The Complete Guide to Growing Onions.

Interested in growing onions?  This onion growing guide will answer all your questions about seeding, planting, growing, curing and storing an onion crop. Wow that sounds dull. How about, HOLY CRAP YOU’RE GOING TO KNOW  MORE THAN ANYONE ABOUT GROWING ONIONS!!!!  Yes. Much better.

Red onions hanging off the drawer of an antique wood cabinet.

If you plant your own vegetables, growing onions is the first thing you get to do in the spring after a long winter of flipping through seed catalogues and binge watching Monty Don. 

If you think it’s a bit early to start onions, chances are it’s actually EXACTLY the right time. Trust me on this. It’s ALSO the exact right time to plant leeks. If you’re growing them this year THIS is my absolute best tip for growing leeks based on an Elliot Coleman technique. 

 

When to Plant Onions


Onion seeds should be started 12-14 weeks before the last day you expect frost in your area.  You can then transplant the onion seedlings outside 4 weeks before the last day you expect frost in your area.


  • If you’re in Zone 6 like me, your last frost date is May 10th.
  • Onion seed planting day is 12 weeks prior to that – February 22nd.
  • Planting onion seedlings outside date is April 19th.

Of course this is all based on predicting weather which is about as accurate as applying makeup while wearing mittens.   Even a big difference in time zones only makes a difference of a week or two. (say you’re a zone 3 compared to my zone 6)

Just hit Google to find your last frost date based on where you live and then click here on my seed calculator to see when you should plant everything from tomatoes to beets.

How do you plant onion seeds?

Like any other seed, you basically just stick the seed in dirt. Or “soil” for the more refined gardeners among you. BUT growing onions is different than growing other vegetables depending on where you live because where you live dictates what type of onions you grow. 

Onions can be divided into 3 categories depending on how much light they need to form a bulb (that’s the onion part.)

Short Day Onions
  • These onions start to form bulbs when there’s 10-12 hours of light a day.
  • Should be grown in areas where the length of daylight remains fairly constant throughout the year.
  • Best grown in areas with a latitude of 25-35 degrees North.
  • Can be grown in more Northern regions but won’t do as well or get large because the extreme length of light per day will trigger them start forming bulbs when their tops aren’t big enough to provide enough energy.
  • Example: Bermuda onions
Intermediate Onions
  • These onions start to form bulbs when there’s 12-14 hours of light a day.
  • Do well in all regions other than South Florida or Texas.
  • Can be grown almost anywhere but do best in latitudes between 32-42 degrees North.
  • Example: Candy onions
Long Day Onions
  • These onions start to form bulbs when there’s 14-16 hours of light a day.
  • Should be grown in Northern States or Canada.
  • Do best in areas with a latitude of 37-47 degrees North.
  • Typically the best storage onions.
  • Example: Copra onions

Karen Bertelsen holding up a small glass vial of onion seeds in front of an antique wood apothecary cabinet.

I  know that’s all very confusing but if you’re buying seeds from a local seed seller chances are they’ll be selling you the proper type of seed for your area.

If you’re really confused look at it this way:

What Type of Onion You Should Grow

If you can wear shorts most of the year, you should be growing short day onions.

If you own a snow scraper, more than one winter coat and rock salt for your sidewalk, you should be growing long day onions.

Everyone in between?  You’re intermediate onioners.

Karen Bertelsen planting seeds in a cell tray.

There are 2 methods for planting onion seeds.

Winter Sowing

If you want to try winter sowing which requires no special equipment you can read my post about winter sowing here.

or

Growing under lights

This year I’ll be growing my onion seedlings under lights.

You can learn about the lights I use and other seed starting tips in this post.

Black, round, onion seeds held in the palm of a hand over a pot of growing medium.

If you have NO idea how to plant seeds read this post I wrote on how to start vegetable seeds first. It describes everything you need to know from how wet your soil should be to the materials you’ll need.

How to Plant Onions from Seed

  1. Make sure you’re planting the right type of onion seed for your region. If you’re planting in a “short day” onion zone, you can plant them directly outside.  If you’re planting “intermediate” or “long day” onions you should start them under grow lights.
  2. Scatter many seeds onto a 4″ pot filled with pre-moistened seed starter mix. (Around 1 tsp of seed)
  3. Cover seeds with 1/8th” of seed starter mix.
  4. Cover pot with plastic wrap or a small plastic tray to stop moisture from escaping.
  5. Set pot on a heating mat or place somewhere warm like on top of a fridge or heated floors.
  6. Check daily on the seeds and once they start to sprout remove the dome. Under the right conditions onion seeds will sprout in around 8 days.
  7. Set the sprouted seeds under grow lights.
  8. For intermediate type and long day type onions leave the grow lights on for 11 hours a day. Don’t leave them on any longer than that or you’ll trigger them to start forming onion bulbs (which you don’t want).
  9. 5 weeks prior to your last frost date, start hardening your onions off outside. You can read about how to harden off plants in my post here. 
  10. 4 weeks prior to your last frost date (and after hardening off) plant your onions seedlings.

