Keeping unheated greenhouses going in the winter

I have an 8x8 greenhouse that I'd like to use year-round but I don't want to use electricity to heat it. Here in Colorado, it can get pretty cold outside, so I'm trying to harness the sun by having large water containers in it and have even built a cold frame inside the greenhouse. I've had some limited success with growing spinach and beets in the cold frame, but that's about it. Any suggestions?

You can also use row cover "blankets" under your cold frame.  We tend to have good success with carrots and tough greens such as kale, collards, turnips, etc.  What zone are you in?

I also have a 'greenhouse' that I just put back up with some help on the weekend. It's actually a plastic car shelter, but I used it very successfully for years as a season extender for tomatoes and other tender crops. I put it up this year thinking I will use it to overwinter my very small pawpaws that I started in pots from seed this summer, my Meyer lemon (which will be bundled up in remay for the cold weather) and probably my potted figs.

I was wondering how damaging the temperature swings that are likely to occur in an unheated greenhouse are for plants. I've never used it in winter before. It was sunny the day we put it up and was getting pretty hot in there.

Is it worse for the plants to be subject to big temperature swings, or would they be better off taking their chances outside? Like Marisa, I also am going to put some containers of water in there to hold some warmth. I think I will stack some large rocks or bricks along the edges where the plastic meets the ground as those will help to moderate the temperature swings as well. The person I bought the lemon from recommends the old-type Christmas lights on a timer wrapped around the remay-ed lemon to add a little heat at night.

Inside our greenhouse we have a knee-high wall of soda cans painted black & filled with sugar water.   The sugar acts as an antifreeze.  It only works when the day before was clear.

When we have had a cloudy day we set a 5-gallon bucket of hot water in the greenhouse with an instant pot sous vide set to 30°c.  I am sure it voids the warranty on the sous vide, but it keeps the greenhouse above 10°c.  The sous vide runs off a solar battery.

Wesley Kohn wrote:Inside our greenhouse we have a knee-high wall of soda cans painted black & filled with sugar water.   The sugar acts as an antifreeze.  It only works when the day before was clear.

When we have had a cloudy day we set a 5-gallon bucket of hot water in the greenhouse with an instant pot sous vide set to 30°c.  I am sure it voids the warranty on the sous vide, but it keeps the greenhouse above 10°c.  The sous vide runs off a solar battery.

Wesley, that sounds like a clever use of the cans. I had only ever seen them used to build a hot air collector vented directly into a building, but unlike your system those don't hold and re-radiate heat at night obviously.

How do you close up the cans? What concentration of sugar, and do you have to do anything to keep the syrup from going off?

Andrea Locke wrote:

Wesley Kohn wrote:Inside our greenhouse we have a knee-high wall of soda cans painted black & filled with sugar water.   The sugar acts as an antifreeze.  It only works when the day before was clear.

When we have had a cloudy day we set a 5-gallon bucket of hot water in the greenhouse with an instant pot sous vide set to 30°c.  I am sure it voids the warranty on the sous vide, but it keeps the greenhouse above 10°c.  The sous vide runs off a solar battery.

Wesley, that sounds like a clever use of the cans. I had only ever seen them used to build a hot air collector vented directly into a building, but unlike your system those don't hold and re-radiate heat at night obviously.

How do you close up the cans? What concentration of sugar, and do you have to do anything to keep the syrup from going off?

We make the sugar water in a 5-gallon bucket.   We put a 5 pound bag of white sugar in the bucket.   It should taste too sweet to drink but not like syrup.  We originally tried wax plugs but the plugs failed.  They got hard in the cold.   However, we figured out that filling the cans in the fall takes only about 1 hour for the entire wall and not having the cans sealed makes for easy removal in the spring when we don't want the heat in the greenhouse.    The cans are stacked in a wood frame with chicken wire on both sides.   Face the drinking hole inward and use a male cat catheter on a large syringe and squirt until water comes out to refill the cans.  In the spring just  tip the entire wall on its side and let the water drain out.   It is very light at that point and can be moved to store over the summer.

What is the greenhouse like?

I think that for cold climates, having insulated walls on the non-sun sides makes a huge difference. Especially (for northern hemisphere folks) the north wall.

And maybe even having a nice large windbreak (ideally including a berm) protecting the north side from the cold winds.

Are you planting in the soil or in raised beds or containers? If you insulate the inside soil from the outside soil it will act as a big thermal mass.

Do you use the water from the containers at all or just continually keep the water in them?

It might be a bit much for 8x8, but another option to consider would be a rocket mass heater. Here's a link to some plans from Ernie and Erica:

https://permies.com/t/64464f95/Greenhouse-Rocket-Mass-Heater-Plans

Having something reflective (ideally a body of water) on the sun side of the greenhouse can allow extra light (and heat) to bounce in.

First off remember that water barrels etc only work if they get warm enough to completely melt out even in your coldest weather.  Many people trying them find that the don't because the heat loss is simply too great.  Day one of cold weather the barrel half freeze and the next day 1/4 of it melts.  Day 2 3/4 is frozen and 1/4 melts.  ... Day 4 the plants freeze.  Or some similar pattern.  So you either need to be able in some form to melt them out or remove them and bring in fresh unfrozen.

Beyond that the first major step is insulation of the sides you don't get sun gain from.   It might be simply the north wall or the north wall and roof.  Or it might be more.  

Second trick would be green house inside green house as layers.  Each layer gets you a tiny bit more.  Each layer is supposed to get you roughly one zone worth of protection.  If you can manually or automatically insulate the inner layers at night it helps more.  So you might have a wall of water inside a cold frame inside a light plastic greenhouse inside your main green house and at night you might throw a wool blanket and a space blanket over the cold frame.

