How to Heat a Cold Frame or Greenhouse For Free
Extending the growing season is key for maximizing food production for home gardeners. When winter lasts for half the year in Southern Ontario, finding a cost effective and safe heating source can make all the difference in succession gardening and seed starting.
We’ve come up with a heat source that as a parent, you probably already have laying around and if you don’t, you can likely pick one up for $5.
Cold frames, greenhouses; any structure designed for growing will be un-insulated and inefficient as far as retaining heat as their purpose is to maximize solar gains. But when our days are short and grey and the nights are long and cold, this means our temperatures are often inconsistent and too cold for plant growth or germination until spring gets rolling.
So what do we do if we want to start growing when the temperatures are still around freezing? We have to heat our structure.
Electricity is not the cheapest commodity in Southern Ontario. Alternative energy sources such as fuels bring about safety concerns. We want this to be cheap AND safe.
Open up the cupboard that houses all of your miscellaneous small appliances and grab your DIY Cold Frame or Greenhouse Heater.
DIY Cold Frame or Greenhouse Heater
Exterior Grade Extension Cord
The one I use is from Canadian Tire but Amazon has similar.
Exterior Grade Timer
The one I use is from Canadian Tire.
Crockpot
Make sure it is not a digital one. In this case the older, dial style works best with the timer. The one I use is Crockpot Brand but any dial based model (keep warm, low, high) will do. They are usually abundant at thrift stores and frequently can be found for $5.
Thermometer
Once again, just a basic outdoor thermometer will do.
Each structure will respond and retain solar heat gains differently which is why the thermometer is important. You don’t want to cook your plants with the heat of the sun plus this heater. How long you’ll need to run the crockpot and at what setting (keep warm, low, high) will change as the season progresses.
For reference, I built my little ‘cold frame’ in early April, about 5 weeks before last frost for my area. Temperatures were still around freezing going up to 4°C during the day, and the nights were around -5°C+/-. You can see the size of the structure I built in the video above. With all of these factors, I set the crockpot to high and had it running from 5pm-11am. It would be off during the heat of the day. As the weather evolved and spring warmed, the timing changed to 5pm-7am, then the setting was changed to low and eventually the heater only needed to be used on the odd, extra chilly nights.
Free Renter-Friendly Cold Frame - No construction required
A quick disclaimer: This structure doesn’t fit within the standard definitions of typical season extending structures. It’s like a hoop house, but without the hoops. It’s low like a cold frame, but without the frame. It’s most closely resembles the definition of a greenhouse. A greenhouse is “a structure with walls and roof made chiefly of transparent material, such as glass, in which plants requiring regulated climatic conditions are grown.” but is not what most would imagine a greenhouse to be. (Check out the video at the beginning of this article for a visual reference.)
So call it what you will, this is how it was made:
Patio furniture
Rally the kids and ask them to build you a fort. Same principles apply to this structure. You’re going to create ‘walls’ or a framework to support the…
Tarp
I utilized a transparent one that we had from camping. Ours is from Canadian Tire.
Tie Downs
I used old bricks and rocks from the garden, but rope/tent pegs, a number of items will do the trick depending on if you are building on land or hardscaping.
What is so great with this structure is that it’s temporary and free. It’s not another item to buy and store. Kids are inherently great at building forts so you can delegate the task as play if your kids are of an age. Sure it doesn’t have fancy vents or doors, but this can also be a great lesson in understanding how heat rises, cross breezes flow etc. An easy science experiment and lesson without it feeling like school. And you get food or flowers out of it!