How Long Does it Take For Onions To Grow?

Green onions can grow to maturity in as little as 30-40 days.

Regular bulb onions take 4-5 months to grow.

Growing onions is just the start of it because after you grow them you have to cure them. It’s easy, don’t worry.

Close up view of individual onion seeds on soil before covering them up.

When planting your seeds make sure they have good contact with the soil. Press them in with your hand.

A soil covered hand drops onion seeds into a soil filled pot.

After scattering a fine layer of soil overtop, tamp that down too so it’s also in close contact with the seeds.

10 day growth of onion seeds sprouting in biodegradable pot.

Within 8 days or so you’ll see the seeds sprouting like this. It’s a very exciting day and never EVER ceases to thrill me.

Sometimes the onion seedlings have trouble popping their little heads up and remain in loops like this. If yours do that you can just cut the loop in half where it bends and the seedling will keep growing. No harm done and it’s safer than trying to dig out the tip of the seedling.

Mid summer raised garden bed filled with growing onions, started at home with seeds.

After planting them out make sure your onions have regular water and if you’re worried about pests cover them for the season with floating row cover. It’ll help control thrips, leaf miners and onion maggots.

Onion maggots are the real killers. They burrow into the leaves, and eat their way down to the forming bulbs leaving a rotting neck and killing the onion.

Healthy pest free onions have straight, strong, solid green leaves.  Onions with pests have scarred leaves.

Pest damage visible on the leaves of an onion.

Pest damage on this crop didn’t ultimately harm the bulbs.

 

Pest damage on onion leaves shows scarring from onion maggot.

But the damage to these onions became known to me when I went to pick them and their necks were complete mush from pests and the bacteria they bring.

The BEST way to prevent any of this damage is to cover your crop with a floating row cover as soon as you plant them.

 

Curing Onions for Storage

If you’ve grown onions from seed you want to make sure they’re going to last and aren’t going to rot away to a disgusting blob after spending all that time nurturing them.

That’s where curing onions for storage comes in.  Curing onions dries them out gradually so the necks properly cure and get dried out which prevents rot later on.


The best temperature for drying onions is 75-85 degrees Fahrenheit.

The best humidity for curing onions is 70%.

To cure onions lay them in a single layer out of the sun, but still in open air like in a garage or porch for 2-3 weeks to cure properly.

The entire neck and stems of the onion will dry out completely. At this point you can cut the stem and leaves off, leaving 1-2″ of stem.

Store cured onions in a ventilated container like cotton mesh bags. 

The best temperature for storing onions long term is between 32 – 36 Fahrenheit. 

Curing onions outside on a wood onion curing rack protected on a front porch.

Get the instructions for how to make this herb/onion drying rack here.

 

Onion Storage Tips

  1. Onions that have very thick necks when you pick them are called “Bullnecks”. The necks of these onions don’t shrivel down and dry well, so it’s best to use them and not use them for long term storage.
  2. If you don’t have room in your fridge for storing your onions all season, store them in the coldest room of the house or (your garage if it doesn’t freeze.)
  3. Don’t store onions and potatoes together. The onions cause the potatoes to rot faster, but the reasoning behind why is mixed. Some people say it’s because of the ethylene gas emitted by the onions and other say it’s because of the high humidity both of the vegetables contain. Either way – just store onions and potatoes separately. 

The Complete Guide to Growing Onions.

The Complete Guide to Growing Onions.

How to successfully grow onions from seed to storage.

Materials

Instructions

  1. Make sure you're planting the right type of onion seed for your region. If you're planting in a "short day" onion zone, you can plant them directly outside.  If you're planting "intermediate" or "long day" onions you should start them under grow lights.
  2. Scatter many seeds onto a 4" pot filled with pre-moistened seed starter mix. (Around 1 tsp of seed)
  3. Cover seeds with 1/8th" of seed starter mix.
  4. Cover pot with plastic wrap or a small plastic tray to stop moisture from escaping.
  5. Set pot on a heating mat or place somewhere warm like on top of a fridge or heated floors.
  6. Check daily on the seeds and once they start to sprout remove the dome. Under the right conditions onion seeds will sprout in around 8 days.
  7. Set the sprouted seeds under grow lights.
  8. For intermediate type and long day type onions leave the grow lights on for 11 hours a day. Don't leave them on any longer than that or you'll trigger them to start forming onion bulbs (which you don't want).
  9. 5 weeks prior to your last frost date, start hardening your onions off outside. You can read about how to harden off plants in my post here.
  10. 4 weeks prior to your last frost date (and after hardening off) plant your onions seedlings.