Next you probably want to see if you can set up a heat source.   Geothermal or compost heat are lowest human involvement.  A convective loop out of either done properly might give you enough but both would be improved with a small fan or small pump or both.  Look up earth battery, earth banking and earth tubes for geothermal.  Basically it is 4" or 6" drain pipe buried at a depth that the soil temperature is stable.  If you put it in on grade you should be able to get it to run convectively at night.  To help with recharge put a window screen active thermal panel in to act as a fan to pull heat down.  For compost heat you may be able to do it inside the greenhouse.  Cold frames and greenhouses are heated with it clear into Alaska.  Or you may have an outside pile.  It might be as easy as wrapping the greenhouse in a compost pile or it might involve water tubes to carry the heat in.

I actually located the greenhouse on the south side of my house, so it is protected from the north wind. And I had made a homemade "solar panel" out of empty pop cans painted black in a frame, but it only helps in daylight hours. I also put a mylar emergency blanket on the north wall to reflect heat/light into the greenhouse better. Again, the biggest issue is at night when the temperatures plummet and there is no sunlight/warmth to capture. Thanks for all of the suggestions though!

Marisa Majemu wrote:I actually located the greenhouse on the south side of my house, so it is protected from the north wind. And I had made a homemade "solar panel" out of empty pop cans painted black in a frame, but it only helps in daylight hours. I also put a mylar emergency blanket on the north wall to reflect heat/light into the greenhouse better. Again, the biggest issue is at night when the temperatures plummet and there is no sunlight/warmth to capture. Thanks for all of the suggestions though!

Empty cans don't have sufficient mass.   Filling the cans gives them the mass they need to supply heat.   We live in Nova Scotia and have a harsh winter.  The cans work as long as the sun was out during the day before.

I also have a greenhouse attached to the south side of my house, in the high desert. I get leafy greens and flowers all winter, and love spending time in there in winter, to bask in the greenery that is absent outdoors, admire new seedlings or blooms, pinch some aphids, search for a weed or two to pull, eat an unseasonable asparagus spear that popped up, browse on the leafies...

I just posted about it on permies a couple of days ago.

I was inspired by Eliot Coleman's two books The Winter Harvest Handbook and Four Season Harvest where he tells how he is able to harvest fresh vegetables all winter in Maine, using cold frames and unheated greenhouses.

At the school I work at nearby, we tested black-painted bottles of water and other things of that type, but didn't find it made a major difference.

In my experience, things that don't mind nights below freezing for a few months include:
Lots of leafy greens: lettuce, spinach, arugula, kale, mustard greens, chard, claytonia, any of the various mustard greens or Asian greens offered by seed companies.
Many herbs: parsley, dill, fennel, rosemary, oregano, thyme, chives, garlic chives, mint, and to a limited extent cilantro/coriander.
Cold tolerant root vegetables: beets, carrots, turnips, radishes. Coleman's advice to plant Napoli carrots in September and eat them in February (almost like a sweet fruit) has worked out idyllically for me (the roads to our region close for the winter, so we can't get any fresh fruit from the market all winter, only when somebody brings them by air, using up their baggage allowance).

What can't be left out in the greenhouse or it'll get frozen to death are anything very frost tender, like basil, tomatoes and the whole nightshade family, cukes and the whole curcubit family, and very tropical plants, like my curry leaf plant, and the houseplants that my friends keep in my solar-heated house for the winter.

Meyer lemon is supposedly pretty frost tolerant, more so than most other citrus, so I think the tree won't get killed by nights below freezing, but I don't know anything about its fruiting cycle or season, and maybe the fruits could get zapped. Maybe wrapping it or strings of lights would be enough. I'm planning to try Meyer lemon in my greenhouse. It's also supposedly a naturally dwarfed tree, so it may fit in my greenhouse better than the random lemon or lime trees I have currently outgrowing buckets that have to be hauled in and out. I'm concerned about having a tree in the greenhouse sporting inch-long thorns, though... Hmm... Not sure.

It is very important to have a way to ventilate excess heat out of your greenhouse. I have a small window at one end, and a door at the other end, and I open one or both when it's getting too hot in the greenhouse. A lot of plants suffer if they get too hot, or can even be killed. I completely remove the plastic glazing from the greenhouse for summer, and attach it from mid-October to early May.

Has anyone considered the idea of simply buying regular Coke and not opening the cans? Seems a cheap way to get sugar water in pre-vacuum/canned storage.

Interested in this subject as well.

I have used 2-liter soda bottles in my cold frame to provide additional heat during cold nights, but I also cover the frame with an old insulated bedspread.  I've never checked the temperatures so don't really know how much either helps.  I do have one of the black plastic preformed ponds that roughly measures 2' x 4' and have considered putting it my next greenhouse build.  The greenhouse will probably be 8' x 10' so I'm wondering if this would be of any benefit?

I have a 12 by 24 foot high tunnel.   I keep about 200 gallons of water in it.  I also have raised beds with the equivalent of 11 bales of composting  straw.  This gives me a night time temperature gain of about 20 degrees. I do have an lp heater in it, but it is rarely cost effective to use.  I have recently installed a small wood stove.  So far, so good regarding that, but it has not been seriously tested. I have Hope's of it getting me through a winter.

We had a small wood stove in our old greenhouse and while it worked good it ended up being a pain because of tending to it every hour or two during the night.  Tried a kerosene heater for a few days but that got expensive quick.