Updated: Apr 24
Onions are a vegetable used nearly daily in many of our kitchens. Onions are a cold hardy crop that grow well in the zone 3 gardening climate of Canada and other parts of the northern hemisphere. Onions belong to the allium family and can be beneficial in your garden as a companion to many plants as a pest deterrent. (See below.) If you plan to grow onions this year read these tips to help you grow larger onions in your garden.
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For many years I struggled to grow onions. Each spring I would plant onion sets around the end of May and by September most would be only slightly larger than my garlic. Thankfully some research, along with learning from mistakes I had made, pointed me to where I was going wrong and last year was my first ever, very successful onion harvest! Let me share with you a few tips for growing onions that I have found out over the past year.
3 Mistakes I Made When Planting Onions
Not making sure I was growing intermediate day, or preferably, long day onions.
Planting onions too late in the spring. (End of May vs. end of April).
Planting the onion bulb too deep.
Let's take a look at the varieties of onions to grow in Canada.
Long Day vs. Short Day Onions
Onions are photothermic. That means they need enough hours of daylight to grow to their full potential.
One of the most exciting tips I have learned for growing onions is that there are long day, intermediate day and short day onions. Up here in Canada in the northern hemisphere (or in latitudes 37–47°), where the summer days are long and winter days short we should be planting long day or intermediate day (sometimes labeled day-neutral) onions. If you are northern Canada, go for the long day onions. Long day onions require 14 to 16 hours of daylight to mature. Near the equator intermediate day onions grow best and in the southern hemisphere, short day onions.
As long-day onion bulbs grow quickly when there are more hours of sunlight in the day, it is beneficial to plant onions as soon as the soil is workable and night temperatures are warmer than -6C (20F). With June 21st being the longest day with respect to hours of sunlight, planting onions earlier allows them to get the most out of the long days.
List of Long-Day Onions
Long Day Onion Varieties grow well in Canada
Here is a list of long day onions that grow well in northern climates.
Early Yellow Globe
Rossa d'Inverno Rubino
Southport White Globe
Southport Red Globe
Sweet Spanish Utah
Yellow Sweet Spanish
Onions are Cool Hardy
Onions are cool hardy and tolerate light to medium frost, and even snow, well. To grow larger onions, plant onions outdoors up to 4 weeks prior to the last expected frost.
I grew onions from seed for the first time two years ago and tested their hardiness by planting some of my onions out on May 1st. They had frost on them multiple times and snow on them 2 or 3 times. I planted other seed onions May 24th. Then onions I planted outdoors earlier were much stronger and healthier than the ones I planted out later. I did not cover my onions during the frost or snow. During that time, it got down to -10⁰C (14⁰F). Although the onion tips received a little frost bite, the rest of the plant remained strong and healthy.
Onion Bulbs are Not Part of the Root System
For some reason, in the past, I had assumed that the bulb portion of the onion was part of the root and was planting my onions too deep. I had thought this would help the bulb grow larger when in fact, onions should be planted no deeper than 1 inch beneath the surface of the soil. As the bulb develops it will push soil away and 1/2 to 2/3 may end up exposed above the soil. Do not cover the exposed portion with more soil.
The onion bulb is technically not root or stem but rather a tunicated bulb, or a modified stem where the fleshy layers of the onion bulb store food and transfer this to other parts of the plant when needed.
Onion Sets or Seeds?
Onion sets are small onion bulbs that have been grown from seed, dried, and packaged to plant the next season. Onion sets are preferred by many as they are easy to plant and work with. If you wish to grow onions from seed, they should be started indoors in mid to late January. (Find my full printable seed starting guide here.) Personally, I have found my onions from seed grow larger than my onions from sets.
Onion seed germination rates go down quickly as they age. It is best to use seeds that are no more than 1 to 2 years old. I have had great luck order seeds from West Coast Seeds. My favorite are the WallaWalla.
Onions are biennial. This means they form seed heads in their second year of growth. Some feel onion sets are at a disadvantage because they may put energy into forming these seed heads instead of bulb growth.
How to Grow Onions From Seed
One benefit of starting onion seeds indoors is that they can be planted closely together in a small container or pot and continue to grow in that until planted outdoors in late April or early May, so you don't need a lot of space. When it comes time to transplant the roots separate easily without breaking.
Plant onion seeds indoors around 10 weeks before your last hard frost. (A hard frost can be considered temperatures below -6⁰C or 20⁰F.) I like to plant my onion seeds around mid January as I plan to plant the onion starts outdoors near the end of April. Sprinkle onion seeds on moistened soil and cover with 1/4 inch of soil. Onion seeds should germinate within 7 to 10 days.
Once your onion seedlings are about 4 inches tall you can trim the tops to keep the tops from falling over and breaking. Trim onion seedlings as needed to keep them around 2 to 4 inches until they are ready to be transplanted outdoors.
Planting Onion Starts or Sets In Your Garden
If possible, choose a full sun location to grow onions.
Plant onions in loose, well drained soil.
Make a hole about the size of the onion.
Plant onions 6 inches apart and 1 inch deep with the root end facing down.
Cover onion with soil.
Water onions immediately after planting and continue to water as needed afterward.
Once onion tops are about 4 inches tall they can be harvested and used as green onions. Choose an onion that has at least 3 green tops and cut one of them about 2/3 the way down. Taking too many onion tops from one plant may stall onion bulb development.
When to Harvest Onions
Harvest onions as you need them anytime throughout the summer. If you are not collecting seeds, harvest and use any onion that starts to develop a flower. A growing onion will stop putting energy into the bulb once the flower begins to form. These onions will not keep long after developing the flower.
Harvest onions a week or two after the tops start to turn yellow and fall down, and before a first fall frost. This year some of my onion tops were still green, but I knew frost was coming. Some people believe the onion will grow larger if you bend the tops over yourself around mid August. Others say doing so makes no difference. I have not done an experiment myself using this method. Maybe I will have to try it this summer. Also allow the tops of the onions bulbs to emerge from the soil and be exposed in the final weeks.
Skip watering for 3 days prior to harvesting your onions and try not to do it if you have had rain in the past 3 days. The onions will store better if they are harvested when the soil is dry.
Curing Your Onions
After harvesting your onions, spread them out in a dry location in a single layer. I like to spread them on a blanket on our garage floor. If you are braiding them, do this now. Otherwise leave them to cure for up to three weeks, or until the tops are completely brown and dried. Put any you find with green tops aside, to use within a few weeks. Trim the roots off and the tops to 1 inch. The onions can be stored in a box with holes in a cool, dark place. Ideally store them in single layers. If unable to do so, separate the layers with paper towel or wood shavings.
Onion Companion Plants
Plant onions next to beets, brassicas (cabbage, broccoli, cauliflower & brussels sprouts), chamomile, cucumbers, dill, kale, leeks, peppers, potatoes, summer savory, spinach, squash, strawberries or tomatoes.
Avoid planting onions near asparagus, peas and beans.
You can find my full printable companion planting guide here.
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