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A Guide to Growing Cold Hardy Plants Under Cover - Unheated Greenhouse Growing

I remember the day I found out that it is possible to grow cold hardy vegetables before and after my first and last frost dates. I was blown away. Here I had always thought outdoor gardening had to be packed into just a few short months and low and behold, I was wrong!

It happens that I have a small 10' x 12' unheated greenhouse that works perfectly for growing early and late cold hardy veggies. The trick has been figuring out exactly how to do this, finding lists of cold hardy seeds, and trying to figure out seed dates.

Winter greenhouse in Canada used as a cold climate greenhouse to grow cold hardy plants.
My greenhouse in March, growing cold hardy vegetables under cover inside the beds.
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Unfortunately I have not got this information completely perfected for you. But I am so excited about what I have found out I don't want to wait any longer to share it so that YOU can get started growing cold hardy vegetables as well!

A lot of what I have learned has come from Eliot Coleman's book The Winter Harvest Handbook. I highly suggest purchasing this if you wish to learn more on growing cold hardy vegetables through the cool months of the year.

How to Grow Cold Hardy Vegetables

Plants protected from the elements can survive much colder temperatures than plants exposed. This is especially true of the wind. To grow cold hardy vegetables in winter months you will need at least two layers of cover, ideally having 3 layers.

The layers necessary for growing cold hardy vegetables are as follows:

  • A greenhouse. (This is not necessary, cold hardy vegetables can be grown using only the following 3 layers)

  • 6 mil Poly. Inside my greenhouse I just use two layers of 6 mil poly right next to each other. If building a poly tunnel for winter growing, it is important to build two hoops with a 4 to 6 inch layer between. This layer of air between the layers of poly acts as insulation.

  • Light row cover. This row cover should be placed 2 to 4 inches above the tops of the plants.

A poly tunnel in a cold climate greenhouse used for unheated greenhouse growing.
Poly tunnel inside my greenhouse with two layers of poly over the hoops.

Hoops will need to be placed for the poly. As mentioned, two layers of poly are needed when growing cold hardy veggies in a poly tunnel vs. inside an unheated greenhouse. Hoops can be made from most pex or PVC piping, using metal strapping to secure. I used this 12.7mm SharkBite pex pipe for my garden hoops. You can check out my story highlights on IG for details on how I made my hoops. Shown in that video are only single hoops used for brassica netting as well as single layer poly for spring crops.

Cold hardy vegetables being grown under cover for unheated greenhouse growing in zone 3 Calgary, Canada.
Row cover is used as the first layer of insulation for growing cold hardy plants under cover in zone 3.

Cold Hardy Vegetables

I am still working on perfecting a good list of cold hardy vegetable seeds. I will share with you a few options I think would do well here. Most of these links are from the West Coast Seed website and are affiliate links. Many of these varieties are also available from other seed companies as well.

A cold weather greenhouse being used to grow frost hardy vegetable plants in zone 3 Calgary, Canada.
Fresh planted seeds inside my unheated greenhouse in late February.

Unheated Greenhouse - Winter Sowing Planting Calendar

When to plant... That is the question. That is the question I am still trying to answer. I may not have it perfected but I do have some rough guidelines. General planting dates are as follows:

  • Spring Planting Dates - Feb. 20 - Mar. 20th

  • Fall Planting Dates - Aug. 25 - Sep. 20th

Plant growth slows considerably with less than 10 hours of daylight. In the Calgary area (on the 52nd or 53rd parallel) we experience less than 10 hours of daylight from around October 24th to February 20th.

The idea with cold weather gardening is that the plants should be mature by the time you reach the 10 hours of daylight and less point. They essentially hibernate under cover and can be used fresh as needed.

Hardy vegetable plants grown inside a cold climate greenhouse by someone gardening in Calgary, Canada, zone 3.
Hardy vegetables in my greenhouse in early April.

With aiming to have plants matured by October 24th, I have come up with a winter sowing schedule. Adjust as needed according to your last 10 hour day of daylight in the fall.

*It is important to note that Days to Maturity listed on seed packages are the days it takes the plant to mature under ideal conditions.

August 25th to September 1st

  • Kale

  • Beets

  • Claytonia

  • Mache

  • Swiss Chard

  • Carrots

  • Turnips

  • Endive

  • Green Onions

September 2nd to 10th

  • Mesclun Salad Mix

  • Pac Choi

  • Spinach

September 10th to 20th

  • Arugula

  • Radishes

I have found my winter plants can survive down to temperatures of -22 degrees Celsius (-8 F). It is truly amazing how hardy plants can be when given protection.

An unheated greenhouse growing frost hardy vegetable plants by someone gardening in Calgary Alberta, zone 3.
Small cold hardy vegetables in March, being grown under cover in my unheated greenhouse.

Your spring cold weather planting calendar is not as specific. Plants can be seeded outdoors anytime after February 20th as soon as your soil is thawed. Days to maturity will depend on how soon you will be able to harvest. Again, remember to expect plants to take longer to mature than listed on the seed packages when the weather is cool.

Using these winter planting methods and growing under cover allows you to enjoy heathy, fresh home grown vegetables throughout more months of the year with little extra cost for supplies.

If you have found this article helpful and would like to see more gardening tips and tricks, subscribe to my blog (the bottom of the home page) and follow me on Pinterest, Instagram, Facebook and/or YouTube!


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