How to Grow Sweet Onions: Growth and Care Guide


Growing sweet onions is a delicate process that changes slightly depending on your geographical location, planting and gardening options, and the season you are planting in. 

To grow sweet onions, you have two options:

  • Plant them in autumn, in rich and acidic soil, to grow through the winter and be harvested the following summer.
  • Plant them in the early spring, in shallow soil that is watered generously and often with a slow-releasing nutrient-rich soil that has plenty of drainage. Harvest in the fall.

Keep reading to learn whether you should plant in the fall or the spring, and even extra care tips for growing the largest and best sweet onions possible.

Spring or Fall Planting?

For sweet onion planting, the season matters.  While most people automatically think of springtime as the only planting season for fruits and vegetables, there is actually another time that onions can be planted. 

“Overwintering” is a widespread practice among sweet onion growers, according to Highmowingseeds.com.  Overwintering is when a grower plants their onion seeds or bulbs in the late fall season and allows them to grow through the winter. 

Since onion bulbs grow underground, when provided the right environment and given the proper care, they can grow very large through the winter and be ready for harvesting come springtime. They are typically planted before the first frost of the year.

This is only possible if you live in a climate that rarely reaches temperatures below 20 degrees Fahrenheit.  This is considered a mild winter climate. If the climate reaches temperatures much lower than that throughout the winter, then you may have to plant your onions indoors to start them off and relocate them to your outdoor garden come spring.

To do this, according to Almanac.com, you can begin with onion sets or seeds, and plant them two-to-a-cell in your selected starter containers.  Keep them in a warm area and water regularly.  When they begin to form bulbs, you can transplant them outside, preferably after the last frost of the spring.

Here’s a quick video of some sweet onions growing in a container garden with some helpful tips:

How Overwintering Works

When planting onions outdoors in the winter, a raised bed is a good option. These are typically filled with a base of compost.  Then the top 6 inches or so of dirt consists of nutrient-rich soil with a sulfur content of no more than 6%, according to Ugaurbanag.com

They recommend re-fertilizing your soil once per month with that sulfur content until mid-February.  Once you have reached mid-February, they recommend searching for a sulfur-free soil to maintain your onion’s sweetness and using that once per month until harvesting. 

For planting your onions, find stalks that are at least as thick as a pencil and then plant them an inch or two into the soil and water immediately.  They should be about 6 inches away from one another as well. 

Keeping Your Sweet Onions Safe in the Winter

Suppose you notice that you occasionally get temperatures further below freezing or worry that your weather is being unseasonably cold. In that case, there are options for maintaining the optimal growing environment for your onions through the winter once they are planted outdoors. 

Those options include:

  • Providing some coverage over the top of the garden
  • Providing coverage completely surrounding the garden on all exposed sides
  • Placing mulch over the top of the buds to keep them warmer
  • Placing other soils on top to add layers for warmth

These can help you have more success with the outdoor overwintering of your sweet onions.

Springtime Planting

Planting in the springtime is recommended for colder climates that reach temperatures below 20 degrees Fahrenheit regularly through the winter.

For springtime planting, choose long-day sweet onion bulbs or seeds.  The long-day onions prefer the sun exposure and warmth of the long summer days.   Be sure to plant in rows half a foot apart, spaced an inch or two between seeds or bulbs.  Make sure the soil drains well and that there is plenty of sun exposure. 

Water the onions generously, and just as with the overwintering, make sure that your soil has a low sulfur content to keep your onions sweet. 

Generally, spring-planted onions don’t get as large as overwintered onions, but they can still be sweet and delicious and ready for harvest come late summer or early fall.

Here’s a longer video with more info showing onions growing in a green house:

How to Make Sweet Onions Sweeter

Sweet onions don’t actually get their reputation for being sweet.  What the hype is really all about is the lack of sulfur in the soil that keeps any bitterness from being added to the onions as they grow. 

As recommended by Gardeningknowhow.com, planting your onions in a soil that is very low in sulfur content, or researching the natural soil in your location to see what its pH levels are, can help you to ensure that your onions grow sweet and not bitter. Often a sandy soil is better than a clay-like soil.

Another helpful tip includes watering your onions often enough for them to grow large, since sweet onions can be pretty big eaters and drinkers.  They don’t grow roots long enough to search out water or absorb much on their own, so it’s essential to keep up with the watering routine so they can get what they need. More tips can be found at Harvesttotable.com.

Harvesting Times

Onions are unique in the edible plant world because they can be harvested and used at any time in their growth process.  Whenever they are the right size for your preference, simply dig one up and add it to your meal. 

They can be pulled when they first have grown buds and haven’t yet bulbed and eaten as green onions.  They can also be harvested when they have just barely bulbed as a spring onion and eaten as is. Then, at any point during the bulb’s growth process, they can be eaten. 

The sweetness has little to do with the size or age of the onion and everything to do with the soil acidity, sulfur content, watering frequency, and onion type, as we discussed above.

Choices for Sweeter Varieties

Two main groups of sweet onion types can make or break your growing experience: long-day or short-day. 

To help you figure out which type you should select, Almanac.com suggests imagining a horizontal line running across the United States, beginning between North and South Carolina and ending near San Francisco. 

They advise that if you live north of that imagined line, plant long-day onions in the spring to take advantage of the long summer days.  Some long-day sweet onion varieties include:

  • Walla-Walla
  • Alisa Craig
  • Copra
  • Sweet Sandwich

These are a few of the types of sweet onions that will fare well in the northern climates.  If you are planting south of that imagined line and live in a warmer climate, then the springtime planted onion type you should select would be short-day onions. 

Some examples of short-day sweet onions include:

  • Yellow Grannex
  • Texas 1015 Supersweet
  • Texas Early Grano 502 PRR
  • Vidalia

When planted in warmer, southern climates, these sweet onion varieties will thrive.

Layers of Sweetness

Once you have selected your onion type and variety, you can get planting.  Choose soil with as little sulfur as possible, and water those bulbs as much as they want.  Whether you are planting indoors in the fall because your winters are harsh and icy, or you’re planting outdoors in the spring for long summer days, you can look forward to some sweet onions at the end of the growing season. 

Robert Sampson

I'm Robert Sampson and I live in Colorado where I spend a lot of time in the backyard with my family either grilling, playing games and sports, or working on a project to make our backyard a better place to be.

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