Help on a low cost greenhouse idea

Eliot Coleman has plans for a really simple, cheap greenhouse: hoops of rebar covered with pvc pipe to build the frame, with a covering of greenhouse plastic.

I can get a 200 square foot greenhouse like that built for $158.

The problem is, with that setup, it freezes most cold nights.

That is all right for Eliot's hardy hibernating salad crops.

(By the way, if you haven't read his books, by all means do. They are great.)

I like salad greens. However, I also want to start a whole bunch of seedlings, and haven't been happy with the way house started plants work.

I know tomatoes like a cool head and warm feet. So I was planning on using either a heat mat, or a compost pile inside the greenhouse, and keeping the air at about 45 degrees minimum.

But, when I used an online BTU calculator, I found that I would need about 26,000 BTUs.

A heater with that output costs about two and a half times what the greenhouse costs. And I still need to buy a vent opener.

So I was wondering: If I added some sort of insulation to the north wall, had a compost pile in the greenhouse, and placed a lot of water filled barrels to collect heat, could I get away with NO heater? I have done a lot of reading on solar greenhouses, and I think probably not. (I am in Zone 5, Colorado. We can get some of our lowest temps in the spring.)

Unfortunately, chickens are out.

What if I, in addition to the setup above, ran a small, 5000 BTU heater, and covered my compost pile/ seedling beds with self venting row cover? (I would not be able to get to the greenhouse every day in time to prevent cooked seedlings once the sun comes out. ) The row cover can gives 4 degrees of frost protection, and lets in 85% of the available light. Where we are in Colorado, that is still a lot, probably as much as some places on the east coast. And I have found by trial and error that tomato seedlings are surprisingly tolerant of low light levels. (Try growing tomato seedlings in the cloudy climate of Pennsylvania, behind a window, with no supplementary light. It works, but the plants are a bit spindly.)

So the heater and solar mass ought to keep the main house from freezing on most nights, and the compost and row cover should protect the seedlings on the few really cold nights.

Eliot Coleman uses plastic greenhouses with an inner layer of row cover. They still do freeze, but less often. And that is in Maine, without heater, compost, or solar mass.

What do you all think?

I had the same questions before I build my greenhouse. What I ended up doing was ordering one of these: Handy Home Solar Shed

It has 2x4 studs for the walls and I was able to add 2" rigid foam insulation to all the walls. I then placed 3 55gal steel barrels filled with water inside as a thermal mass. Last winter (I live in Seattle for reference), it was constantly 10 degrees warmer every morning than the outside temps. On the coldest nights I ran an extension cord out to it and plugged in a heading pad. My greens never had a problem and grew all winter long (well not much growth, but they stayed alive).

I do not think it would be possible to make a plastic covered hoop house winterized enough to be a good alternative. After a few years, the cost of running the heater would exceed the extra cost of builduing a more substantial greenhouse that could deal better with the cold winters.

If you wanted to stay cheap, I'd consider a underground or pit greenhouse.

This is taken from another thread:

I heated a greenhouse with the sun. I picked up a 55 gallon plastic drum from a guy for 10 bucks. This was filled with water and an 8 watt fountain pump was added. The pump moved the water through 500 feet of black PEP irrigation tubing which was coiled in the driveway. The pump ran off a timer so that when the sun hit the hose, the pump would start up then stop when the sun went behind the palm trees. The sun hits the hose, warms up the water, which gradually raises the temperature in the barrel. I'd get the tank up to 100-120 most of the time. Come morning it would have cooled to 40-50, but kept the space warm enough. Never lost a single thing to frost. The power to run the pump cost about 12 cents per month.

50 gallons of water at 120 degrees dropping to 50 degrees releases 28k BTU.

Tom OHern wrote:
I do not think it would be possible to make a plastic covered hoop house winterized enough to be a good alternative. After a few years, the cost of running the heater would exceed the extra cost of builduing a more substantial greenhouse that could deal better with the cold winters.

If you wanted to stay cheap, I'd consider a underground or pit greenhouse.

Or you need to figure out a cheap heater -- like the rocket stoves that are well discussed on these forums. If you build a rocket stove out of bricks in the greenhouse and bring a big pot of water up to boiling, that's a lot of BTUs to be released during a cold night. Remember the definition of a BTU, a pound of water releases 180 BTUs going from boiling down to freezing. So to release 26,000 BTU, you need to have about 145 pounds, or about 18 gallons of water. If you have a 55 gallon drum at the boiling point, that's almost 80,000 of stored heat.

Thank you for a lot of great replies!

I should have clarified that I can't build a better greenhouse, because it is uncertain how long I will be on this site. Also, there is a high water table. Otherwise I would do a Walipini.

But thanks again; you really have some great ideas!

we built a greenhouse and were able to keep it above freezing in Longmont on all but the coldest/windiest days.  it held up to the 100 mph winds just yesterday ( i drove by to see, as we sold the property about 4 months ago.
we used a slope roof kit from here:
https://www.ebay.com/itm/153831758508?hash=item23d1165aac:g:HKMAAOSwWQdfRbZS

1 in electrical conduit.   we purchased the kit with the sides and a 10x30 to get an extra  frame.   cut the horizontal members to 6 feet for extra strength for about 21 x 10 total space covered in heavy duty plastic Agtec Super Strength Clear Woven Greenhouse Film 12mil.  we had 12 55 gal plastic drums on the back wall full of water for thermal mass, an inkbird controller for around 1000 watts of heat tape and some box fans. The plants were in hog troughs with old sheets and curtains around the outside and the heat tape under them, the top we covered with a heavy duty row cover fabric like a tiny hoop house. we were able to keep many plants through the winter like snap dragons, bee balm, greens, carrots, herbs, and elephant ears ( yes the tropical zone 10 plant).  we also started our tomato's in march with no problems!

we insulated the back wall with 1.5 polyiso with the shiny side into the greenhouse with the Barrels in front of that.  depending on how you attach to the ground it would be very easy to move when the time comes.

if you are interested in more details, let me know.  it was really not too expensive for such a robust and useful structure.   the air temp got below freezing sometimes, but the real thing for plants is the soil temp, and that never got below freezing.  I used wireless tags for temp monitoring, but would probably choose another system for that aspect next time.

you have to own the space you have, if you keep saying "can't" you are limiting the amount you learn and grow, both food wise and personally.   We knew we would not be in the old place forever and are very grateful for its purpose in our lives. we did invest a lot of money and time, but we got so much out of it that we can appreciate its impact to our lives and move on happy to starting over at a new property.

i have no idea how an 8 year old thread showed up on top of my list.  odd. hopefully this info is still welcome.

adam hman wrote:

if you are interested in more details, let me know.  

I would love to see you start a thread outlining your build process, including as many photos as you would be willing to take.  I'm a visual learner, so pictures along with the written description really help me.

Sometimes old posts get resurrected.

I built a similar greenhouse myself, and I think the expectation needs to be created properly. Overwintering salad greens and cold hardy plants is very different than starting seedlings or trying to grow year round. For less than the $158 I was able to get some grow light LED's and a shelf. I can start ALOT of my seedlings indoors (which I'm already heating). Then the greenhouse is where I send them late spring to get hardened off before going in the ground.

If I had to keep things warmer, I would look at layering the layers. Eliot Coleman talks about it in his books. Put up a greenhouse in Maine, and your plants think they are in New York, put a row cover over them and suddenly the plants think they are in North Carolina. You might also consider straw bales with an old window or door over the top for an easy, inexpensive cold frame. Put that inside the greenhouse and you can have a small warmer spot inside.

Just some thoughts for people who stumble on this post down the road.