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Winter Growing: Heating Greenhouses

Keep your winter greenhouse productive with these heating and heat-conserving ideas.

Winter GreenhouseGreenhouses are wonderful places, especially in the spring when benches are filled with brilliant green starts, and in the summer, its doors and roof vents propped open, with cucumbers trailing from the ceiling and tomatoes ready for picking. But in winter? Not so much. Overwintering herbs and potted plants cluster together for warmth. A few brown, leafless cucumber vines hang from an overhead trellis. Kale and spinach are over-picked and the seeds you planted have yet to sprout.

It’s a winter-time fact in most parts of the country: greenhouses, even those that might be attached to the house or garage, need some kind of heat source (of course, supplying appropriate light is equally important). My first, only partially realized, greenhouse up in the rain forests of Washington’s Olympic Peninsula needed heating just to keep the humidity down. But in most parts of the country, cold is the problem. You may have built your greenhouse with visions of supplying your family fresh, year-’round greens. But winter growth and germination are difficult when soil temperatures seldom climb out of the low 50 degree level. Sure you can use a heating mat to encourage germination. But even the hardiest green grows slowly — very slowly — when nighttime air temperatures plunge.

The germination mat is one kind of way to bring the temperatures you need to your greenhouse. There are as many ways of heating your greenhouse as there are greenhouses, and some of the new energy-conscious heating techniques (fuel is expensive!) are promising if not proven.

Whether you’re trying to grow food year-round or bring crops to market, we’re here to help. From nursery pots and potting soils to fertilizers and plant supports, Planet Natural has what you’re looking for!

One fuel-avoiding, sustainable-friendly method is to build a trench down the center of your greenhouse and, after covering it with palettes or some cobbled walk-way, make compost in it. This might be limited to a small hole in the center of a hobby-sized greenhouse. Even at that, the compost will help moderate temperatures in the greenhouse and you’ll always have a ready supply of garden gold. And the daytime temperatures in the greenhouse should encourage your compost to heat up. Click on our composting guide to learn more.

Now. . . full disclosure . . . we haven’t tried this. Our greenhouse was attached to the south facing side of the house in an attempt to get what little sun we had to give us some solar-generated temperature gain. And it worked, a bit, even on some cloudy days, the concrete grow box and stone walkway serving as heat sinks, giving off warmth to the bathroom and the kitchen porch at night. But an open pile of compost situated (technically) inside the house wasn’t considered to be a good idea.

Another way of creating heat sinks that will absorb energy during the daylight hours and give it up slowly in the cold dark, is to place 55 gallon barrels (or whatever is available and convenient) in corners and other practical locations in the greenhouse. They should be painted black for maximum solar gain. Even buckets of water in a hobby-sized greenhouse, will moderate temperatures just enough to make a degree or two difference, a difference that might be critical. We know a guy who wrapped black garbage bags around packaged tube sand and laid those out in his small house. The bags slowly disappeared as he and his son needed more weight in the back of their pickups.

Electric room heaters are the easiest and probably most popular way to heat a winter greenhouse overnight. Be sure that you follow all safety instructions and makes sure your heater is stable and away from any flammable material. Also take care if you’re running an extension cord out to your greenhouse. Make sure all connections, especially those inside the house, are snug.

Heat circulation is important when using an electric heater. Moving the warm air around will prevent hot spots (and their contrasting cold spots) as well in reducing condensation that heating will encourage. Some heaters have built-in fans, some need additional circulation.

Enjoy your garden longer! DeWitt’s 1.5 oz. Frost Fabric (N-Sulate) is a medium weight, permeable, UV treated cloth designed to protect plants from frost, cold and freezing temperatures. Protects tender plants by raising the temperature beneath the fabric by 6-8°F.

We’ve been surprised at the number of greenhouse operators using wood heat to warm their buildings. With propane becoming more and more expensive, wood and pellet stoves, again with provided circulation, are operating effectively and more cheaply than other fuels. Large, commercial-sized greenhouses are finding wood a viable alternative to expensive gas and petroleum products. When installing a wood stove in the green house be sure to follow all your local code requirements. Stand alone pellet stoves are especially easy to load and operate and most come with some kind of temperature control. Some have blowers to circulate heat.

