Incan irrigation and hotbeds could transform our polytunnel and greenhouse woes by Fiona Nevile

There was a fascinating article in Permaculture magazine (no. 66 Winter 2010) using an Inca technique to self water a greenhouse. Basically, water is harvested from the roof of the tunnel or greenhouse into a central semi lined gulley. This seeps in between small stones beneath raised beds and waters the plants from below. The soil on the top remains dry – so is no longer a nirvana for slugs and also counteracts the nasty problem of moulds. The gulley has a wooden slatted walkway above that forms the pathway through the tunnel.

I got very excited about this idea. With a bit of work and a small initial outlay, most of the onerous summer watering could take care of itself.  Then, needless to say, I was distracted by other jobs in the garden and the idea was shelved for the time being until now. It might be just a tentative flicker but I had a rare “light bulb” moment last week. I just need to fill you in on the rest of the background first.

The Polytunnel Book: Fruit and Vegetables All Year Round by Joyce Russell gives plans for a simple hot bed. A wooden frame on top of the soil, measuring 1.5’ x 4’ and 1’ high. She digs 0.5’ of soil from beneath the frame adds 1’ of manure, puts the soil back on top of this and plants up after two days. The life of this hot bed is about one month.

This became a must-have for my new Solar Tunnel the moment that I read about it. I did a swap – a bird lavender bag with The Chicken Lady for straw and horse manure mix. TCL, S and I went to the steaming heaps of muck. We just managed to squeeze seven 20 litre bags into the boot of The Duchess. And I roared home in a fever of anticipation with a warm and whiffy boot.

I decided to do a bit more research on hot beds, from the Victorians to the present day. There wasn’t much information out on the Internet. But I did discover some important facts – the manure mixed with straw generates a lot more heat for longer than just pure manure. There were two simple styles – a mound or a pit – both with a cold frame on top. The cold frame can be opened on warm days and closed at night. Even a pit with a simple glass cover would do.

Clearly a brick built contraption would hold a lot of heat too because bricks hold the heat. But I was impatient to begin. The muck bags were lying in an expectant heap in the Solar Tunnel. They had cooled since being unloaded from the boot. Would they heat up again?

Then I found this article . Roy Martin’s hotbed is outdoors and retains heat or about two months. He also clearly has access to a digger. Sob. I also found this article useful and liked the idea of letting the manure/ straw mix increase in temperature for nine days before adding the final layer of compost and soil. Described here.

So I dug a small pit 4’ x 2’ and 2.5’ deep. It took a long time. Well, about four hours when I decided that grave digging was not for me. I lined the base with 3” of yew hedge clippings and filled the hole with 140 litres of straw and horse manure. This pit is about 3” wider than the cold frame that I’m making out of an old window and some offcuts that I found in the brick shack.

The only financial investment that I made was a very sturdy Soil and Compost Thermometer. I do tend to be quite hard on tools. This one has a metal casing and alarmed Danny when I waved it at him while he was on a conference call to Bangalore.

The build up of heat was fast. 55 degrees Fahrenheit on day two, 65 on day three, 75 day four, 95 day five and today 111! The temperature will eventually drop and plateau for two months. It was then that I came up with the idea of using this method of heating the green house and solar tunnel.

Maybe I could install the summer self watering system in both of them. Then I could have minimal watering duties during the hotter months and in winter I could drain the system and fill the irrigation channel with straw and manure!

No massive bins encroaching on the borders. No paraffin heaters. No electricity. Free heat that breaks down into good organic matter to dig into the garden in the spring. Perfect.

You might be wondering about the possible whiff? Well it’s not ammonia and gas masks on. Just a gentle straw and horse smell that intrigues the Min Pins and could not offend anybody.

So that’s the plan. As this is the start of my hot bed experiment I have no idea how long the small bed will retain its heat. But if it lasts for two months, I reckon that an irrigation channel of a similar depth would just need two full top ups each winter. Staggered to guarantee a constant heat.

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