Learning how to grow onions from seeds can seem a bit daunting. Not to worry! I’m going to walk you through everything you need to know: What type of onions to grow, exactly how to sow onions indoors and out, how to transplant onion seedlings, and what spacing to use. All of this will help you grow the best onions and largest you’ve ever had.

In order to go into depth on how to grow onions from seeds, I’m going to break this down into two different parts. Part 1- How to grow onions from seeds and Part 2- How to harvest, cure, and store onions.

What type of Onions should you grow?

Before you do any seed shopping, you need to find out what type of bulb onions work best for your area. If you are growing green onions, any type will work!

I personally found it helpful to know the “why” behind why you grow certain types of onions in different areas of the country and the world. Here is the most important thing to know:

When growing onions, they only start to bulb when they reach a certain amount of daylight hours. 

Different areas of the world are able to reach different amounts of daylight hours. When talking about onions, it’s easiest just to line up where the latitude of your location is with the type of onion you should grow.

Start with finding your latitude then line it up with the type of onion you should grow described below.

Don’t know your latitude? HERE’s a cool little latitude finder.

There are three types of onions: Long Day Onions, Intermediate day onions, and Short Day Onions. 

Long Day Onions:

• Grow best at latitudes of 37–47°
• 14-16 hours of daylight it begins bulbing

Intermediate Day Onions (Also called day-neutral onions):

• Grow best at latitudes of 32–42°
• 12-14 hours of daylight it begins bulbing
• This type of onion can also be grown successfully in other areas of the country.

Short Day Onions:

• Grow best at latitudes of 25–35°
• 10-12 hours of daylight it begins bulbing

Still not sure? For more information on which onion is right for you, Johnny’s Seed Company has a wonderful map and detailed information for US residents. You can find it HERE.

Now that you know which kind of onion to grow, you need to buy seeds. Usually, the seed packet will note the type of onion and where it should be grown.

Where should I buy seeds?

First, you need to find a good source for onion seeds. Some companies have consistently high quality seeds and I personally believe that for all the work that goes into growing something, it’s worth it to pay an extra dollar or two for quality seed stock.

I recommend- Baker Creek Heirloom Seeds, High Mowing Organic Seeds, and Johnny’s Selected Seeds.

There are definitely more good seed companies out there, but I haven’t branched out yet because I’ve been happy with the variety and quality of those mentioned above.

Onions are very different than most other seeds I start. Here are some key differences.

1. You can and should sow more than one onion seed per cell. Onions have a small footprint and starting one seed per planting cell on the tray would be a waste of space.
2. You don’t even need to plant them in cells. All the little plastic separator boxes aren’t necessary!
3. Onion greens need trimming. More on that below.
4. Onions seedlings experience very little transplant shock. Lifting them completely out of the soil you grew them in to plant them in their final growing spot is fine!
5. Onion seeds should be new and fresh. They have a very short shelf life, usually only lasting one year.

Seed Starting Onions Indoors

Onions are often the very first plant to sow indoors. I sow mine 16 weeks before our average last frost date.

How to grow onions from seed -indoor starting

1. Prepare your planting medium (aka seed starting soil). Often store bought mixes have a wetting agent so you can put the soil directly in the container you choose dry. If you made a homemade mix, you’ll want to wet it down before putting it in your container.
2. Sow the seeds by making a half inch deep holes in your seed starting mix. I try and make them a half inch apart (this prevents the likelihood of damping off, a soil or seed borne fungi). I usually sow about 9 seeds per 2 inch cell or by scattering the seeds in an open container.
3. Cover the hole or furrow with soil. Water the seeds and put underneath a grow light or next to a very bright window.
4. Plants will emerge over the next two weeks. A very important part of growing onions is that they need to have a trim when the plants reach 6 inches tall, trim the tops off (like a haircut) to about 3 or 4 inches above the soil surface.
5. As my onions get bigger, I transplant them into larger containers that allow the onion roots to grow deep. Last year, I used the peat pots shown below.They are 4 inch pots.

How to Transplant Indoor Sown Onions

The goal is to get onion seedlings to about the width of a pencil before transplanting (moving from indoors to outdoors to it’s permanent location in this case). However, they can be transplanted earlier if necessary.

1. Harden off plants (get them use to the outdoors slowly) over a two week period starting about 8 weeks prior to your average last frost. Use your best judgement since some years the weather can prevent it. Onions can tolerate light frosts and should be planted outdoors about 4-6 weeks before your average last frost date. Onions are very resilient and can be lifted entirely from the soil at transplanting. If the onions aren’t short and stocky, you may want to give them another trim of the green tops again.
2. The purpose of trimming the tops is to keep the onions as erect as possible. You don’t want them (the entire onion seedling) flopping over on the ground. When onions flop on the ground they usually don’t end up being the best quality in the end. You may need to go out and fix a flopped over onion after rain.
3. Onions should be spaced in rows 10 inches apart in a raised bed or for intensive spacing. Just like most plants, that may seem like a lot of space at first but they really fill in. Within each row, I space the plants 6 inches apart. See the diagram below.

How deep to transplant onion starts or seedlings

Here’s one of the most important parts of transplanting onions: Do not plant them too deep! I am speaking from experience on this.

One year,  I transplanted my onions. They were about a  half inch deep (this is just about right). Immediately after, I covered the soil with two inches of wood bark mulch (adding mulch is where I went wrong).

A few weeks later I noticed that some onions weren’t looking good. They were literally starting to rot under all the mulch.  I pulled the mulch away from the plants and was able to save the rest of them.

With a heavy mulch like bark, the key is to add mulch later in the season. You want to wait until the plants are larger and even then don’t pile it up against the onions.

Every year since then, I wait about a month to add mulch and add it only in between the rows. It’s been a very successful method.

So long story short, plant your onion starts about a half inch deep. If you do need to mulch right away, keep it away from the plants at first.

How to Grow Onion Seeds Outdoors

If you choose to start onion seeds directly in the garden,  you need to plant in very early spring, as soon as the soil can be worked.

Or in more southern regions, you can sow seeds in fall to overwinter. Either way, make sure your planting in loamy soil with a ph of around 6.

The benefit of planting onions directly outdoors is less work and equipment! And yet there is a major drawback, it is dealing with weed seeds as your tiny onion seedlings grow. Also, in the far north, you may not have a long enough season to plant onion seeds directly in the soil.

To plant seeds outdoors, make a half inch deep furrow in the soil. Sow seeds ½ inch apart in the furrow and cover with a half inch of fine soil. Furrows should be ten  inches apart.

A few weeks after seedlings fully emerge, they should be thinned to 3-6 inches apart depending on variety.

Onions grown outdoors will also benefit from a trim when the plants reach 6 inches, especially if you find they are flopping over like I mentioned above.