Season Extension: Compost and Thermal Mass


Rye cover and our beloved 25x11 portable Hoop House

I love the Hoop House at our Market Garden.  I love that I can harvest spinach in March.  I love that I can plant my tomatoes in late April and still be picking into October.  I love that it was 85 degrees in there today and 70’s in January are a reality in my little microcosm.  But as I mature, it is the more ephemeral things that matter… how it provides an ‘anchoring’ structure in our little .2 acre plot; something more substantial to draw the eye and provide contrast to row after row after of vegetables; the smell and feel of warm soil in January.   The picture above shows how the rye/feild pea cover crop is coming in.  Got a late start (as usual – “real” jobs get in the way ya know?), but it is doing alright.  The tilled strip in the middle was the last 200#’s of potatoes to come out of this plot, and has a .5″ high stubble of winter wheat coming in.

In years past I have planted spinach and mache in the hoop house, but always put it in so late that I never get a harvest until March and this year is no exception.  I typically get good growth and about 3 weeks prior to harvest (it takes forever with the short days in the winter) we get nailed with the Deep Freezes in early January when it gets down to -10 or so.  This kills everything flat.  The roots regrow in Febuary for a good harvest, but I would like to see what I can do to take the edge off that one or two weeks.


Left: 1st week of compost Right: 250 Gallon "Pond"

I also have another winter problem.   I have arrangements with some local restaurants to compost their organic waste.  This nets us 100#’s or so a week… every week.  From November through April that means I have several cubic yards of slobsicles in my compost bins.  Building up my capacity to handle 4 months of gorp was the primary driver of our new Bin Of Dreams.  That bin is located on the north side of our garage and without direct sun, it takes a LONG time for 1500#’s of gorp to thaw out.  Will Allen at Growing Power composts year round in his hoop houses… perhaps I could as well.  So this year I am trying to kill both these birds with one stone.    The windrow is designed to be 4′ wide and has a 24″ tall fence to contain it somewhat and give me a 3-4′ height without a 8′ base width.  Eventually this will be 20′ long.  I have dreams of following the thermophyllic composting bacteria down the windrow (no turning) with composting worms.   Can I extend both my composting and greens season with the simple movement of 1500#s of gorp into the hoop house?  Time will tell.


Its not a ghetto Koi Pond.... Its thermal mass!

In the past 2 winters I have had buckets and trash cans full of water in an attempt to add some thermal mass to the hoop house.  As I have said, it gets to be 80+ in there with 50-60 degree temp differences to outside air on good days.  But with only a single layer of plastic it sheds btu’s like a sieve.  The thermal mass in previous years has not been enough, it simply freezes solid eventually and effectually adds a cooling effect to the hoop house.  Rat Farts.  This year I have added a large fiberglass crate that hold several hundred gallons of water.  As you can see in the picture, to help this out, I have surrounded it with a foot thick layer of leaves for insulation on 3 sides, and will plant the spinach directly to the south.  In a further attempt to extend the season I amy dig out some row cover I found laying around and cover the spinach/compost/crate to keep the warm air around the plants.  Finally the entire Hoop House will get a 4′ wide “foundation” mulch to keep the frost from creeping in as long as possible while the north wall will get as many leaves piled against it as I can find.  Will this be enough to combat the utter lack of R value in the Hoop House?  Time will tell.  What is likely is that a double walled Hoop House with a bubble insulation system will be the way to go, combined with these techniques, if I want to go 4 season.

On a side note, I am trying to function stack the thermal mass.  I have added about 5 gallons of leaves and some finished compost to it to make the well water a bit more nutrient rich.  Then I walked down to the river and scooped up 5 gallons of river water and made sure to get a bunch of sludge from the bottom.  This water is full of critters and microorganisms.  I dumped this into the crate to “inoculate” the water and in a week or two I will add a handful of feeder goldfish.  Now it is very likely in my first stab at aquaculture I am dooming these goldfish to a cold death as fish cubes, but if they somehow don’t freeze solid the 250 gallons of microbe rich ecosystem should keep them alive.  And if the temps allow them to stay alive I will have also proven that I can sustain temps high enough to overwinter lake perch. Fish Fry anyone?



14 Responses

  1. Pretty cool, Rob. Single sheet plastic is tough. I wonder whether you could fight the cold with successive massive batches of compost designed to get hot. If the piles were big enough, and especially if they could warm that water, it might keep things pretty cozy in there. But you’d probably have to watch it carefully and move one pile as soon as it cooled down so that it could be replaced with another. Things for me to think on too…

  2. Thanks Kate. If you are serious -I’m not overly much – this project is as much to keep my compost going as it is to grow spinach an month sooner – here are some ideas from one of my all time favorite Gatherings of the Weird and Brilliant: The New Alchemists!

    • Great link! This quote from it seems especially important:

      “Ammonia will damage plant leaf tissue in concentrations as low as 10 ppm (Thomas
      1951) and must be scrubbed from the exhaust stream before it enters the greenhouse atmosphere. A
      number of experimental composting greenhouses have suffered chemical bums of leaves by NH3 that
      was released directly into the greenhouse atmosphere without being filtered.”

  3. Rob, you really should get ahold of Eliot Coleman’s latest book, The Winter Harvest Handbook. You might be a little less inclined to go with thermal mass: it’s not heat that makes winter crops grow or just merely survive, it’s light. That your stuff just sits there in mid-Nov makes perfect sense: you’re under 10 hours of light a day, so they go into a bit of stasis. If you get them big enough before this happens (by planting earlier, like, in Sept. outdoors and transfering in, or direct-seeding in early Oct.) then they should be able to last, and not die back to the ground. And double coverage really, really helps; invest in some Reemay and cover the beds. I am in a warmer zone than you but Coleman isn’t.

