Your success would be dependent on your location, and other factors. Could you close it up airtight on freezing nights?

Airtight I am not sure! I have never owned one of those and never saw up close, but I do think it can be airtight if I put sand / soil on the sides of the plastic tarp. Both ends are closed with zippers (or cheap models have ropes you have to tie). I suppose the zipper would be better ;)

My location is Quebec, Canada, zone 5b. I said the word "greenhouse" but it's more like a high tunnel. My intentions were to have several beds in the car shelter, and cover those beds with floating row cover.

I'm not too concerned about the heating specs of the car shelter, but mainly I wonder if it would be handy or viable as a greenhouse. I am not sure what could be a flaw in that design, but maybe I need to be able to roll up the sides to vent it? Maybe the structure is too high and it won't heat well or the air will not move and create problems for my plants?

I found a Quebec retailer that converts some car shelters in greenhouses!!! I have not seen any comments from people that bought it, though.. that worries me somewhat, and the price is super high, but still it is the idea that I had:

http://www.abrisharnois.com/en/shelters/green-house/

If only I could have comments from people who bought that model, I would know if they found any flaws! By the picture, I am worried about air flow since the back window doesn't even open fully. In spring / summer it must be crazy warm in there. And if I invest 1000$ in a shelter or any sort, I expect to use it all year long!

Yes you will have to replace the "tarp" with a poly cover. You can buy rolls of poly at larger garden supply outlets. Here's a list of some greenhouse coverings along with a brief description.

various greenhouse coverings including poly.

http://www.hobby-greenhouse.com/coverings.htm

I don't know where you live but if you get lots of snow or any snow for that matter you are too far north to grow anything in the winter and if you try the heating costs (no insulation) wouldn't make the endeavour worth the time and money. Not many plants thrive with only 8 or 9 hrs of sunlite in Nov. Dec and Jan.

Your greenhouse would be better suited for 3 seasons.

I live in zone 6 and did a hoop house out of PVC conduit with home-made frames with special connectors that made the top more onion shaped (vs perfectly round). The ribs were probably 3ft on center. This successfully survived with produce (spinach, lettuce, kale, etc) all through my winter (tried on the winter 2013-2014). I bought special poly for greenhouse for the outside - I buried the edges down. I also on the edges went around with 2" foamboard insulation dug down about a foot. It wasn't air tight since I had a door and window (I use it in the spring/fall as well). I added lots of water to the back row (lots as in probably 500 gals) and then some water under my floating row cover. I also added 1.5" foamboard insulation to the back wall under my hip board for added temperature stability. I had xmas lights under the floating row cover for "emergency" heat. It was fine. I don't know how much snow you get but some say it is okay to use a broom on the inside of the hoop house to brush snow off of it. I didn't and didn't have problems. I need to see if I have any pictures of my setup (I took it down this summer since I went all in to buy a greenhouse - a RIGA). That winter is the only winter I used my hoophouse for - I didn't try before then or since then (for personal reasons I did not winter garden last year). So it might be due to a mild winter that I had such success. I say try it!

BTW - I can't open your link but have read many places that a truly round roof hoop house will struggle against snow load since the top is practically flat. That is why I went with special connectors to make more of an onion shape so the very tip was ridged vs flat.

We're going to moving in the spring and so for a temporary gh we bought a 12x17 canvas portable garage from Harbor Freight. Just the frame is up and going to add grow lights and a heater here in zone 8. Plan to put cinder blocks along the bottom to keep cold air and our six 4 legged girls

I had missed a few comments here, sorry.

I passed on the car shelter this year because it was too costly. I just build low tunnels. As for the round shape of the roof, maybe I'll have a problem there. Mine is round, but I thought of it as a "pronounced" arch, so I didn't think the snow would stay on it. We'll see, I guess ;)

At the moment the plants are thriving. I added water barrel beneath the inner layer. The tunnel never froze yet, while it has frozen about 7 times outside (and twice it was pretty bad, all the grass was frozen until 9h30-10AM!).

I think it's good for me to try with a temporary and cheaper shelter like this before I invest in something bigger. I still haven't figured out many things about winter gardening.

The first problem I anticipate is the tarp freezing on the ground or under the snow so I won't be able to open it. I can't put salt there, there is a garden row just beside the tarp..

Once I'll have lived a winter with my tunnels, I may think about something more costly ;)

We were able to buy the garage on sale and with moving in the spring it'll come in handy for storing things like the riding mower etc. Wish you all the best with your tunnels and they should work.

Sabrina - you might want to figure out a solution for the tarp freezing to the ground before it happens... I had a doorway in my high hoophouse that I would snowplow/shovel to and the doorway was ~ 2" higher than ground level (used a board on the ground to step over). This worked great. Not sure if you could get a board of some sort on the ground and then fasten the tarp to another board so that you could lift it up or something. Once ice sets it'll be a bear to lift (i'm picturing melting it, via pouring hot water etc.). What are the dimensions of your low tunnels!

BTW - I agree with trying a cheap solution the first year before going all in! I did a PVC hoophouse as my starter and then bought myself a RIGA! :-)

Yeah I'd like to figure a solution but no ideas come to mind hehehe. Well no, I'm lying.. I thought if the tarp was not too long (IE not go down the sides of the tunnel and then sit on the ground) then it wouldn't freeze.

The only way I thought of making this work while stopping the air from getting in was to put a long velcro all along the tunnel! My tunnel has a wooden base, so I could attach the tarp on the base so it would never touch the ground.

I don't understand what you mean with the board sorry :( I can't picture it in my mind. I'm also picturing the hot water and the re-freezing and it looks like a pain!

I have two tunnel. One is 4X8 foot.
The other is 4X16.
I'm kind of experimenting with those two, wondering if the size was a bid factor or not!

I should post a pic of my tunnel, it would help you visualize how it looks.

Here is the tunnel's structure. Basically pallets, then rebars and a cattle pannel on top. The rebar are spaced 4', so I think it's going to be weak and probably not support a lot of snow.

And with the tarp:



Since then I have sewed doors on each side :) I can unzip it to open the door and zip it back when it's colder. I love the idea.

So of course I'll have to cut the tarp since it's way too long now, it will be covered in ice soon.

Here is the same setup in spring! I am not sure why I used the white tarp for winter... I thought it was thicker and would retain more heat at night. Originally I wanted to setup 2 tunnels of the same dimension, one with a transparent tarp and the other with the white one to compare but I didn't have enough plants to fill the two tunnels and felt too cheap to buy some without knowing if they would all die.