I don’t think I would put a wood stove in a plastic covered greenhouse. Stovepipes can get very hot and the risk of melting or igniting Visqueen and other plastic covers seems great short a stove vented out through a masonry foundation or some other such careful planning. I had hoped to put a small, slow-burning wood stove in my attached glass greenhouse, both to heat the growing things and the rooms on that side of the house; one of the unrealized dreams in my green house.

What about a plastic covered greenhouse? Insulation is a great way to conserve heat without expending fuels. I’ve seen it recommended that those with Visqueen covered houses line the inside of their plastic with bubble wrap. Now,again, I can’t say we’ve tried this but it makes perfect sense that the bubble wrap, with its large bubbles of air, would preserve indoor heat while letting sunlight through. Hmmm…anybody else try this or have other ideas on heating greenhouses? You know we’d love to hear about it. In the meantime, here’s a guide to heating greenhouses from those resourceful, greenhouse-happy British. Notice they put the bubble wrap idea first. If you don’t have a green house and the idea seems impractical for whatever reason . . . how about a hot box cold frame?

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39 Responses to “Winter Growing: Heating Greenhouses”

  1. Kimberly on December 28th, 2013 at 9:57 am #

    A rocket mass heater is a great option for a greenhouse. The rocket stove burns hot and fast, with little or no emission. It then heats the mass which generates heat for up to 12 hours. One small fire in the evening would keep even a large greenhouse warm all night. Imagine a small firebox in one end and the mass portion extending as a platform down the length of one wall. It could be used to set seedling mats on to germinate, and plants like tomatoes that need more warmth. sorry I don’t have a link to any pictures, but I have seen one being built in WA state.

  2. Tammi on December 28th, 2013 at 12:23 pm #

    We use a small wood burning stove in our hoophouse and it works wonderfully! We have a gravel floor but I’m considering digging a pit and putting a small compost pile in the center for added heat.

  3. Pat G on December 28th, 2013 at 2:18 pm #

    With regard to your bubble wrap for warming the greenhouse. When the power went out here, I was concerned for our tropical fish which must remain between 72-80 degrees. I wrapped their tank in a single layer of bubble wrap and placed a towel over all. In the morning the house was 52 degrees, but the fish tank was still at 72 where it had been.

  4. Ken Frick on December 28th, 2013 at 7:22 pm #

    What about using hot water heat? As a boiler system for the gren house for heat? Moisture & Heat combined. Baseboard heating registers?.

  5. Lillian McCracken on January 3rd, 2014 at 10:46 am #

    With night time temperatures double digits below zero & day time temperatures that stay below freezing for several months our pit greenhouse requires supplemental heating for nearly 3 months most years. We have a fan system that collects the heated air in perforated pipes buried in the earthen floor which acts as mass to store some warmth which is later recirculated. The one advantage we have at 8000 ft altitude in the high desert is plenty of sunlight most days. Cool weather leafy green crops & sunflower sprouts seem to thrive in this environment.

    • William Carr on March 10th, 2014 at 4:46 pm #

      A Pit Greenhouse was the design I was considering; with a Soap Bubble insulated roof.

      Do you have a website showing your build with the perforated drainage pipe heat sink system?

    • Amjad on November 21st, 2015 at 4:20 pm #

      I am in the same situation, my first winter in Toronto. just built a 12 x 8ft Palram essence greenhouse, using poly-carbonate rigid twin sheets. I don’t think it can provide any effective insulation when outdoor temperature drops much below zero centigrade. There is plenty sunshine during the day, but nights are very cold. I plan to plant carrots and lettuce.. but don’t have much experience. I wonder if any one could help.
      I am in Mississauga, Ontario Canada.

      • Marta on March 12th, 2017 at 11:56 am #


        Watch the above YouTube from “one yard revolution”

        He grows all year round in his greenhouse without supplemental heat. He’s in zone 5, very similar to Mississauga, ON Canada

      • Gavin on July 21st, 2017 at 5:21 am #


        Just curios to how well your greenhouse is going? I am in Brampton and curious in getting a greenhouse.

