How Much Depth & Space Do Onions Need? (3 Key Things To Know)


It’s no secret that onions are one of the most common vegetables for people to keep in the kitchen at any given time. But many people don’t realize how easy they are to grow, as long as you adhere to proper growing protocols. 

Depth and spacing are two of the most important considerations for growing onions. Bulbs should always be planted at least 2 inches apart with the pointed end facing up, and about an inch below the soil’s surface. Never pack the soil tightly over the top of an onion bulb or around its roots, as they need plenty of sun and air for sufficient growth.

While onions are relatively forgiving and vigorous growers, there are other important guidelines to follow. In this article, we’ll talk more about the depth and space of onion crops and discuss ways to improve your growing process. 

How Much Depth & Space Do Onions Need To Grow?

Depending on the variety, onion bulbs should be planted 2-3 inches apart. Larger bulbs can be farther apart than smaller sets to avoid overcrowding. 

onion leaves growing
Plant onions 2 to 3 inches apart, depending on the variety. Larger bulbs will need more space at maturity.

Plant your onion sets no more than an inch under the soil’s surface. Onions need full sun and prefer lightly acidic, very fertile soil for best results. 

How Deep Do You Plant Onions?

Onions can be grown from seeds, but most home gardeners start them as bulbs, which are dormant bulbs also known as “sets”. It’s more difficult and labor-intensive to grow onions from seed. This article will focus on growing onions from bulbs, as it’s the most common method.  

onion seedling sprouts
You can grow onions from seed, but this takes quite a bit more time and effort than buying bulbs (sets).

According to The Farmer’s Almanac, onions only need to be buried about an inch deep. Before you plant the onions, consider adding an inch of compost to the bottom of each row. Onions need a constant flow of nutrients to flourish and produce large bulbs. 

When you’re ready to plant the bulbs, gently push them into loose soil at a maximum of 1 inch, ensuring that the pointed end of the bulb faces up.

onion set
Onion sets should be pushed into soil at a maximum depth of 1 inch.

After a few weeks, you can fertilize your onions with a nitrogen-based fertilizer. After the initial application, continue to feed your crop every 2-3 weeks. You can stop fertilizing once you see the onions emerge and start to push the soil away. 

What Happens If You Plant Onions Too Deep?

The bulbs may not develop correctly when onions are planted too deep into the soil. The bulbs need sufficient access to air and sunlight to grow, so excessive depth could lead to small onions or no growth at all.

small onions
Planting onions too deep may lead to small bulbs, due to lack of air or sunlight (or both).

You should also avoid pushing the bulbs down into the soil, as this might compact the soil around the roots. Instead, make sure the hole you’ve prepared is the appropriate depth so that you can simply set the bulb in there, and loosely cover it with soil. Water the plant immediately after planting, and don’t press the soil down any further. 

It should be noted that if your purpose for growing onion bulbs is solely for the green tops, you won’t have to pay quite as close attention to depth. 

onion leaves
If you only want the green onion tops for cooking, then depth won’t matter as much.

If you’d like the best of both worlds and have already planted them too deeply, there’s still a way to fix it. Rather than uprooting the bulbs, let them grow where they are for a couple of weeks.

Once the bulbs begin to expand, you can carefully push some dirt away from the sides of the plants to expose them to the light and oxygen they need to grow larger.

How Far Apart Do You Plant Onions?

According to Ohio State University’s Extension, you should place the onion bulbs 2-3 inches apart. When the onion plants start growing, you can thin them so that they’re 3-4 inches apart.

Rows Of Onions In Garden Early April 2022
Start onion bulbs 2 to 3 inches apart, and thin them to 3-4 inches apart as they grow. Use thinned onions as greens onion.

Fortunately, you can still get some use out of the onions you’ve thinned; use these as green onions, which are great for salads and cooking.

How Far Between Rows Of Onions?

If you’re growing multiple rows of onion sets, the spacing between them is important. The University Of Minnesota Extension recommends that rows be 12 to 16 inches apart.

onion plants
The recommended spacing between rows of onions is 12 to 16 inches (at least 1 foot).

Consider placing a layer of straw in between your onion rows, which will help to maintain a moist environment and prevent weeds from growing. 

(You can learn about other options for mulching around onions here).

If you’re planning on growing different types of vegetables, consider companion planting to reap the pest-deterring benefits of onions.

straw bale
Straw between onion rows can help to prevent weeds and retain moisture.