Notes

  • Don't forget to water! Onions (or anything) will die if you don't water them.
  • You can cut the tops of your onion seedlings off when they're all around 3" high. This makes them less tangled and messy which improves their growth  habit.
  • Fertilize the onions as soon as you plant them with a combination of bone and blood meal. (your box or bag will have guidelines for how much but it's basically just a sprinkle around each plant)

Karen Bertelsen smiles as she pushes soil into a small pot prior to planting onion seeds.

 

Look at how dopey and happy I look planting onion seeds. Don’t you want to be that dopey looking too??  Just grab a packet of seeds, some soil and a container and get your hands dirty and you’ll be on your way.

If you really want to up your dopey face game now is also the time for you to start peas, leeks, parsley and spinach.

 

→Follow me on Instagram where I often make a fool of myself←

 

The Complete Guide to Growing Onions.

40 Comments

  1. Mary says:

    what is the rack that you use to dry the onions?
    Great info. thanx

  2. Stephan Emswiler says:

    What’s your process for planting them outside?

  3. Sarah says:

    I am now officially in luv with you and this site. I’ve been looking all over for EVERYTHING about how to grow onions in one place and you just provided it. Except fertilizing requirements but no biggy. No onions this year, kitchen remodel instead is taking up my early garden time. I will plant from seed for the first time next year for sure!!

    Thanx thanx thanx!

  4. Marci says:

    Karen,

    Your seed calculator is awesome but if we don’t plant seeds and put in plants instead should we use the “set out date” for planting small plants from a nursery? Also, can we just plan a sprouting onion from our pantry?

    • Karen says:

      Hi marci! Yes, you can absolutely just use it for the setting out date with your nursery plants. And you *can* plant your sprouting onion, but you won’t get a bigger onion from it. What you WILL get are seeds for that onion. After growing a bit in the spring and summer, the onion will flower and then develop seeds that you can gather. But if the onion is a hybrid you won’t get an exact replica of your original onion. ~ karen!

      • Kathy B says:

        Oh didn’t know that about getting seeds from sprouted onions, I usually just throw them in the compost. Will give that a try this year.

  5. Andrew DK says:

    I love onions but I’m trying a different method this year. I just can’t do the gentle teasing apart of the lil roots without losing my mind. I still start bunching onions All together in cups but this time the bulbing ones are started in the 3/4” soil blocks one seed at a time. Once they’ve got a good root structure I’ll pick out the best ones and put them four to a cup so they have plenty of room and then plant out all four at once.

    Hopefully they can get big and strong by transplant time as well as making it much less frustrating!

    Good luck! 😄

    • Karen says:

      I’m using soil blocks as well, but think I’m probably still going to do my onions all in one pot like normally do, mainly to save space. Let me know how yours do! ~ karen

  6. Jennifer says:

    You are my hero! I was just getting ready to plant onions for the first time and kind of overwhelmed. We had a large crop growing when we bought our house that overwintered and I collected the seeds last fall. I also bought onion seed and some Egyptian Walking Onions for this year. Can I grow them? We will see!

    • Karen says:

      Good luck! ~ karen

    • Norma says:

      After you get the Egyptian Walking Onions started and growing, plant them out in a place where you want them year after year. I have them in my front flowerbed (because they’re deer proof) and they come up every year – both from the base roots and also from the odd onion bulb that remains and falls into the dirt. From now and forever more, you shall have onions for yourself and to share with any passing neighbour!

  7. kre says:

    I’m jealous of your last frost date! I just looked mine up: June 21-30.

  8. Robert says:

    I hate to tell you, but I use Dutch sets here in Zone 3b and have never failed to get great onions! I plant them in late May or early June, depending on soil temperature. Great onions come September!

    Question though. What’s the purpose of bending the leaves/stalks over?

    • Karen says:

      That’s great for you Robert! If it ain’t broke, don’t fix it. Dutch sets aren’t normally very good. Their biggest drawback is their tendency to bolt (flower) much easier than a seeded onion. ~ karen!

    • Norma says:

      The onions are ready to harvest when the tops fall over naturally – if you’re eager to harvest them earlier, you can bend them over and the bulbing process will stop.

