Simple DIY

Passive Solar Coldframe

Using packed earth tires as a thermal mass, this simple coldframe extends our season by 1-2 months in the spring and fall.

What is a coldframe?

A coldframe is essentially just a mini greenhouse. It protects plants from cold temperatures and can help growers in northern climates increase the length of their season.

Thermal Mass Wall

This design adds a passive solar element. By having a large, heavy wall (on the north side) made of tires filled with dirt, we are capturing sunlight and heating the tires. At night this heats is radiated back into the growing space keeping the plants warm.

The build

This was a very simple (and not very elegant) build. We used all recycled materials (even the hinges), and so it only cost a couple dollars for screws.

Planting

We decided to make a mini hugelkultur bed because we were low on compost and soil. We laid down a few logs in a trench and then covered them with leaves, sticks and some unfinished compost.

As an experiment to see how lazy we could be, we didn’t add any finished compost or soil and just threw seeds directly on top of these leaves and unfinished compost.

You can see kale seeds strewn randomly in this rough unfinished compost.

It seemed like this couldn’t possibly work because we weren’t even planting into soil. But the plants sprouted and took root, their roots snaking down to the moist and rapidly decomposing logs below. We had fresh salads and baby greens by the end of April, barely after the snow had melted. Overall this was a massive success. The lessons from this coldframe will be applied to our full size passive solar greenhouse.

Results

In the spring nighttime temperatures would routinely reach -5C, but the coldframe never had a killing frost, and didn’t even appear to freeze at all. The cold hardy plants (kale, mustard, lettuce, swiss chard) all established themselves from seed and grew well. This cold frame allowed us to significantly extend our growing season and have fresh greens earlier.

Update 2024. We decided to dismantle this and focus our efforts on consolidating our garden space, but this was a cool experiment showing the power of thermal masses collecting and reflecting heat into a space.

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