Onion Plant Growth Stages | Life Cycle

Depending on who you ask, onions may or may not be a staple of the home vegetable garden. While these vegetables are easy enough to get your hands on at the grocery store or local farmers’ market, there are plenty of reasons to try growing them yourself!

Whether you’re interested in growing traditional bulb onions (Allium cepa) or bunching onions (Allium fistulosum), I’ll walk you through the key onion plant growth stages and tell you everything you need to know to cultivate these veggies from seeds or sets.

Conditions for Growing Onions

Onions are moderate growers that form bulbs in spring or summer. Many gardeners stagger onion starts over the course of a few weeks to extend the harvest later in the season.

Onions need a sunny spot to grow. Most cultivars require at least 6 hours of sun per day but more is always better. Avoid planting onions alongside vegetables that will tower over them throughout the growing season.

Soil is arguably the most important factor in a successful onion harvest. Onions should be planted in loose, fast-draining soil with high amounts of sand and organic matter. Rocky or compacted soil will interfere with healthy bulb development. 

According to Ohio State University, onions grow best in neutral or slightly acidic soil with a pH of 6.0 to 6.8. Alliums are sensitive to overly acidic soil so I don’t recommend erring below this range.

Onions are heavy feeders that need consistent nutrition to develop properly. Amend the soil before the start of the season with aged compost or manure. Per the Old Farmer’s Almanac, supplement with a high-nitrogen vegetable fertilizer until bulbs start to form — i.e., you notice the soil cracking or lifting around the plant base.

I think onions are some of the best vegetables to grow in raised beds. Not only does this give you greater control over the soil content and drainage but it also eliminates issues with embedded rocks and other debris. Just be sure to select beds deep enough to accommodate good bulb development. As a rule, plant onions in containers or raised beds that are at least 10 inches deep.

onions in raised bed

Growth Stages Of An Onion

The Allium genus includes a number of pungent vegetables, including onions, garlic, shallots, and chives. All of these plants are primarily grown for their edible bulbs, though some are also cultivated for their stems and foliage.

Bulb onions generally require more than 90 days to mature from seed to harvest. Bunching onions can be harvested in as little as 60 days but are fully mature at around 110 days.

Bulb onions are categorized by the amount of daily sunlight required to trigger bulb development. These categories include:

  • Short-day onions need 10 to 12 hours of sunlight per day to trigger bulb development. These varieties are typically recommended for gardeners in USDA zones 7 and higher. According to Oregon State University, 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. They’re known for being milder and sweeter than other onions. You can grow day-neutral onions in all but the hottest regions but they perform best in zones 5 and 6. Day-neutral onions are usually sown in the fall and harvested the following spring.
  • Long-day onions require 14 hours or more of daily light for healthy bulb development. I recommend these varieties for anyone in zones 6 and colder. Start long-day onion seeds indoors in late winter.

Garden onions can be started in a few different ways. The most popular methods are by sowing seeds or planting onion sets. 

Onion seeds are no different than any other type of seed. They must be germinated and cultivated through the seedling stage before they can start producing flavorful bulbs or stems. 

Onion sets are immature bulbs that were started the previous year and then stored in dormancy through the wintertime. Basically, onion sets offer a way to ‘jump start’ your onion plants in the spring. They’re also affordable and widely available at the start of the gardening season pretty much anywhere you can purchase vegetable seeds.

onion sets
Red Baron onion sets

You’ll run into proponents of both onion seeds and sets throughout the gardening world. I think sets are significantly easier to grow in most climates — an opinion that’s shared by the University of New Hampshire. The biggest advantage to starting onions from seeds is that you can choose from nearly countless cultivars for your home garden.

1. Seed Germination 

Onion seeds may be started indoors or directly sown in the garden depending on your climate and the type of onion you’re growing. For example, as a cool-climate gardener, my experience is of starting long-day onions indoors in late winter or early spring.

For a standard mid-summer harvest, start onion seeds indoors 8 to 10 weeks before your last frost date or outdoors as soon as the soil is workable. Plant seeds just ¼ inch deep in rich, well-draining soil.

According to Utah State University, the ideal soil temperature for onion germination is 75°F. Place indoor seed starts underneath a grow lamp or in a bright window. If using a grow lamp, be sure to set up a daily timer or you might accidentally trigger premature bulb development.

In optimal conditions, onion seeds germinate in 7 to 10 days. Note that onion seed viability drops significantly after 1 to 2 years, so think twice about using old seeds left over from previous seasons!

Starting onion seeds too early will impact germination rate and growth. On the other hand, starting onions too late in the season may expose the plants to high temperatures and trigger bulb development before they’re ready. Timing is crucial to producing high-quality bulb onions at home.

