How to Grow Onions from Seed

Growing onions from seed can be difficult, but it doesn’t have to be! Onion seeds take a long time to grow and must be started indoors during mid to late winter. It would be nice if you could just sprinkle them over your flat and be done with it, but they will turn into a tangled mess before long. So let’s learn the tips & tricks to grow onions from seed all the way to harvesting big, beautiful bulbs, plus how to cure and store them for using over the winter. 

Grow Onion Seeds or Sets?

Of all the seeds I start each year, onions are my favorite to grow! Starting a new garden in the dead of winter has a way of making the dreary days seem to go by more quickly. That’s reason enough for me! But there are other advantages as well. 

When growing onions from seed, you have a greater choice of variety to meet your flavor, storage, and day-length needs. 

I’ve tried growing onions from sets several times. I’ve sourced them from reputable online sellers as well as from local Amish garden centers. In both cases, the onions I’ve grown from seed are always healthier and more productive. The little onion sets are often dry and look half dead when you plant them. While they’re “coming back to life” they may need replanted if they pop out of the soil. It seems as though they don’t root as deeply as onions grown from seed. They may go straight to flowering without producing a bulb. Or they may simply never produce a bulb at all. 

All About Onion Day Length

Which varieties of onions you should grow completely depends on where you live and how much sun you get in the summer. Why? Because onion growth is triggered by the number of hours of sun they receive in a day. Some onions need more sun to thrive and require longer days. 

In a nutshell, Short Day Onions do best growing in the south. Long Day Onions are a better choice for those living in north. 

A few varieties on Long Day Length Onions you may like to grow are: 

  • Ailsa Craig
  • Red Wing
  • Yellow Sweet Spanish
  • Copra
  • New York Early

Some examples of Short Day Length Onion Varieties are: 

  • Texas Early White
  • Red Creole
  • Yellow Granex
  • Red Grano
  • Burgundy

A few varieties of onions are less picky and may work in either area. These are called Intermediate Day Length Onions. 

  • Walla Walla
  • Red Candy
  • Zoey
  • Cabernet
  • Valencia
Onion Seeds: How to Grow Onions from Seed

How to Grow Onions from Seed

I’m going to try to be as detailed as possible while describing how I successfully grow onions from seed. Please don’t let my over-describing discourage you! It’s not really that difficult as it may seem and is very rewarding. Give it a shot this spring and I imagine you’ll learn that it’s worth the effort! 

Pre-Sprout the Seeds

Pre-sprouting (or “chitting”) seeds is where you get them to germinate BEFORE you plant them. Pre-sprouting onion seeds has two primary purposes. First, you know for certain the seed you are planting in the flat will germinate. (Because it already has.) Since the light & possibly heat you will be using to start seeds isn’t free, you want to make sure there is no wasted space under the glow of the grow lights.

If you have a 75% germination rate, that is 25% wasted space that you could be using to grow more food. While pre-sprouting is an extra step, it is 100% worth the work! After all, we’re Homesteaders of America! This isn’t play gardening, we take the work of feeding our families and communities seriously!

The second reason to pre-sprout onion seeds is that the seeds are fairly small. And very black. It is a nightmare planting them one by one in your flats when you can’t even see what you’re doing. Transplanting the sprouted seeds is careful tedious work, I won’t deny it. But at least you won’t end up with eye strain and still not be certain they’re evenly spread out. 

How to Pre-Sprout Onion Seeds

Chitting onions is simple. All you have to do is scatter them over a wet paper towel (or I suppose a rag would work) and lay another paper towel (or rag) carefully over the top. Keep them flat and slip it into a gallon zipped bag so they stay moist. 

Whether you set them in the sun or keep them in a dark place to germinate doesn’t matter. Just make sure it’s warm and check on them every day or so. You will see a tiny white tail come out of the side of the seed and then it’s time to transplant them!

Newly germinated onion seedlings: How to Grow Onions from Seed

Prepare Your Potting Soil

Normally I prefer to start my seeds in soil blocks. There are many benefits to soil blocks, but I find that they are difficult for planting onions where multiple seeds will share one cell. So for onions I use a standard plastic 4-cell pack. The cell packs are set into a tray with no holes in the bottom. 

(While plastic isn’t the most eco-friendly seed starting option, many DIY hacks are too small to hold enough soil that will be needed to feed the plants over the next few months. Compostable “Jiffy” pots dry out far too quickly, stressing tender seedlings, and, frankly, compost poorly during the first growing season. I’m content with treating my cell packs & trays with care and reusing them year after year.)

You can use your favorite potting soil mix to grow onions but also add in some blood meal or bone meal for extra fertility. Since onions will be in the medium for months they will go through the available nutrients in their little cell and need a boost to make it through to transplanting time in the spring. A good source of nitrogen is also important for onions because it is what a plant uses to produce leaves. And the more leaves on an onion plant, the bigger your bulbs are going to be!

I just eyeball it and probably use a cup or so mixed into enough soil to fill one tray. 

Wet the potting soil so that it is saturated, but isn’t so wet as to be soupy. Pack your cells with the mix, gently pressing them to remove excess air, but not so much that they’ll be a brick or you’ll have trouble transplanting the pre-sprouted seeds. 

Transplanting the Pre-Sprouted Seeds

Once the seeds have their little tail (but before they are long and easily breakable), it’s time to transplant.

I use a pair of tweezers and find something to work as a dibble. A pencil is probably too thick but a framing nail or something similar would work well. 

This part requires a gentle hand and is painstaking if you’re not a slow gardener. Get yourself set up with the latest Homesteaders of America podcast and settle in. You’ll be done before you know it. 

