Use Compost to Heat Greenhouse-Increase Grow Season Length in Zone 5

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Winter is just around the corner and we’ve already done fall clean-up on our vegetable garden. All hot-weather vegetables were removed and transferred to the compost pile. There are still some cool weather vegetables (kale, swiss chard, and peas) growing even after the first frost– they are covered when the weather forecast projected at 33°F.

I wanted to further increase the growing season and since I’m already covering the plants in my mini greenhouse, the next step would be to heat it. I read an article once that a Wisconsin farm composts manure in their greenhouse to keep it warm all winter to grow their vegetables. I have a lot of stuff need to compost anyway, why not use it to heat my mini greenhouse?

If you Google “use compost to heat greenhouse,” you’ll find tons of ideas and suggestions. I admire this simple yet effective project the most: She uses compost to heat her greenhouse. But I’m far less a handy lady than her, so instead I’m thinking to just put the compost pile inside the greenhouse– just warm enough to keep the veggies growing for several weeks longer. Here is my non-handy solution. I’ll report back in December.

My mini greenhouse is a covered raised bed (3’x6′) ordered from Costco in Spring 2019.

When the raised bed was built, it was filled with compost layers, 6-inch layer green and 6-inch layer brown then green and brown, etc. (if you have no clue what are green and brown, just Google “compost brown and green”) and topped with raised garden bed soil. The plants loved it.

Today is October 24th and the temperature last night was 32°F. Half of the raised are kales. The other half only has one Swiss chard and about 50 USUI pea plants that were sewn on September 16th. This was part of my USUI pea shoots/pea tips experiments. The center is empty– just the right spot for my compost bin– a 10″x13″x16″ carton. I drilled some holes using a 3/8″ drill bit hoping the worms are able to wiggle into the box for food.

Bury the box but left about 5″ above ground because I was too lazy to dig down too deep. Also, it might help heat the air a little.

A website suggests two parts brown and one part green, so that’s what I did. I added some wood chips as brown (it’s supposed to slow the breakdown, producing more heat over time) and a small amount of manure. Layer green and brown until it’s full then cover the carton flapper. The project is done! It’s time to wait for results– I’ll update in December!

Update Apr 2021:

About 10 Pea plants and a Swiss chard disappeared. All the kale plants in the box are still alive. The best one look like this:

The worst one look like this:

What to do with the boxful of food scraps in Spring? We just remove the bottomless box and mix the half processed compost with the dirt in raised bed. Cover a thin layer of compost from our compost bin. It’s ready for cool weather crops–super rich and continue to keep the crops warm.