s neck start to droop. Shortly after, the bulb surfaces to the top of the soil, as if taking a well-deserved bow.

After 90 days of sowing the seeds, the bulb reaches maturity. The leaves, no longer needed for photosynthesis, dry out and turn from lush green to pale yellow or white. Meanwhile, the bulb swells in size.

If you see that 20-50% of the leaves have fallen and dried off, get your baskets ready – your onions are ripe for harvesting. If you’re growing onions from sets (small bulbs), expect this stage around 80 days, with the same signs of maturity.

4.2 Time for the Harvest

Green onions are ready for harvest once their leaves stand tall at 6 inches (15 cm). These fresh greens are perfect to add a zing to your dishes. When the onions have matured and the bulb is fully developed (typically after 100 days of planting seeds), you can proceed to harvest the bulbs.

To do this, gently dig around the edges of the bulb with a garden fork. Be careful not to dig too close to the bulb, as you want to avoid piercing it. This can lead to rot and bacterial infections later on.

As you loosen the soil around the onion, grasp it firmly and pull. The onion will easily come out from the ground. Shake off the excess soil from the bulb and roots. At this point, avoid the temptation to wash them with water.

Allow your harvested onions to dry for 1-2 weeks before using them. You can do this by placing them on a tray in a well-ventilated area. Alternatively, you can hang them up using their dried leaves, a method preferred by some gardeners.

After the drying stage, the tops of the onions will be completely dry, and the outer layer will turn brown and crisp. Cut off the dried leaves from the tops of the onion, leaving about 2 inches (5 cm).

To store, place your onions in a wire basket or bag in temperatures between 32-40°F (0-4°C). This can help them last for up to three months. Remember to avoid storing onions in humid areas to prevent rot.

As someone with a green thumb, I recommend leaving a few onions in the ground to continue their lifecycle until the following year. Onions are biennial plants, meaning they will start to flower in their second year and produce a batch of seeds that you can save for the next planting season. This not only saves you money but also allows you to cultivate a variety of onions that have adapted to your garden’s conditions.

When the onions have fully matured and the leaves have completely died off, leave the bulbs in the ground over winter. You can add a light layer of mulch for added protection from frost and insects. Come spring, as temperatures warm up, they will sprout new leaves and grow a flower stalk. After flowering, they will produce small seed heads, which you can collect and store for future use.

5. The Flowering Stage

If you’re an average vegetable grower, you might not often see your onions reach the flowering stage since the bulbs are typically harvested beforehand. But if you’re interested in seed collection, then the flowering stage is critical.

Onions, which are edible, produce clusters of white, umbrella-like flowers. The first sign of flowering is a papery bud that emerges from a central stalk. If your goal is to harvest the bulb, you need to nip this bud in the bud – cut it off as soon as you spot it.

Onions are biennials, meaning they usually flower in their second year. However, environmental stressors such as extreme heat or cold can trigger early flowering, known as bolting. Bolting is undesirable as it diverts the onion’s energy from producing a flavorful bulb to generating flowers and seeds. The only time you would want your onions to flower is when you’re aiming to collect and save seeds.

6. The Art of Pollination

While onion flowers are technically self-fertile, they usually need cross-pollination to produce seeds. This is due to the timing of their reproductive process: the male part of the onion flower releases its pollen before the female part is ready to be fertilized.

In most garden situations, wind and insects play the major role in pollinating onions. If you are growing onions specifically for seed collection, remember that only alliums within the same species will cross-pollinate. To prevent cross-pollination between different onion varieties, the Seed Savers Exchange recommends a spacing of at least 800 feet.

For a closer look at the flowering process, watch the following time-lapse video that showcases an onion flower forming and opening to reveal a stunning array of tiny white flowers.]

7. Seed Development

The magic of nature truly unfolds with successful pollination in onions. Each of the tiny white flowers, contributing to the larger umbel cluster, evolves into individual seeds. Just a quick reminder: don’t be fooled by the appearance of a single bloom from a distance, an onion flower is actually a fascinating ensemble of multitude of separate flowers!