2. Seedlings

Onions are monocots, meaning that they start with a single proto-leaf. This leaf is called a cotyledon and looks a lot like a blade of grass. All foliage that appears after the cotyledon is mature foliage. 

Onion seedlings are not as hungry as their full-size counterparts but still need a consistent nitrogen source. According to Pennsylvania State University, most potting media will support seedlings for 2 to 3 weeks before additional fertilizer is required. Wait to apply fertilizer until after the onions’ first adult leaves have emerged.

You may have heard that trimming onion plants at this stage encourages stronger, more compact growth. I don’t feel passionately one way or another about this practice but some gardeners swear by it. 

If you do choose to trim back your seedlings, be sure to use a sharp, sanitized blade. An article from Iowa State University advises cutting back onions to 4 inches tall once the seedlings grow to at least 5 inches.

This is also the perfect time to thin out the seedlings if they’ve grown a bit crowded. It’s common practice to heavily sow onion seeds since germination rates can be hit or miss. Thin seedlings so they’re spaced at least 2 inches apart (larger cultivars may need to be over 4 inches apart for proper bulb development).

3. Vegetative Growth

After the seedling stage, onions focus on producing leaves. This is true of both bunching and bulb onions.

Healthy foliage is incredibly important for bulb onions even though it won’t be harvested. These onion types use their leaves to store energy that then goes into bulb development. Lacklustre vegetative growth will have a negative impact on your final onion crop.

Bulbing onions will direct energy into vegetative growth until the day length triggers bulb initiation. Because of this mechanism, the number of days between sowing or transplanting and mature vegetative growth can vary quite a bit.

4. Bulb Formation

Again, when the daily sunlight reaches a certain length, your bulbing onions will redirect energy from vegetative growth and begin bulb initiation. While this development occurs below the soil’s surface, you should notice some lifting or cracking of the soil when the bulbs start to form. Do not disturb the plants in an effort to check bulbing progress.

Because of the way onions utilize stored energy to create bulbs, the size and number of leaves at the time of bulb initiation directly correlate with the final bulb size. 

5. Flowering

Edible onions produce white, umbel flowers that are clustered together. The first sign of an onion flowering is a papery bud emerging from a central stalk. This bud should be cut off as soon as possible if you intend to harvest the plant’s bulb.

Onions are biennials and, when allowed to grow naturally, tend to flower late in their second year. Environmental factors like extreme heat, cold, or general stress can trigger early flowering, a phenomenon known as bolting.

Bolting is a bad sign because it means your onions are using valuable energy to create flowers and seeds versus a delicious bulb. The only time onion flowering is desirable is when the goal is to collect and save seeds.

Onion Plant Growth Stages - flowering onion
Beautiful onion flower

6. Pollination

Onion flowers are technically self-fertile but, in practice, cross-pollination is needed to induce seed production. This is because the male part of an onion flower sheds its pollen before the female part is ready to be fertilized. 

Insects and wind are largely responsible for pollinating garden onions. When growing onions for seed collection, keep in mind that only alliums within the same species will cross-pollinate. To prevent unwanted crossing between onion varieties, the Seed Savers Exchange recommends spacing of at least 800 feet.

Onion Flower Timelapse Video

This short video shows the onion flower forming and opening to reveal a beautiful array of tiny white flowers.

Onion Flowers Blooming Time Lapse

7. Seed Development

With successful pollination, each of the small white flowers that make up the larger umbel cluster will develop into individual seeds. Remember: onion flowers are made up of dozens of separate flowers even though they look like a single bloom from a distance!

If you want to collect the seeds for future use, allow them to mature and start to dry out on the stalk. You can harvest the seeds when the flower head starts to dry out. 

When To Harvest Onions

Harvest bulb onions when you notice the above-ground stems begin to yellow and fall over. This signals that the onion has used all of the available stored energy and converted it into a bulb. Depending on your climate and the onion variety, this should happen after at least 90 to 100 days of growth.

Bulb onions that show signs of bolting should be harvested and used within a couple of days. 

You can start harvesting bunching onions once the stems grow several inches tall. Most cultivars reach a harvestable height within 60 days of sprouting. 

The best way to harvest bunching onions is by cutting individual stems as needed for your culinary needs. This technique extends the harvest and encourages additional growth later in the season.

FAQ Onion Plant Growth Stages

Citations

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Ben's horticultural interest grew when graduating from Hertfordshire University in 1997. Having contributed to numerous publications including Better Homes & Gardens, Garden Design Magazine, and The English Garden. He is also the author of Propagating Houseplants Made Easy.