I built a standard Eliot Coleman style greenhouse: a simple plastic covered hoop structure, without insulation or thermal mass. However, Colorado is not Maine, and when the outdoor temperature changed from sixty degrees and sunny one day to below freezing for the next three days (with lows below zero) even kale and Swiss chard plants died.
So, I need some way to stabilize the temperature. Reducing the humidity would also be nice. I looked at a climate battery, which is a system of pipes every two feet underground in several layers, the deepest being at about two feet, and a big fan to run hot air down there during the day.
But the greenhouse is already built, the perimeter was not insulated, and the soil is full of rubble. The group running the project does not have much money. So a standard climate battery is out.
What if I built a whole bunch of above ground planters, and used small computer fans to move air into each one through perforated tubing? Would this help any?
I actually need to build a bunch of wicking planters anyway. The soil in the greenhouse is infested with poison hemlock, so growing salad greens in ground is not possible. (It even came back through sheet mulch.) And we don't have the ability to water every day. Wicking planters will solve these problems, and since they are movable, we can keep warm weather crops growing in them till frost, and then move in new planters which we have started cool weather crops in outside. (Otherwise there is not time or space to start cool weather things, with our abrupt autumn.)
So if these wicking planters could be climate batteries as well that would be great. And I was worried about more freeze thaw in the above ground containers, too. This would solve that.
Eliot Coleman uses a second layer over crops in the greenhouse when the outdoors temperature is going to go that low. He uses floating row covers, which are much easier in the greenhouse than they are outdoors, because you don't have to anchor them against wind damage. Here, we've sometimes used old greenhouse plastic for an extra layer loosely propped over the beds in the greenhouse during the coldest part of winter, and sometimes when we're ambitious we cover the beds in the evening and open them every morning.
I think trying to heat a huge mass such as earth garden beds using forced hot air sounds impossible. Sisyphus.
About moisture in the greenhouse, I think I read recently in Coleman's book, that due to the way the hoophouses etc retain all the moisture, he used to not water at all in winter, for about 3 months. But he got aphid problems occasionally and read that that can be caused by nitrogen excess causing juicy growth, so he decided to water deeply once a week in order to flush the nitrogen down.
But that says to me that you could try watering a whole lot less frequently than you currently do, and when you do water, only water one section of the greenhouse on a given day, so as not to raise the humidity too much.
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.
Climate batteries (Pushing hot greenhouse air through the soil) have been used in quite a few greenhouses. They work not so much by direct transfer of heat from the air to the soil, but by condensation of moisture in the soil, thus releasing all the heat used to evaporate it in the first place. In Aspen Colorado there is a greenhouse high in the mountains that depends almost entirely on a climate battery, and they can grow bananas and other tropicals. The temperature there drops to -30 F regularly.
But, you might be right that it would not work in above ground planters.
Also, I think the plants were killed by thermal shock more then anything else. Most places don't get our wild temperature swings. It is not unusual to have 60 degrees Fahrenheit and -10 degrees Fahrenheit in the same week, and sometimes even in the same day. Row covers would probably help with that, but still, the plants don't get a chance to harden off. Colorado tends to have two season, summer and winter, and they alternate for a while in the Spring and Fall.
Gilbert, I think the question is whether the wicking beds will have enough mass to hold enough heat to buffer the swings for your plants. Adding the row covers when the cold snaps hit might be the trick. A little extra warmth, contained right around the plants.
And yeah, I think CRMPI has pretty well documented the performance of their version of passive annualized heating
I have a hoop house in similar or harsher growing conditions, and I am wondering if this might not be solvable without a whole lot of intervention.
Our hoop house now: roll up sides, sides not secured very well. (we might get to it, much more relaxed this year), already down to 8 deg
Our hoop house last year: much tighter closure, but still open during the day because we keep chickens in one end, hit -10 last year (we did throw some blankets over the veg that night).
Before I had a hoop house: 18" low tunnels made from some Home Depot visqueen and some 9 gauge wire, held together with clothespins (this super ghetto, totally not-snow-worthy system was in SLC, max low one year was 0. No venting, in fact the coldest year I ignored them from Christmas until March, when I was surprised by so much lettuce I gave it away by the sackful to neighbors.
In all of these systems, chard and kale thrive. The problem, if any, is that it gets too hot on mild days (+80 deg). The arugula started to flower in November this year! I haven't got any fabric for a second layer, though I keep meaning to get some.