  6. Anne Kiley on January 3rd, 2014 at 11:27 am #

    I have a Northern Star freestanding 10×20 greenhouse in Western New York, covered with rigid, double-walled plastic panels. The first few years after we put it up we tried to heat and insulate. Electric heat was prohibitive and the bubble wrap insulation we tried on the north wall kept falling down because the tape froze. We also tried barrels of water, and they froze. Here’s what we do now: we plant the greenhouse beds with kale, parsley, chard and escarole… especially kale. We use 3×3 raised beds… and cover the beds with the leftover bubble wrap insulation, although there is really no need to cover the kale at all. If you want things to grow in the greenhouse in the winter, it’s not the cold you have to be concerned about… it’s the light. The days are just too short for anything to grow very much at all, so you need to get your plants to a decent size so that they will hold through December and January. I harvest a little from the greenhouse during those months, but mostly rely on sprouts and shoots I grow in the house.

    • angela carrera on July 8th, 2016 at 3:07 pm #

      I wonder if your raised beds sit on the ground? earth? or on the floor of your greenhouse. my floor is made of pressure treated wood, would have loved to cut out a piece of it, put it on hinges, so in the winter I could open it up and compost it or plant in it. so new at this. learning every day!! thanks!!!

  7. Cliff Williams on January 3rd, 2014 at 12:11 pm #

    This is my passion!!! Getting this right, that is growing year round, is the key to the ultimate in health. I have recently published a book that features a winter greenhouse at the center of a new sustainable lifestyle. 5 chapters are devoted to building a winter greenhouse. My first winter greenhouse was also in the Pacific Northwest, and it can be seen at http://youtu.be/Xsln4I_2_W4 , or a link from my website UrbanCrofting.com You have a few more steps to go though, for germinating in the winter I use a greenhouse in a greenhouse. I build a small temporary greenhouse inside that is just big enough for half a dozen flats, this can be heated with a small ceramic heater. As far as the heat, I like a wood stove, but the problem lies more with minimizing the heat loss.

  8. Cathy on January 5th, 2014 at 8:52 am #

    We have used bubble wrap for several years. It is placed on the outside and protects the cover as well as helps with the insulation. You can get pool covers that you can piece together or buy one huge one. I look on ebay to compare prices and shipping and you can get several weights as well. Also the use of row cover can get you thru some cold nights.

  9. Electric Greenhouse Heater on October 20th, 2014 at 8:03 am #

    I use bubble wrap every year and I have found that it helps regulate the temperature somewhat. On the especially colder days and nights I do have an electric greenhouse heater but I have had to use that a lot less since I discovered the bubble wrap trick.

    I have found the larger the bubbles, the quicker the heat is lost, so smaller bubbles seem to work best.

    • Anonymous on January 13th, 2015 at 3:09 pm #

      Where are you located? I have 10 55-gallon, black, drums filled with water for
      Temp moderation but paying too much $$ per month for electricity here in SE
      michigan….. bonnie

      • Ross Murphy on November 7th, 2015 at 8:16 pm #

        I am building a 9 X 9 X 33 feet tunnel greenhouse, low end about seven inches below high end. My greenhouse is made of eight 20-foot cattle panels, which cost $21 each. I thought independently of buying small bubble wrap, then covering it with vinyl. I spoke two days ago to a salesman from a bubble wrap manufacturer. He told me that it wasn’t a good idea, probably so, because it would cost him a sale.

        As for the drums filled with water, I have about six of these, not used yet. My idea is to stand these on the sunny side of the house, painted black. By doing so I could lay all-weather plywood across the tops to use for planting shelves; heat would rise to warm them. In addition I had planned to run a PVC pipe into the bottom of the first barrel, out the top and into the bottom of the next one, until I had all six connected. From the top of the last, I will run a PVC pipe to the floor, into a four-inch deep trench. Then continue the trench back and forth until I had about six runs of it. This will heat the floor with the heat rising to heat the whole house. From the end, I would connect it to my wood fired water heater.

        I am west of Kansas City. It used to get as cold here as in southern Michigan, but with global warming, our winter have become milder. For the past three years, we have picked ripe red tomatoes on the 15th of November. We are doing so now, including green beans, four kinds of hot and sweet peppers, cilantro and Rosemary, and our zinnias. cosmos, Vero Beach Daisies and borrage are still attracting bees to their blossoms. My five bee hives are thriving, and with milder winters, I no longer have to wrap the hives with insulation. It used to be we had snow by this time.

        So tell me, how are your water-filled drums doing? Please tell me all you can.
        By the way, I am also a writer, having sold 185+ free-lance magazine articles over the years. If you want to see one published in May, search google for “courage and persistence.”

