How to Plant Onions and Get Big Bulbs

Learn how to plant onions in order to harvest big beautiful onions. This post tells you when and how to plant onions in your garden.

how to plant onions

Onions are a cool-season crop and can be planted as soon as the soil is workable; as early as 4 weeks before the first frost free days. In Manitoba, our average first frost free date is May 24, so onions could go in around April 20th if the soil is ready. Even if the green tops start poking out, onions can take frost or even a layer of snow.

Why Plant Onions Early?

Did you know that onions are photothermoperiodic?  Cool word, but what does it have to do with planting onions?  It means onions are sensitive to light and temperature, in other words, onions need to get enough hours of daylight at the right temperature for optimum growth.

Long-day onions (white, yellow or red cooking onions) – the kind we grow here in Manitoba – need about 13 to 16 hours of light daily during bulb formation (early May to July). We need to get our onions in the ground as early as we can to meet those daylight requirements. Remember the longest day of the year is June 21 and daylight time decreases after that.

Get onions in as soon as the soil can be worked and overnight temps are higher than -6°C (20°F) to take advantage of those long days. I usually plant mine sometime between April 20 and May 5 – depending on temperature, soil temperature, soil moisture, etc.

If you’ve ever planted onions and wondered why you’re bulbs were so tiny – not enough daylight is likely the main culprit.

How to Get Bigger Onion Bulbs:

  • Ensure they aren’t shaded, as stated above, onions need the maximum number of hours of daylight to grow big bulbs.
  • Provide adequate space between onion bulbs, 3-4 inches (even though the bag may say 1 inch, as mine did).
  • Remove weeds that may compete for light and/or moisture.
  • Ensure sufficient moisture during bulb formation especially – onions won’t look like they need water (ie they won’t wilt), but they do!
  • Plant early enough in the season to ensure onions will get enough hours of daylight.
  • Plant the right variety for your area – long-day (north), intermediate-day (central) and short-day (south). Buy local and check the label to be sure your onion sets are sourced from your region – Canadian sourced onion sets will most likely be long-day onions unless specifically labelled otherwise.
  • Loosen the soil so the soil is not too compact for the bulb to grow.

How to Plant Onions

It is easiest to grow onions from onion”sets” – immature onion bulbs. If you prefer to start from onion seeds, they must be started in doors very early in the year. One little onion set will grow into one big onion, so plant as many sets as you want to harvest. Put the onion with the root side down and the pointy side up in the soil.

How to plant onions

When to Plant: Early, 4 weeks before last frost free day (end of April to mid-May in Winnipeg) – when soil is workable

Days to Maturity:75-110 days depending on variety, green onion tops can be harvested within 3 weeks

Where to Plant: Full sun is best in well drained, fertile soil

How to Plant Onions: With pointy tip up, place sets in soil and cover with 1″ (2.5 cm) of soil. Don’t go too deep!

How to plant onions

Spacing Between Sets:space 3-4″ (8 cm) apart so each bulb has plenty of space to grow

Spacing Between Rows: space rows;12-16″ (30-40 cm) apartHow to Plant Onions

Depth of Set: 1″ (2.5 cm) deep

Companion Seeding: Seed onions next to cabbage, broccoli, kale, kohlrabi, brussel sprouts, tomatoes, celery, carrots, beets, peppers, spinach, parsnips, strawberries.  I like planting onions next to my carrots with the hope that the onions keep carrot flies away – I’ve never had them, so I keep this partnership alive!

NON- Companions: Avoid planting onions next to beans, peas, sage and asparagus

Here are some random photos of onions growing in our garden.

onion row mature - watermarked

By end of July onions are flopped over and ready to harvest. You’ll want to read the following:
How to Harvest and Cure Onions
How to Store Onions
How to Dehydrate Onions
How to Freeze Onions

How to Plant Onions

More Tips for Planting Your Garden

How to Plant Corn

How to Plant Leeks

How to Plant Carrots

How to Plant Peas

How to Plant Beets

How to Plant Tomatoes

How to Plant Garlic

Top 5 Herbs for Your Garden

Grow Your Own Seasoning Blend

Grow Your Own Herbal Teas

How to Grow Chives

When to Plant Different Vegetables

Sign up to get articles by Getty delivered to your inbox. You’ll get recipes, practical tips and great food information like this. Getty is a Professional Home Economist,  speaker and writer putting good food on tables and agendas.  She is the author of Manitoba’s best-selling Prairie Fruit Cookbook, Founder of Fruit Share, a mom and veggie gardener.

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5 Comments

  1. I found your site while looking for a reminder of when and how deep to plant onion sets, and I am pleasantly surprised that you are in the same growing zone (I’m in Rocky Mountain House, Alberta). I have now read most of your veggie tips and have bookmarked this site. It’s often hard to find a site from Canada, let alone the same exact growing zone.

    Question on onions… you mentioned buying local onions because of the day length. Too late for me this year as I’ve already purchased from home hardware (in bulk), but is there a way to know which varieties are the right day length?

    1. Hi Mara,
      So glad you find us. How wonderful to have the Rockies as a backdrop to your garden!

      As to your question, coming from Home Hardware, I suspect your onion sets are just fine. Their online packaged onion sets have a label that says they’re sourced from Canada so I assume their bulk onions are too.

      I don’t know if you can see a noticeable difference between day length varieties in onion sets. I rely on the label and where they are sourced from. Onions sourced in Canada are going to be long day onions unless specifically labeled differently. If you’re ordering from an international big box store or from an online retailer – you’d really have to know what you’re getting. Onion sets from the US, Mexico or elsewhere could be different day lengths.

      Hope this helps and happy gardening!
      Getty

      1. Yes, thanks, that does help. Plan for today: get my onions in the ground!

  2. Hi Getty, I started the seeds for “Long white stalk” bunching onions (Burpee brand) about 15 days back. I did not realize until recently that these were bunching onions. I actually was hoping to grow onion bulbs. Now I am wondering if the bunching onions actually bulb. Did I start wrong seeds? On the package it says 120 days full maturity but can be picked after 60 days. This is kind of giving me hope that may be full maturity means bulbs. Any clue if these would bulb?
    I am so glad to have found your website as it is very hard to get good, clear and detailed information about gardening in this cold zone. Thank you for all your good work!

    1. Hi Snehil,
      Glad you found the site and are finding it useful, thank you for your kind words.

      My guess is that you’re won’t get very big bulbs from your bunching onions – maybe a wee bit of bump, but likely not much more. You may want to consider planting onion sets this year if you feel like starting from seed might be too late this year. Onions are sensitive to light conditions so be sure you get them in the garden early in a nice sunny spot so they can take advantage of the long sunlight hours of later spring early summer.

      Good luck!

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