Heating Our Greenhouse with Manure

Starting seedlings on hotbeds

And we’re off. ‘Shit Day’ has come and gone. A harem of young hens arrives tomorrow afternoon, and we’ll start pressing seeds into soil in the greenhouse, embarking on a season-long wild ramble. I’m ready. I think. Sure, I didn’t finish a third of the jobs I wanted to get done over the winter, but I had plenty of mornings filled with buttered pancakes and bacon, and watching the Great tits¹ (actual name of the birds) nibble pig lard and stale bread out the window.

We’ve had ‘Shit Day’ marked on the calendar for months. Last Sunday I called up Max. He makes saddles in a little workshop warmed from wood, lit by the sun, which radiates the intoxicating peeled-bark smell of leather. Old fruit trees hang over the sides of the dried quarry that backs up to his cottage and barn where four horses take up residence. It’s what passes through them that I’m after.

Max lives in a village just outside the city limits of Prague, the name translates to ‘little frost’. When the whole city was shut down and people weren’t allowed to leave without the right kind of paper with the right words on it, you bet I rolled the dice . I tossed a cowboy hat on and high tailed it through the checkpoints. We needed that shit.

We heat our greenhouse with it. Seeds need higher temperatures to germinate than they do to grow. Heating a greenhouse in late winter creates the warmer conditions most vegetables need to germinate. Without it, our season would be at least a month shorter and summer crops like tomatoes and cucumber wouldn’t be worth growing. 

The whole concept is very straightforward - pile manure and straw into the corner of a greenhouse. The decomposition of the materials gives off a lot of heat. So hot, you can’t keep your hand inside the pile for more than a few seconds once it gets going, and that takes about 2-5 days depending how cold it is.

For a compost pile to generate heat, a critical mass of about 1 cubic meter (9 cubic ft) is needed. Our hotbed is about 2 cubic meters and is made out of scavenged pallets and boards. Anything less than this and not much heat will be given off for any extended period of time. I like to imagine a mound of manure as its own universe. The drama of thousands, perhaps millions, of civilizations of bacteria battling it out, feasting on the remains of their foes. Or maybe it’s like the Serenghetti, wildebeest herds of amoeba roam the great endless plains of the tip of a straw.

I inoculate the pile by lasagna-layering compost from the chicken yard into the manure, adding a layer of biochar on top. I dance on the turds like I’m making wine to pack it down, especially in the corners. Better to not stomp on it too much though, you don’t want to over compact it and have the pile go anaerobic. That gets smelly, and doesn’t produce a quality compost.

After three days our pile hit stride. I could see steam pouring off the top. It got a little hard on the nostrils in the closed space until I put the charcoal on the pile. You can hardly smell anything now. Charcoal is extremely porous. A crushed tablespoon has the surface area of a football field. It acts as a carbonic sponge absorbing the vaporized gasses. The slight ash content means the resulting compost won’t be too acidic.

We start thousands of seeds each year on our hotbed. Once they germinate, we rotate them off the bed to the tables. Before planting into the beds, we harden the seedlings off in a glass cold-frame outside the greenhouse. This avoids transplant shock and lets the plants get acclimated to the much less hospitable climate outside.

To top it off, all the manure, compost and charcoal, becomes a gorgeous chocolate cake by the following February, an ideal substrate to start seeds in (or make compost castles). 

We mix half composted manure with half organic substrate we buy. Cut’s our costs by half and the young starts always look healthy, no doubt well fed from the compost.

If I was smart I’d run a hose through the pile and connect it to the watering barrel and allow thermal-siphoning to heat the water, which would then act as a battery of heat like we do in our house. There are people out there that shower all winter on water heated by a big pile of manure and wood chips. Even alligators make a nest out of piles of rotting leaves to keep their eggs warm while they’re off sunbathing.

Tomorrow will be cold. Thanks to what drops out of the business end of Max’s horses, I’ll be warm as a gator in the greenhouse.

Good growing to you all,

Brett

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1. Sýkora koňadra (Parus major)