        Ross Murphy, Shawnee, Kansas

  10. Sheri on November 5th, 2014 at 8:32 am #

    I tried bubble wrap last year but it was a ton of work and seemed expensive for something I would have to buy each year. Also I used several kinds of tape to put it up and later had a terrible problem with ladybugs getting stuck on the tape anywhere the stickiness was exposed, they do get everywhere! I use three small electric heaters, each one set to come on progressively as it gets colder. There is an insulation product which is bubble wrap with silver Mylar on both sides and I use that to line the 18 inch high cinder block foundation walls. I have two 80 gallon fish ponds for thermal mass, plus the exposed floor is paved with stone. This year instead of the bubble wrap I bought double walled panels from a greenhouse supply website and am just installing them over the existing panels. It is expensive, but they should last as long as the greenhouse and save me money in the end.

  11. KJMClark on November 5th, 2014 at 10:48 am #

    The problem with putting compost in the hoophouse is the source of nitrogen. Can’t be manure, since that’s putting raw manure too close to your crops. If it’s not raw manure, it’s not likely to heat up all that much. So that means either finding a source of green legume in the dead of winter, or adding something expensive like blood meal.

    We only have solar power in our hoophouse, which is pretty slim (the solar) in the winter in SE Michigan. So we put a 4″ corrugated pipe in a trench outside our hoophouse, with a thick mound of horse manure on top of it. I hooked up a small computer fan to an Arduino microcontroller. The controller only turned the fan on when the temperatures dropped to just above freezing. We had two low tunnels inside our hoophouse, and ran corrugated pipe from the fan to the two tunnels.

    It all worked pretty well. The air coming from the pipe was usually a few degrees warmer than the air in the hoophouse on the really cold nights.

    However, come the thaw, and that trench we had put the pipe into became a stream flowing into our hoophouse! Destroyed the fan, of course, and gave us some bailing duties as well for a few weeks. This winter we’ll probably do the same thing, but without the trench part – just lay the pipe on the ground and put the manure on top of it.

    • Ross Murphy on November 7th, 2015 at 8:32 pm #

      I may have just responded to you, but I will continue nevertheless. There is nothing wrong with nitrogen in a greenhouse; it’s harmless. The air you have breathed all your life consists of 72 per cent nitrogen. You may have been thinking of methane, with is what rotting vegetation produces. Too bad you couldn’t put a cap over your compost pit and burn the methane for heat!

      For several years I have used cold frames I made, the tops leaning up against my small barn. The glass tops are contributed double pane sliding glass doors. When the weather report warned of freezing nights, I put pans of burning charcoal inside them, which kept them quite warm until the sun took over. Despite this, they never worked very well.

      I put started plants into these, but they required considerable care. Usually on most early spring days, the solar heat inside had to be watched very carefully. If I failed to open them at the proper time, the plants would burn up. This happened often because I had so many other springtime chores to do that I couldn’t watch their thermometers minute by minute. So, I stopped using them.

  12. kAi on December 3rd, 2014 at 3:02 am #

    Eliot Coleman and his un-heated/heated greenhouse commercial crops. Business? Here are more than a few ideas if you are in the cold climate. This is terribly interesting. 2 hours in duration. Please leave comments and get a thread going.


  13. sammy on May 17th, 2015 at 7:12 pm #

    I live in Georgia, so temperatures around here don’t usually get that cold. I was just wondering if there was any simple way to heat greenhouses without using heaters, because that’s not really an option for my buget. if any of you have any advice for building a small greenhouse to grow medical herbs (not crops like corn and tomatoes) it would be greatly appreciated. heating, ventilation, or just building the greenhouse in general would be amazing.

  14. Andronetta Douglass on June 12th, 2015 at 3:13 pm #

    Has anyone tried solar water heaters with radiant floor heat?

    • Steve Yawn on January 21st, 2016 at 1:08 pm #

      I have a radiant floor heat system in my house using a propane boiler. I have a 10X20 greenhouse not connected to the house. I am going to pipe the exhaust (waste) heat from the boiler to a floor vent inside the greenhouse to provide heat and carbon dioxide. I am aware of the hazards involved regarding carbon monoxide and CO2 possible poisoning. The greenhouse is not sealed tight so there should be plenty of exchange of air. I will put in a CO alarm and the greenhouse has a ventilator fan for heat exhaust in the summer that I can ventilate when I am working inside.
      How does this plan sound?

