No matter what part of the country you live in, at some point the growing season will come to an end as winter draws near. The days get shorter, the sun gets lower in the sky, and the ground cools and sometimes even freezes. One good hard frost is all it takes to wipe out most garden crops. There is a way, however, to extend your growing season by planting in a cold frame.
A cold frame is basically a box which is slanted and positioned toward the south for maximum sun exposure. The sides can be made of wood, rocks, bricks, metal, plastic, or even hay bales, with a glass or clear plastic lid. Sunlight enters the cold frame and is trapped in the box, keeping your plants much warmer than they would otherwise be out in the garden, allowing them to thrive when they’d otherwise freeze.
Cold frames can be used to start seedlings early to get a jumpstart on the garden. They can also be used to continue growing cool weather crops, such as leafy greens, year-round.
If you live in an area that experiences mild winters, you shouldn’t need to artificially heat a cold frame. As a matter of fact, you’ll need to vent it during the daytime so that you don’t fry your plants when the sun is high (been there, done that!). If, however, you live in an area that experiences hard winters and extreme freezing temperatures, you might want to look into alternative ways to add additional heat to your cold frames to keep them above freezing- especially overnight.
Here are 10 ways to heat cold frames over winter…
1. Light it up!
Light bulbs can add much needed heat to a cold frame during chilly winter days. Even a string of Christmas lights will add some warmth. You can hang them in the box to dangle over the plants, or weave a string of lights between the plants to keep them nice and cozy.
2. Compost is hot stuff.
As compost breaks down, the fermentation process creates natural heat. Dig a trench and fill it with layers of “brown” and “green” materials: leaves, sticks, compost, manure, grass clippings, etc. As the “lasagna” layers decompose, they release heat which steams up from the soil and keeps the plants warm under the cold frame’s glass.
3. Insulating with bubble wrap.
Insulating a cold frame will help to hold the sun’s natural heat in. Thick bubble wrap taped either to the outside or the inside walls and lid of a cold frame (or greenhouse, as shown) acts as an amazing insulator.
4. Lots of leaves create heat.
This guy shows how he uses a four foot thick pile of leaves to heat his greenhouse during the winter months. The process could easily be converted to use with a cold frame by making the backside of the frame out of chicken wire covered with plastic with holes poked in it, and stacking leaves to insulate this northern wall of the frame. As the leaves break down they’ll introduce the heat of fermentation to the cold frame, warming your plants and helping them to grow.
5. Double it up!
By covering a cold frame with an arched plastic hoop house you can trap in even more solar heat, building a nice little microclimate to grow in.
6. A homemade terracotta pot heater.
I’ve seen these homemade terracotta heaters made several different ways, but the idea is to use tea light candles to warm up the terracotta pots, which then absorb and radiate the heat for hours after the candles have burned out. This idea could potentially be used in a cold frame to add warmth. Just be sure to keep it away from flammable materials, as this could definitely be a fire hazard if you aren’t careful. Not something I’ve tried yet, but could be worth experimenting with.
7. Keep it toasty with a rocket stove.
The guys in this video show how they used a wood burning rocket stove to heat a greenhouse. A large row of cold frames could benefit from a similar setup.
Some additional ideas…
- Cover the cold frame with a thick blanket overnight to keep in some warmth.
- Build the north side of the frame (the back wall) using bricks, blocks, or stones to help absorb the heat from the sun.
- Add water bottles painted black to help hold thermal mass inside the cold frame.
Do you have any other thoughts on how to heat a cold frame? Please share in the comments section below.