If you’re keen on collecting seeds for future planting, allow them to mature and start the drying process while still on the stalk. Harvesting these seeds becomes feasible when the flower head begins to dry out.

Growing Onions: A Comprehensive Guide to Conditions, Harvesting, and FAQs

Onion bulbs ready for sowing on soil with shovel, Planting material. Vegetable background.

The Best Conditions for Growing Onions

Onions, the humble yet versatile root vegetables, need specific conditions to thrive. Depending upon the variety, these moderate growers form bulbs during the spring or summer months. To prolong the harvesting season, smart gardeners stagger their onion planting over several weeks.

For optimal growth, onions need a good dose of sunlight. Make sure your chosen spot has at least 6 hours of sunlight daily. Do not plant onions next to tall vegetables that might overshadow them during the growing season.

The soil plays a critical role in determining the success of your onion crop. Onions prefer loose, fast-draining soil with ample sand and organic matter. Hard, rocky, or compacted soil can hinder the healthy development of onion bulbs.

According to Ohio State University’s research, onions thrive in neutral or slightly acidic soil. The ideal pH range is 6.0 to 6.8. Steer clear of overly acidic soil as alliums (onion family) are sensitive to acidic conditions.

Feeding onion plants with ample nutrition is crucial for their proper development. Prior to the growing season, enrich the soil with aged compost or manure. The Old Farmer’s Almanac recommends supplementing with a high-nitrogen vegetable fertilizer until the bulbs start to form. You can identify this stage when you see the soil cracking or lifting around the plant base.

Raised beds are excellent for onion cultivation. They offer better control over soil content and drainage, and eliminate issues with rocks and debris. Ensure the container or raised beds are at least 10 inches deep to facilitate good bulb development.

The Right Time to Harvest Onions

Knowing when to harvest onions can make a difference to your produce. For bulb onions, wait until the stems start to yellow and droop. This indicates that the onion has utilized its stored energy to form a bulb. Depending on your region and the onion variety, this typically happens after 90 to 100 days of growth.

Onions that show signs of bolting (premature bolting to seed) should be harvested and consumed within a few days.

Bunching onions can be harvested once the stems reach several inches in height, usually within 60 days of sprouting. To extend the harvesting period and stimulate growth, cut individual stems as per your cooking needs.

Common Challenges in Growing Onions: From Pests to Diseases

While onions are known for their ability to ward off pests from other plants, they’re not entirely immune to their own set of problems. Various factors, from bacterial to fungal infections, can hamper their growth. Additionally, certain pests can also pose a threat to these potent plants.

Now, let’s delve into the specifics so you can know what to watch out for:

Soft Rot: The Bacterial Bulb Bane

Ever noticed your onion tops turning pale, appearing water-soaked, and feeling mushy? Your onions may have fallen victim to soft rot. This bacterial issue typically affects mature bulbs and can be triggered by heavy rains, overhead watering, and pesky insects.

In extreme cases, you might even be able to squeeze out a foul-smelling liquid from the onion’s neck. Beware, soft rot can also strike after harvesting, so proper storage at correct temperatures is crucial.

Preventive measures for soft rot include avoiding overhead irrigation and using pesticides to deter insects from spreading bacteria.

Downy Mildew: The Fungi That Feasts on Foliage

If you spot lesions on the older leaves that start off as pale green and later turn yellow, your onions might be dealing with downy mildew. This airborne fungal infection can cause your onion to lose all of its leaves, severely impacting the bulb formation stage.

Downy mildew is often seen during spring when the weather is transitioning from cooler, wet conditions to warmer, dry days.

To keep downy mildew at bay, use a fungicidal spray to prevent the spread of spores. Washing seeds before planting and maintaining a clean area around your plants that’s free from leaf litter and rotting debris can also help.

On Guard Against Onion Blight

Leaf blight is a troublesome fungal infection that targets onion leaves. It manifests as small, yellow to white flecks, roughly 1-5mm in diameter. Over time, these flecks sink into the leaves, causing them to die within 5-12 days. The loss of foliage unfortunately leads to irregular bulb formation and inhibits the onion from reaching its full maturity.