    • Greg on March 3rd, 2016 at 9:37 am #

      Now days you would be better with a PV system, running an electric water heater.

  15. Bev Spehar on September 21st, 2015 at 7:19 am #

    I live in the Upper Peninsula, and we get some cold winter days and nights. I have a greenhouse and we were thinking about heating it with hot water heat from our sauna its close by and we can run the peke pipe from there, but I was wondering if we would run the pipe in the soil or on the bottom of the box? We would be heating with wood then, does anyone have any suggestions for me? thanks

  16. Abby Danner on September 22nd, 2015 at 8:20 pm #

    We’re going to be setting up a hoop house in central IN. I’m hoping to use a couple of techniques. Joel Salatin’s compost blanket, a combo of hot manure & wood chips in layers, a multiple-barrel heat sink, & possibly, the soda can solar heater. We’re going to be doing aquaponics in 250 gal tanks, with catfish, so I’m hoping the tanks act a little like sinks themselves. Also thinking about Will Allen’s compost corners, which he uses in Milwaukee winters.

  17. craig on June 24th, 2016 at 6:51 pm #

    Here in Utah alot of us have basements, try using these window wells as greenhouses. Outside lay a few 2×4 across the well and cover and seal with plastic then easy access the well from inside your home to water. Leave the window open a few inches allowing the heat inside your home to heat the well. This is a cheap way of using resources you already have.

    • Anonymous on April 13th, 2019 at 9:01 am #

      The IRC doesn’t allow use of a cover that requires more force to remove than what’s required to operate the window

  18. Mike on October 15th, 2016 at 7:12 am #

    Has anyone ever tried heating rocks (made for this purpose) and carrying each evening into a small greenhouse (in central ny)?

    We located the small kit greenhouse itself within our garage, which is unheated. Or, how about throwing sheets of clear plastic over the entire greenhouse?

  19. Cathy on October 28th, 2016 at 2:38 pm #

    Kimberly, do you think concrete cinder blocks would provide enough mass? I am in zone 6 and have access to rocket stoves, but don’t want a permanent mass. I am just trying to get plants started about 2 months early to put in the hoop house, not necessarily grow year round. Could I run the pipe through the cinder block holes?

  20. Marc Allyn on December 4th, 2016 at 11:35 am #

    I have a 300gal aquaponics system with 3ea 2ft x 10 ft x 12″ deep beds. I cycle water through each one at a time so I don’t empty my pond. I have 30 hybrid bluegill. I put 2 – 55 gallon drums of water under each bed and have 5 drums upright around the pond. This water is separate from pond and is pumped with a 12vdc pump through a multi tube 2ft x 20ft solar pool heater. The thermal mass is not enough if there are more than 2 days of rain/cloudy. I installed a Chinese propane instant hot water heater in the loop but it will only stay on for 10 min (safety circuit). This really helps because the propane is heating the air and also heating the water barrels. The heater is only $68 dollars on Ebay. Now I’m going to try to heat up the pond by using a radiant heat lamp on the rocks in the pond. I really feel like that should do it. My tomatoes are 24″ tall and have flowers. Brussel sprouts and broccoli are getting there. Sugar snap peas shoosting. It breaks my heart to think I may loose it all to several days of below 20 temps. Trouble with thermal mass is when it looses its heat. Getting the heat back. By the way it is all enclosed in a homemade green house all powered by solar (with grid backup). Half of it is insulated. Sooo, thermal mass, thermal mass and thermal mass… then figure out a method to heat up the mass during the day. Radiant heat will work better than latent heat (heating air). I like the wood heater idea. Love the brainstorming on this site.

  21. andy anderson on November 18th, 2017 at 8:19 am #

    Comments are interesting. I have a 10′ x 7′ x 18′ greenhouse with thermo-glass plates [10] and for the base, inside the greenhouse, 18 bales of hay for lower insulation. My planters are on top of the hay bales and are at waist level for easy planting, weeding and watering. I heat my greenhouse with recycled candles, donated to me by churches, senior centers and friends. I have a small wood stove outside and boil the old candles down and pour the wax into 10 oz soup cans with a new wick. I had a metal shop build me a 12′ x 12′ x 18′ galvanized stove with air holes around the base — hole on top for a 3″ stove pipe which vents outside. The stove pipe gets very warm, but not hot enough to burn the wood exit on the wall. The real test came last week Nov 15 [NE PA] when the night time temp dropped to 15 degrees. I had placed ten 10oz candle cans inside the stove. They burned for 4 days maintaining 42 degrees minimum. To use candles without an effective filter system would result in the candle fumes covering the green leaves, thus choking the plant. The 4′, 3 ” galvanized pipe heats the entire greenhouse.
    Photos are available if interested.

