A 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. 

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.


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

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.

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.


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.

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.

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

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

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.

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 on this crop didn’t ultimately harm the bulbs.


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. 

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. 



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.


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  1. 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.

  2. 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.

  3. 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.)

  4. 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!!

  5. 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!

  6. 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!

  7. 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!!!

  8. Nancy says:

    Where do onion seeds come from?

  9. 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!!

  10. 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!

  11. 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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