Insects aren’t fans of the fragrant sulfur compounds that onions emanate, so planting other crops nearby could be beneficial. Broccoli, carrots, and lettuce are some good examples of common companion plants for onions.

Although onions naturally repel pests, there are two main culprits that are a major threat: maggots and thrips. Onion maggots are small flies that lay eggs in the soil near onion plants.

The larvae of these pests feed on onion roots and can cause the bulbs to rot during storage. To reduce the risk of maggots, don’t plant your onions in the same spot year after year, and refrain from planting them in manure or other undecomposed organic matter.

thrips tomatoes
Thrips are tiny insects that affect onions.

Onion thrips are tiny, almost microscopic insects that drain the fluids from plant leaves. Over time, the leaves will start to die off.

Thrips are more likely to be found in hot, dry weather. They often target grains such as lima beans, and move onto onions once the grains have been harvested. Avoid planting onions near your small grains to prevent thrips.

What Happens If You Plant Onions Too Close Together?

It’s never a good idea to plant vegetables closer than the recommended amount of space. When plants like onions are too close together, they start to get overcrowded, leading to competition for nutrients.

With insufficient nutrients to go around, some of your onions will be stunted, and some sets may not grow.  

onion plants
If you plant onions too close together, they may start to get crowded and compete for water or nutrients.

If it’s too late and you’ve already planted and crowded your bulbs, the best course of action is to thin them out once they start growing. Remove any growth that is less than 2 inches away from its neighbors. 

Conversely, it’s important not to over-correct and space the onions out too much. When onions are too far apart, not only do you waste space, but the yield won’t be as successful. You must strike a balance between the two extremes to reach the sweet spot.

What Size Grow Bag For Onions?

Onions are a great candidate for container gardening since their roots don’t take up much space and they can be grown in clusters. Instead of plastic containers, you can use grow bags, which offer several advantages. 

Grow bags are large and porous, so air flows through them more efficiently than other types of containers. The bags also drain very well and absorb extra moisture, preventing over watering issues that can end up killing your crop. 

Grow Bag
If you want to use any type of grow bag for onions, you don’t need much volume(maybe 10 gallons).

Onions should be planted in a 10-gallon grow bag for the best yield. Here are some tips to increase your chance of success:

  • Since the root space of your onions will be slightly limited, you may want to give the plants slightly more nutrients to grow larger onions.
  • You can set the planted grow bag on a pallet to maximize drainage.
  • If you don’t have a grow bag, a large empty soil bag will suffice, with some modification. Poke drainage holes in the bottom of the bag, and cut the top of the bag in a way that allows for the most surface area.  
  • Since air circulates through grow bags, and they will be in the sun, the soil will dry out quickly. You may end up having to water your onion plants every day.

You’ll know it’s time to harvest your onions once most of the tops on the plants have wilted and fallen over. To harvest, gently pull the bulbs out of the soil with the tops still attached. Once you’ve gathered your crop, the top of each plant should be cut an inch above the bulb. 

Next, let the bulbs dry (a process known as curing) until the outside layers become paper-like and thin. Curing can be done outdoors as long as the temperature stays between 75-90 degrees Fahrenheit, and the area is well-ventilated. The curing process can take anywhere from 2-4 weeks.

onions
Let onions dry after harvest (known as curing) until the outer layers become like thin, crinkly paper.

Once you’ve harvested all the onions and cured them, store them properly so that your hard work won’t go to waste. They should be placed in a properly ventilated container like a wire basket or mesh bag.

Keep them where the temperature can be maintained between 32 – 40 degrees Fahrenheit. If the temperature creeps above 40 degrees for too long, the onions will begin to sprout. 

You can get some ideas for what to do with extra onions from the garden here.

If you can’t use your onions fast enough before they sprout, you can learn about how to plant them here.

Conclusion

With so many varieties and ways to plant them, growing onions can be a fun, easy process. As long as you follow the basic guidelines, you can harvest about 100 days after you plant your bulbs.

You can learn about the best onion varieties to grow (short-day, day-neutral, and long-day) here.


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About the author:
Kathryn is a plant enthusiast and freelance content writer who specializes in home and garden topics. Based in New York, you can get in touch with Kathryn at https://kathrynflegal.journoportfolio.com/.

Kathryn F.

Jon M

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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