Finally getting an unheated greenhouse is such a big milestone for gardeners. Still, there’s scant information on the internet about how to actually use one in Zone 3 without killing all your plants.
When you first get started, there are so many questions!
Can you grow anything in an unheated greenhouse in winter? Will an unheated greenhouse protect plants from frost? How many degrees warmer is an unheated greenhouse? How do I keep my greenhouse warm at night?
Unfortunately, there are very few easy-to-find answers.
While I don’t consider myself an expert in unheated greenhouses, here’s what I’ve learned so far about using one in my Zone 3 climate. I hope to update this post yearly as I learn more about the ins and outs of unheated greenhouse growing.
So far, I have used my unheated greenhouse as a place to harden off my seedlings and seed start a few later plants. I have not yet attempted to grow flowers or vegetables over the winter, but the knowledge I share will be from what I have seen other Zone 3 gardeners accomplish.
Want to take a look at how I made over my unheated greenhouse for $50? Check out this video:
Realistic expectations for unheated greenhouse gardening in zone 3 & other cold climates
I know I will burst some beginner gardener’s bubbles, but it is simply too cold to grow plants in an unheated greenhouse over the winter. Unless you’re going to add heat, even the most cold-tolerant kale plant is not going to survive -40 temperatures.
That said, it’s not all doom and gloom.
You can investigate the world of passive solar greenhouse gardening and fulfill those winter gardening dreams. I highly recommend Arkopia’s YouTube channel, as this family lives within a 30-minute drive of our acreage and has an amazing year-round garden with their passive solar setup.
If you’re unable to invest a lot of money, you can extend the season in your unheated greenhouse with row covers or adding a bit of heat to get you through the odd unseasonably cold day or night, if you choose. Depending on the year, this strategy could have you gardening well into December or whenever the first stretch of -30 degrees Celsius hits. However, you will not be able to grow any heat-loving plants and must stick to the cool-loving varieties. Think spinach, kale, lettuce, and bok choi.
Now that we’ve got those disclaimers out of the way, let’s take this question by question, shall we?
Can you grow anything in an unheated greenhouse in winter?
In short, yes. But as discussed above, unless you have a heat source, you’ll be limited by that first stretch of really cold temperatures. To stretch your growing season as long as possible, there are three strategies to use:
- Plant cold-tolerant plants
- Use frost fabric and other row covers
- Make use of simple and cheap passive solar methods
Can you grow vegetables in an unheated greenhouse through the winter?
Before you get excited, remember that -30 temperatures at any point mean your winter vegetable growing is likely done for the year. However, some vegetables can tolerate mildly cold temperatures, including…
- Asian Greens
- Collard Greens
Keep the plants covered with frost fabric and/or plastic row covers to keep in the heat. This should keep your cold-tolerant vegetables alive until somewhere in the -10 to -15 degrees Celsius mark. If you’ve been able to keep vegetables alive in colder temperatures than this without heat, I’d love to hear about your experience in the comments.
Finally, you can try to keep more heat in your unheated greenhouse by using one of the cheapest passive solar methods possible: an old milk jug or other plastic container painted black and filled with water. I have not experimented with this method yet, so I don’t know how many degrees of heat this will add to your greenhouse overnight. However, I plan to experiment with this method this spring and will update the post to reflect my findings.
How Many Degrees Warmer is an Unheated Greenhouse?
To know exactly what the temperature is inside your greenhouse and get some actual data for yourself, it’s wise to invest in either a maximum-minimum thermometer or a wireless temperature monitoring system. This way, you’ll know exactly what your microclimate is without having to guess and being unpleasantly surprised by frozen plants.
During a reasonably warm day, temperatures in your greenhouse can easily reach scorching summer temperatures. Make sure you keep the vents or the door open, so your plants don’t die. You may also need to water twice daily, so keep checking to ensure your plants haven’t dried out.
In the evening, an unheated greenhouse does little, if anything, to keep your plants warmer.
Will An unheated Greenhouse protect Plants from Frost?
An unheated greenhouse will only provide extra heat during daylight hours and maybe a few after. It will also cut the windchill. However, it won’t automatically keep your plants from freezing. This leads to the next question…
How do I keep my greenhouse heated overnight?
When there is frost in the forecast, I cover my seedlings with frost fabric, even though they are already inside the greenhouse. If the overnight forecast is colder than I think my frost fabric can handle, I either run a space heater in the greenhouse or bring everything indoors for the night. Either option is fine. I personally choose the heater because I have a lot of seedlings to move, and my house is not super close to the greenhouse.
Want to learn about gardening with a high tunnel? Check out this post: How to Build a High Tunnel in a Northern Climate
If you liked this blog post, find me on Facebook, TikTok, and Instagram for more cold-climate vegetable gardening tips, delicious recipes, and cut flower goodness! I also make weekly videos over on my YouTube channel. I hope to see you there!
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