How to Grow Onions

I've not been super-successful growing onions in the past, but then, I never really dug in and learned how to grow them well. 

In this article, I'll share what I've learned about growing onions, and we'll both have a great crop this year! 

This turned into a long article - if you want the summary, scroll to the bottom.

 

Types of Onions

Onions are an old, old crop. Humans have been cultivating onions for at least 5000 years! Read more at the National Onion Association website article Onion History.

Since they've been grown and eaten all over the world, many types of onions have been developed. Most people are familiar with the onions found in grocery stores: yellow, red, white, and sweet varieties like Walla Walla and Vidalia. 

There are also cippolini onions, which grow in a "flatter" shape. Bunching onions are varieties that don't form a bulb and stay small. These can be grown from seed and produce green onions or scallions. (See article How to Grow Green Onions from June 2021.)

Shallots are also a type of onion, along with chives, ramps, and leeks. 

Onions are Unique

As a crop to grow, onions have a few important characteristics you need to understand if you're going to be successful growing them. 

Onions are biennials. This means they take two years to complete their life cycle when grown from seed. It works likes this:

  • Seed is planted and year 1 starts. The onions grows foliage (the straight tubular leaves) and creates a bulb. 
  • At the end of the growing season, the foliage dies back and the bulb remains to go dormant over winter. 
  • In the spring, the warm weather "wakes up" the dormant bulb. The dormancy period that just ended triggers the plant to grow a flower. This is year 2.
  • Once the flower blooms and the seeds are created from the flower, the onion plant is done and it dies. 

The onions we grow and eat are the first year onions. They are planted in the spring and harvested in the late summer and early fall. 

Occasionally, you'll get a flower in year one. Onions grow in response to light and temperature, and when the unexpected flower appears, it indicates that the onion plant was exposed to fluctuating temperature. It "thinks" it's in its second year.

If you see a flower stalk forming (it looks different from other foliage) clip it off before it matures, unless you want to grow seed. If the flower stalk matures, the onion bulb degrades in quality for eating and it can rot. 

Onions growth responds to the length of daylight in the summer. Longer summer days trigger the plants to start forming bulbs. 

Since onions grow all over the world, varieties have been developed that grow well everywhere. Northern and southern latitudes have very long days in summer. Closer to Earth's equator, day length doesn't change very much. 

 Varieties of onions are categorized by where they grow best:

  • Long-day onions: for northern areas where the days in mid summer are very long with up to 14-16 hours of sunlight. 
  • Intermediate-day: for the middle of the United States where the mid summer days are long, but not extreme. Daylight lasts about 12-14 hours. 
  • Short-day: for the lower southern part of the United States. Daylight in mid summer lasts between 10-12 hours

There's also a category called "day-neutral," which are varieties of onions that will form bulbs regardless of length of midsummer daylight. They can be grown successfully in any part of the country. 

When selecting your onion varieties, check which type you should grow based on your location. Johnny's Selected Seeds offers a comprehensive guide to the different types in this article: Onions Form Bulbs In Response To Daylength.

You might wonder why we would even bother with different day length varieties, and just grow day-neutral onions all over the country. Varieties in each category have distinctive and desirable characteristics - for example, the sweet Walla Walla onion is a long-day onion. There is no day-neutral Walla Walla, so if we want that particular variety, we keep growing long-day onions in the right location. 

Main source for this content: Onions growing guide from K-State Cooperative Extension Service. (free to download)

How to Plant Onions

You can grow onions from seeds, plants, or sets.

Seeds are obvious. The challenge with growing onions from seed is that they take a long time to mature. If you are planting from seed, you will likely start them indoors before the weather is warm enough outside for the seedlings. Then transplant them in the early spring. 

Plants are basically young onion sprouts. Someone (a greenhouse) has done the steps in the previous paragraph for you. When the baby plants reach the desired size, the greenhouse people pull them, trim off their tops  and let them dry a bit. They go dormant. Then, the greenhouse people package them in bundles of 50 or 100 for sale.

In early spring, you transplant the little plants into your garden, much like you would for your own transplants. 

Sets are baby onion bulbs. Again, a greenhouse has started growing the onions and when they reach a certain point (the small bulb) they harvest the baby onions, trim them and dry them. You plant them in the garden in the early spring.

The challenge with the sets is that they have already started the bulbing process when you get them. The bulbs don't tend to grow much larger.

How to Grow Onions

Onions have shallow roots and as they grow they seem to push up out of the ground. By the time you harvest them, they look like they are sitting on top of the soil, rather than being in the soil! 

To get good size bulbs, in addition to planting them at the right time, apply fertilizer as they grow. Ideally, put down some all-purpose fertilizer before planting. Then 3 weeks after the plants start to grow, apply a higher nitrogen fertilizer. A third application can be applied when the plants have 6 to 8 leaves. You may want to put down another application of fertilizer when the plants have 9 to 11 leaves, but only if the bulbs have not started to form. 

How to Harvest Onions

Onions are ready to harvest when the foliage turns brown and falls to the ground. By this time, the bulb of the onion has popped nearly to the top of the garden soil! You can dig them out or simply pull them.

Storage

There's nothing wrong with eating your onions fresh out of the garden.  Fresh onions should be stored in the refrigerator until used in just a few days. 

To store onions for longer periods, they need to be dried, which is a process called curing. All of the onions you see in the grocery store have been cured. It's simply leaving the onions out in a well-ventilated location for 2-4 weeks. 

The curing process dries out the outer layers into the papery "wrapper" that seals the onion and keeps the inner layers moist. 

To dry the onions, lay them out in a single layer in a warm well ventilated place, out of direct sunlight. The onions shouldn't touch each other. It's helpful to place them on a surface that allows air circulation, like a screen or nursery plant tray. Turn the onions a few times so they will dry evenly. 

Sometimes the drying doesn't quite work and the onion will turn mushy and rot. Throw those onions out, of course.

Once dried, trim off the stems and trim the roots. Use a soft brush to remove any remaining clinging soil, and be careful not to nick the bulb. A nick could allow organisms in the bulb which can spoil the onion. Store onions in a cool dry place and they will be good for months. Do not refrigerate cured onions. 

Bottom Line - Summary

Onions are used and enjoyed in most kitchens around the world. To grow them successfully, follow these steps:

  1. Choose the right variety of onion for your growing location
  2. Choose the right type of plant stock for how you want to grow them - seed, plants, or sets
  3. Get them planted early in the spring season
  4. Tend to their needs with water and fertilizer
  5. Harvest when the foliage dies back and falls over
  6. Dry the harvested onions (curing) for storage
  7. Enjoy onions all year long! 
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