Blight thrives in environments with high humidity and heavy rainfall, spreading through airborne spores. To tackle this affliction, apply a fungicidal spray as soon as you spot the first signs of infection. This will help prevent the spores from spreading. Additionally, maintain a clean garden area, free from rotting debris and leaf litter, to minimize the chance of fungal growth.

Beware of Onion Maggots & Flies

Onion maggots and flies are two of the most common pests you’ll encounter when growing onions. The larvae resemble tiny, white grains of wheat and live in the soil during winter. As the weather warms up in spring, they mature into adults and wage war on your plants.

These pests aren’t picky about their targets; they’ll attack onion plants of any size or maturity level, even destroying seedling crops. To control an infestation, apply a pesticide pellet or spray. Also, ensure you harvest every onion once they’ve matured; leftover onions can perpetuate the problem into the following year.

Thrips: Small But Mighty

Thrips are thin, pale yellow to brown insects about 1/12 of an inch long. Their small size makes them hard to spot, but their damage is unmistakable. These pests thrive in the tight spaces of onion foliage, leaving behind tiny holes or streaks where they’ve fed. As they continue their assault, the foliage will turn brown and yellow, eventually dying off and stunting the growth of your onion bulbs.

To manage thrips, maintain good garden hygiene by removing any rotting debris from around your plants. Apply an insecticide spray to penetrate the foliage and deter these tiny destroyers.

While chemical fungicides or pesticides can be effective, consider trying an organic option first. Neem oil, for instance, can be used as a spray to deter pests and reduce the spread of fungal spores on your plants. It’s also safe to use on the soil to disrupt the lifecycle of larvae.

Another organic option is pyrethrin spray, which can paralyze bugs and insects on contact. Using these organic treatments can ensure your vegetable crops are free from harmful chemicals.

Frequently Asked Questions About Onion Growth and Troubles

planting onion seedlings in the soil

How Many Onions Will One Bulb Yield?

A solitary onion bulb, often referred to as a set, will nurture a single plant. These sets are nothing but immature bulbs that have been stored to induce dormancy. When you plant them in your garden, they awaken from their slumber and resume their growth, maturing into full-sized onion plants.

What is the Time Frame for Onions to Mature?

Onions are considered slow to moderately-paced vegetables and require a minimum of 90 days to mature from seeds. If you are growing onions from sets or transplants, they will be ready to be harvested after approximately 80 days. However, Bunching onions, grown from a seed, can be harvested after about 60 days.

How Can You Determine When an Onion is Fully Grown?

The maturity of your onions can be gauged by the number of leaves they have and when the leaves start to droop. The perfect onion is said to have thirteen leaves, and you can confirm the size of the bulb at this stage.

What Conditions are Ideal for Onion Growth?

Onions thrive in cool conditions. Hence, you should plant onion sets when the soil is pliable, and the temperature is not expected to drop below 28°F (-2°C). If you are transplanting seedlings grown indoors, wait until the temperatures reach 50°F (10°C) to transplant them outside.

What is the Average Height of a Fully Grown Onion?

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

Final Thoughts

Onions are a staple in the kitchen, adding depth and flavor to a plethora of dishes. If you have ample space in your garden, growing a bountiful onion crop can supply your household for a significant part of the year.

Most gardeners aim for 16-20 plants per square meter, yielding one bulb per plant. This makes them an excellent choice for a winter crop when all other plant growth slows down. Once planted, onions are relatively low maintenance. Starting the seeds indoors allows you to grow a variety of onions.

Onions are an excellent choice for beginner gardeners. This guide should give you a head start in understanding what to expect while your onions grow.


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Adam White is the founder and chief editor at CraftedGarage.com. He has years of experience from years of Gardening, Garden Design, Home Improvement, DIY, carpentry, and car detailing. His aim? Well that’s simple. To cut through the jargon and help you succeed.