    • Karen says:

      Oop. Sorry, I forgot your bending over question. When the leaves naturally start to fall over, it’s usually time to bend them all over yourself. This signals to the bulbs their job is done and helps the necks start to dry out for storage. ~ karen!

  9. julie kasner says:

    Great information!
    We planted 200 hundred this year. I hope we have a great crop.

  10. Jessica says:

    Thanks for the tip on the length of time to have the lighting on for long day onions. I just went and changed my timer setting.

  11. PMMK says:

    A couple of years ago, maybe more, we had onions that would not form bulbs. It turns out that, when stressed, onions will go into reproductive mode and bolt instead of forming bulbs. When this happens you end up with thick, bitter stalks and no big, round onion. It turns out it was our particularly chilly spring weather that was to blame. The lesson learned? Don’t plant your onions too early; use a cold frame and keep those row covers handy. Here in zone 5, May 2-4 can be too early for us; the ground is often not warm enough yet and the risk of frost too high.

  12. Jane Baker says:

    I was told years ago by one of the old Baba’s in Saskatchewan when I lived there that planting carrots with onions prevented carrot fly. But I also found that it seemed to keep pests away from the onions.
    I always had great success mixing my plantings and when I got really wild and mixed wildflowers in with my veggies, it seemed to keep pretty much all of the pests away. However, I also rotated my crops from one box or ground to another year to year. Keeping plant types out of the same ground year to year might have been more effective than anything. (In other words, don’t plant a brassica in the same soil as you had a brassica the year before.)

  13. Susan D'Achille says:

    I had the same dopey looking face yesterday, I am sure. Planted a flat each of leeks, chives, cooking onions, spanish and red onions. Love playing in the dirt!! Next week peppers!!

  14. Ron Benvegna says:

    We love to cook with fresh green onions but are ridiculously expensive at the grocery. So, I grow my onions in large (nursery tree size) containers. The pests in Florida seem to eat everything so this is why I went to the large black containers. I cheat and buy onion slips at the local nursery instead of seeds. They are about 2-3″ long in a bundle of 100 for 1.99. Planted last week put some natural organic fertilizer on them and watered well. They have shot out of the soil and are about 6″ this week. We go outside grab 2-3 for whatever we are cooking when they mature. (Yes – to question above – green onions are just onions that haven’t matured fully)

    • Karen says:

      $1.99 for 100?! That’s cheap! Good for you. re: the green onions, they’re actually a specific type of onion that doesn’t bulb. You can grow it forever and it will never form a big bulb. You can however pick regular onions prior to them bulbing and use them as green onions. ~ k!

  15. Mary W says:

    Your dopey face is too cute. It reminds me of when I was in high school and near the annual picture taking day. I would spend lots of time in front of the mirror trying to get just the right smile. Of course, on the actual day, I never used the chosen ‘smile’ – but yours is a winner. In FL it is bound to frost one more time and kill all the snow storm of flowers on my plum trees but that is FL. Lots of good info on onions – thanks!

  16. Rachael says:

    I’ve always struggled with onions and never planted them from seed- always from the tiny onion bulbs sold at the local nursery. I will definitely try the row covers because my onions are always stunted and have slimy damaged greens. I just thought I overwatered but I bet I’ve got a parasite issue. I was told all onions from seed we’re biennials? If I plant from each year I can actually get bulbs? Mind blown!!

    • Karen says:

      If you’re using Dutch sets “those small onion bulbs”, STOP. They’re the worst way to grow onions. They have a tendency to bolt and once they do you won’t get an onion. Always plant either from seed, or from tiny onion seedlings to improve your chances of getting bulbs. :) . ~ karen!

      • Sandi Remedios says:

        OMG I didn’t know that. I always buy the Dutch sets and I have the worst luck with onions. Now I know!!! Thanks!!!

  17. Nancy says:

    Where do onion seeds come from?

  18. Kirsten says:

    I have a container garden that is 4X8 and gets so overcrowded every year because of all I try to stuff in it. I love onions but quite frankly dont want to take up the space in my bed. Can I get some suggestions as to what I can plant/ harvest and then replant again? I always do a couple tomato plants and I like some lettuce so I can make fresh salads for dinner…… I’m boring…. help!!

  19. Sandra D says:

    I’ve always wanted to grow green onions. Confirm, please – are these really just baby onions?

    • Karen says:

      Not quite but almost. Green onions and regular onions are indeed two very different things. Green onions are non bulbing onions. However, you can just plant regular onions and pull them when they’re young. ~ karen!

  20. Margaret K. says:

    Thanks for spelling out which onion types to plant where! Pretty sure you mean planting / hardiness zone, not time zone, though.

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