    • Vicky on February 24th, 2020 at 9:46 pm #

      I live in NC and the winters are just cold enough, long enough, to kill the lemon trees I am trying to grow. I have an all electric home which is too expensive to heat, so I am looking for a way to heat a greenhouse using other than gas or electric. Your candle idea is the best that I’ve seen in researching heating sources for greenhouses. Could you please send photos for your galvanized stove and pipe system?

      Thank you!

  22. Audrey Estev on September 25th, 2018 at 8:19 am #

    I agree with you. I saw that, some growers replace their plastic every year just to get a few percent higher light level when growing during the short days of winter.

  23. Mark on October 26th, 2018 at 2:14 am #

    This year I will be implementing the bubble wrap. I’m working on a heat mat, floor heating project. I have purchased a piece of foam that fits on top of my 8′ tables. I will be cutting strips of foam just short of the length of the large piece of foam. I will glue these pieces on the big one creating channel. I will run incandescent rope lights back and forth through these channels. A temperature gauge will tell me what needs to go on top. I suspect a thinner layer of the insulation will have to go on top. The flat of plants will sit on top. My 8′ table fits perfectly in a 10′ walk-in greenhouse. Along with the bubble wrap, I suspect it will be nice and toasty. I live in Victoria BC, which is zone 9, but the killer is the dampness and the lack of sunlight. When I lived in MN, we could start Fuchsias in February and have them ready for spring, with all the sunny days. In Victoria, Fuchsia plants are started in November. The greenhouse has side vents, but I will probably have to have a fan running all the time. I am planning on overwintering my succulents that are not being used for propagation, which takes a full room of the house. I am excited to see how this works. Just earlier today, I said I was going to bring all the plants inside. I just found the Greenhouse that is perfect for my space.

  24. robertsmith on November 1st, 2018 at 1:37 am #

    Thanks for sharing your article on greenhouse heating. It is really informative. Electric heat was prohibitive and the bubble wrap insulation we tried on the north wall kept falling down because the tape froze. We also tried barrels of water, and they froze. Here’s what we do now: we plant the greenhouse beds with kale, parsley, chard and escarole — especially kale. We use 3×3 raised beds and cover the beds with the leftover bubble wrap insulation, although there is really no need to cover the kale at all.

  25. Daniel T Hannon on February 11th, 2019 at 4:35 pm #

    I have a small (approx 8×14) twin wall polycarbonate greenhouse that I would some day like to use in the winter.

    So…. I salvaged some Slant Fin, an old 20 lb propane tank and built a small wood burning stove surrounded by the slant fin, enclosed in a small metal stud shed (approx 3x3x4), also surrounded by salvaged sheet metal. I piped w/ 3/4 type M out of the top of shed, filled the fins with water, lit the stove and WOW, boiling hot water bubbled out in minutes.

    The plan is to leave the shed outside, pipe into the greenhouse, then back into elevated slant fin to finally a large elevated reservoir of H2O.

    Note: you’ll need to fill the reservoir occasionally and leave the end piping open to prevent the need for a pressure relief valve. Not sure how often I’ll need to fill the stove w/ wood but will find out soon.

    wish me luck.
    Fun project.

    • Daniel T Hannon on February 11th, 2019 at 4:40 pm #

      I forgot to mention that I pitched the slant fin downward and installed two drains so as not to freeze the piping should the stove not be in use for whatever reason.

  26. Clay on December 12th, 2019 at 12:55 pm #

    I am surprised that nobody has said much about geo-thermal greenhouse. Everyone has 45-55 degree heat below their greenhouse for free. Mine is 8 feet down and a trench works good for the pipes. Pull some air through it to collect the heat and push it into the greenhouse. No matter what the outside temp is. It will greatly decrease your energy cost no matter what you use. I have used about $16 in propane in 4 months buy having my greenhouse half underground and using geo-thermal air for heat. Look into it.

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