What I am noticing makes the most differences: 1) variety and species selection. This year's kale isn't so robust (white and red Russian kale. I can't remember the best one just yet, will add it later to this thread when I find the notes). Black-seeded Simpson lettuce goes forever. Tatsoi is unkillable, Joi Choi a little less so but they are still looking good today. I haven't tested enough chard to know whether the Bright Lights is more tender than the Fordhook, but I think it might be. Bulls Blood beet is extremely tolerant, we use that for color in the salads. Most of the radishes got pithy, but there was some really hot days in October. The harukai turnips are still doing good but I don't expect them to go much longer.Right now they are so sweet I can eat them like an apple. The arugula is getting a little spicy but shows no damage.
Variable #2 is equally important: when you plant. Last year I planted early October, around our first frost date and that was too late. This year, I planted Labor Day weekend. I started some of the transplants around August 10. I could have started earlier, like August 1. I especially wish I'd gotten carrots in earlier, I suspect they will just linger at pencil size all winter then bolt. Eliot says the plants should be fully grown when they hit the really cold temperatures, and just go into a hiatus. They do keep growing (as observed in the low tunnels I abandoned one year). So maybe to 80% of full size is a good target for first frost. I just am not going to grow cute mesclun baby greens in the winter. I expect them to be robust, full sized and strongly textured but sweeter than the same variety grown in the spring salad mix. For us, I think it really helps that the roots have a chance to grow a little deeper.
Variable #3 is also critical: it's going to really heat up in there on sunny days in our climate. We don't have Maine's maritime cloud cover. I need to pay attention to vent every day. As soon as it goes above freezing, I'm venting if it's sunny. I don't worry about it going from 0 to 60 outside, but 0 to 85 inside is a problem. I don't care if it stays at 40 all day. If it gets hot, I water. I don't want the plant roots drying out, and they will stay cooler if wet on sunny days. Our native aridity and all that venting will counter some of the excess moisture. I still got aphids last spring, should have rolled up the sides sooner. I need to build a goose proof screen door for one end so I can vent even better. The dang geese have figured out where the salad bar is, so we are keeping that end door shut for now which makes it harder to keep it cool.
I have a neighbor with one of those fancy climate batteries (although that's a new phrase for me) and I'm sure it works fine. But we are getting plenty of productivity without it. There was a learning curve however, especially as we scaled up to the full sized walk-in system. Hope that helps.
Given the credit some give to the use of 55 gallon drums as a heat sink, why not 55 gallon sub irrigated planters?
I was inspired by someone on this site to build one, mine uses a peforated bucket and 4" PVC wrapped in weedbarrier for the water supply area,next time half a 30 gallon drum will be used.
Lots of thermal mass plus each barrel could be covered with more plastic.
Barrels that have a pipe leading into them down to just above water level from the peak of the roof could capture the heat as warm moist air is pumped by the fan down the pipe and hits the waters surface. Extra water over flows to the SIP 's keeping them topped off and their overflow pipes should go from the bottom of their resoivars to the bottom of the condenser barrel.
Wrap the barrels in insulation?
That would prevent direct solar gain.
Maybe foil painted black on one side, reversible for summer?
So, bootleg dehumidifier feeds captured water to to giant SIP 's.
My company has been advising people on climate batteries for several years now. Yes, they are possible above ground as all you really need a huge store of thermal mass. That is more difficult to create, but doable. One of our clients who couldn't excavate below ground built a large insulated box of soil and gravel and ran the tubes through it as his battery. I haven't gotten data / feedback from him yet, but can check.
Note that it's hard to match the heat storage capacity of the soil underground ... there is a lot of it, plus soil underground is stabilized by deep earth temperatures (the ground below frost line stays a warmer average temperature year-round).
Alternatively, people have used large water tanks as a thermal mass source and engineered a heat exchanger that stores excess energy in the water tanks.
Couple other thoughts:
-insulating your soil should be your first step unless it is really impossible. Even with rocky soils, this is usually doable given an insulated 'swedish skirt' foundation. (We just did a blog on this... "Resources" page at ceresgreenhouse.com). This prevents heat loss through the floor
-sorry if you've already mentioned this... have you added standard thermal mass yet? (water in the greenhouse). I would say this should be the second step to extending your season, plus adding additional coverings.
Oh! Forgot to mention... climate batteries are natural dehumidifiers. They take hot humid air and lower the temperature, forcing that humidity to condense. Just make sure you have perforated drain pipe or someway for the water to exit the pipes.