    • You are exactly right El – light is the limiting factor for winter growth. I’ve read all Coleman’s stuff and am re-reading his New Market Grower now – the guy is amazing. What I am trying to avoid is the total ‘top kill’ of the spinach growth I do get. I will contest one of your statements, light makes them grown but temps below 10 degrees will kill the top growth to the ground, though not the roots. Losing that growth sets me back several weeks. Thermal mass combined with the thermophyllic bacteria generated heat, if I can keep THEM going, An early Oct. seeding would be ideal and combined with some of these tactics, and coverage, would do it.

      • I was going to suggest Coleman, too. 🙂 One thing he always does is double-layer protection: row covers just taller than the plants inside the greenhouse. I’ve actually got a cold frame inside my greenhouse right now – with green beans just about to bloom. Don’t really expect to get a crop, but they’re a good marker for the temps in there. Outside got down to 23 last night (coldest night yet this year). Inside the greenhouse, it was 27, and inside the cold frame was 34. Even the peas with some row cover tossed over didn’t lose a flower at 27 degrees. So I would definitely suggest some cover over your spinach to help prevent total die-off, especially if your other props help keep the soil from freezing.

      • Well Rob I can’t doubt your 10*: my greenhouses have never gotten that cold!

        I will say that spinach has never been a terribly successful crop for me in the greenhouses in winter. It’s been good but not great. Other things that grow like gangbusters and seem oblivious to the cold are arugula, mache, claytonia, minutina (which is a skinny-leaved plantain; also called buck’s horn plantain) and Italian dandelion, and all varieties of escarole.

        But like Emily and the esteemed Mr Coleman I double-cover, as soon as the temps indoors drop (daytime) below 50 and stay there if it’s cloudy.

  4. Rob, this rocks. Awesome, permacultural ways of generating heat. But I’m going to predict that the Wisconsin winter will still kill the spinach. I think this is an ideal solution for a greenhouse with better insulation, or a somewhat warmer climate, or if you were willing to burn something for heat on the coldest nights.

    The compost will produce lots of steam, and I imagine it will condense on the walls and freeze. I’m not sure what the overall effect of this would be.

    In my climate, I would expect a hungry raccoon or a possum to try to find this way in to explore the compost, but they may be hibernating in your area.

    The blog is always an inspiration, I hope you also keep us up to date on your friends’ renewable energy project.

    • Thanks Matt. Will Allen harvest greens all winter (he is in 5b thanks to Milwaukee’s heat island Effect and the Lake) with giant compost piles in the corners of his green houses and double, even triple, coverage. His piles are hot year long, even outside, but the outside piles are HUGE – like 9′ tall and 20 ft wide 100′ long (the bottoms are all vermicompost only the top foot is active and his worms are in the top 3″).

      To El’s point, he gets most of his growth before November, and merely harvest all winter. But he is abel to keep them alive sans aux. heat.

  5. Rob,
    Cool experiement, thanks for posting on it.

    I was wondering about your snow load out there? Do you worry about collapse?

    Everyone around here seems to take their hoop houses down in winter, last year we saw 4′ of snow in 24 hours, with a full on rain storm the next 24 hours. Imagine, 3′ of ice on the roof, anyone not around to keep that off their hoop houses lost them.

    Also, I wonder if you insulated the bottom of the water tank? Did you use some type of foam board to keep things a bit above ground temp? Without insulation underneath, it won’t take long for the ground to bring that water into equilibrium temperature.

    Did you have an ideal temperature for the water mixture in mind?

  6. Hey Rob, here is what I am trying this winter in my 10 x 20 hoophouse in MT…I, like you, tried to grow a winter crop, but it gets cold here, and no amount of double row covers, etc,etc, etc could keep anything alive after 5 -10 nights of -25 or colder. This winter I have a small flock of layers in the hoophouse and no plants at all. They are on a deep mulch of hay and have a roost post mounted about 4 feet off the ground on one side, over a growing bed. Under the roost I am accumulating all the “played out” hay mulch into a windrow that is as long as the roost post (8 ft). As the days grow shorter, they spend more time on roost, and of course poop more straight down onto this heap of future compost. I incorporate the droppings into the pile as needed and also watering a bit. Hopefully the pile will heat up to not only make compost, but radiate some heat up to the birds on those really cold nights. Come March or so, the birds will go outside, the compost should be ripe, and the planting beds should be in good shape for early crops. Certainly some wishful thinking involved here. We will see how it goes. BTW, hens would love those restaurant scraps.

    • Do you suppose there will be enough browns at the surface to keep ammonia from rising into the hoop house in harmful amounts?

  7. Your readers with late season herb and vegetable gardens may well find that they will grow more than they can use, preserve or give to friends.

    They may want to visit – a site that helps diminish hunger by enabling backyard gardeners to share their crops with neighborhood food pantries.

    The site is free both for the food pantries and the gardeners using it.

    More than 1000 food pantries nationwide are already on it and more are signing up daily.

    It includes preferred delivery times, driving instructions to the pantry as well as (in many cases) information about store bought items also needed by the pantry (for after the growing season). enables people to help their community by reaching into their back yard instead of their back pocket.

    Lastly, if your reader’s community has a food pantry, they should make sure the pantry registers on Its free.

  8. […] spinach.   But Hoop Houses can do more than just grow greens – they can build your soils. A few posts ago I regaled you with my attempts to begin composting in our small Hoop House at our Market […]

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