Simply poke 4 holes in each of the cell with your makeshift dibble then use the tweezers to tenderly plant the seedling into the hole. If your seedling sent out both the root and a shoot, the seed case will stay attached to the shoot. Plant the seedling with its white root pointing down and the shoot with the seed casing up.

(I messed this up the first time I started onions from seed and had to replant all of them right side up!) 

Once your seedlings are all transplanting cover them with a dusting of dry potting soil mix or vermiculite. I prefer using vermiculite because it helps prevent damping off disease. 

Onion seedlings under grow lights: How to Grow Onions from Seed

Grow Onions Indoors

Like all plants, your onion seedlings will need 4 things to grow: light, warmth, and water… and a little TLC.

Light

You will need to grow onions under a grow light. During the short winter days a windowsill will not provide enough daylight for them to thrive. 

You can use a timer to automatically turn your lights on and off each day. In years past I have set my timer to 14 hours of light but will be experimenting with increasing the light to 16 or 18 hours this year. 

(It’s a long story, but my onion seedlings were gloriously thick and sturdy last year after a timer was accidentally turned off and they had constant light for a week straight. I suspect the extra light may be the cause for the strong onions. I’ll update this post with what I find, but if you experiment along with be sure to let us know how it goes! If you’re not the experimental type, stick the 14 hours to be safe.) 

Warmth

If your growing space is chilly, consider growing them on a heated mat. We have a large seed starting space in our unheated basement where heating cables are spread out in a wooden frame, covered with sand, and the flats placed on top of the sand to grow. 

Water

Your onion seedlings won’t appreciate soggy feet, but don’t let the soil dry out either. It is so difficult to get potting soil to rehydrate once it has dried out. And they’ll struggle from that point on. 

My favorite way to water all seedlings is to underwater into the bottom tray. That way the seedlings can decide for themselves how much water they need. Every few days I’ll check on them and refill the tray almost halfway as needed. 

Caring for Your Onion Seedlings

Onions do need a little maintenance besides watering them a few times a week. Their leaves will grow long and tangle together, making transplanting not only a nightmare, but a near impossibility. Cutting them back to 2-3” height as needed will make stronger, healthier leaves. 

Fertilizing Onion Seedlings

You may consider fertilizing your onion seedlings with some fish emulsion for extra nitrogen about halfway between sprouting and transplanting. The addition of bone meal to the potting soil is probably adequate, but since I fertilize my other seedlings with a foliar spray, I hit the onions with it too. 

Onions ready to be transplanted

Transplanting Onion Seedlings

Onion seedlings are hardy and can be transplanted as soon as you can work the soil in the spring. Last year, I planted mine about 4 weeks before my last frost. 

One of the reasons why I choose to plant 4 seedlings per cell (besides preventing a tangled mess and over taxing the soil nutrients) is because I use a transplant method I read about in The New Organic Grower. In the book, trusted market gardener Eliot Coleman tells how you can grow 3-4 onions in the same hole without compromising plant growth and yield. This is a fantastic option for those of us who want to grow a lot of onions on a small scale! 

I plant each cell of 4 onion seedlings in a 10” grid in the bed about 8-10 weeks after I started the seeds. 

Onions in a garden

How to Grow Onions

Onions are fairly shallow-rooted so they make a great choice if you garden in raised beds or soil that has been amended on the top few inches but sits on clay. Again, they don’t like soggy feet, and can be prone to rot, so be sure the soil does drain adequately. 

Keep the soil from drying out (straw or hay mulch works wonderfully for water retention) and use a foliar fertilizer, such as fish emulsion, every 2-3 weeks until they start to form bulbs. 

Onions do not do well with competition from weeds so you will need to keep the weed pressure down. Another great reason to consider using an organic mulch. 

When to Harvest Onions 

The day you transplant seedlings in the garden is the day you start counting your days to harvest. So for example, I grew Bridger onions from seed last year. They are a 90 day onion and were started from seed on February 19th and transplanted on April 25th. 

Onions are ready to harvest when the tips of their leaves begin to turn brown and fall over. The neck, right above the bulb, will be starting to shrivel up, but not fully dry. The Bridger onions were ready and I harvested them on July 27th. 

Curing Onions

How to Cure Onions

Choose a time when there is a warm, sunny forecast before you when harvesting your onions. They cure best when they can spend a few days in the sun before heading to a dry shelter to finish curing. Last year, I tried the system where the leaves are fed through a wide, wire mesh table and the bulbs sit on top, not touching. It worked wonderfully, saved space, and kept the onions from rolling and touching each other. 

How to Store Onions

Once the leaves are fully dry, even at the neck, it is time to store your onions. Trim the dry leaves off at the base and store them in mesh bags or flat on trays that promote air circulation. I purchased a few commercial stackable dishwasher racks and like that a whole lot better than digging around in a bag for a nasty rotten onion that needs to be removed! (Rotten onions are inevitable and are one of the worst smells! Which, let’s face it, is saying something cause I live on a farm. Things can stink around here from time to time.)

Place your trays or sacks in a cool, dry place until ready to use. (The experts say about 35 degrees F and about 65% humidity.) 

How long your onions will last in storage depends on how well they were cured and stored, but also the variety. Some onions just last in storage longer than others. 

Onion Varieties that Store Well 

  • Red Wing (8-10 months)
  • Patterson (10-12 months)
  • Copra (8-10 months)
  • Cortland (9-12 months)

Plan to regularly check your onions in storage for signs of rot so you can remove them early.

How to Grow Onions from Seed: Seed Starting, Caring for Seedlings, Transplanting, Growing, Harvesting, Curing, & Storage

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