The problem with passive solar and electric heat is that all that mass works against you when the heater is trying to raise the temperature. It warms up the concrete in the morning, and all that energy is released, but in the light of day the sun is heating the greenhouse, so it's wasted energy and money.

I think you'll have your best luck by using several layers of plastic and the natural heat of the ground. With a double layer over the structure, plus a low tunnel, plus a frost blanket, you will be able to grow kale, brassicas, and cold-hardy greens. It is a lot of work dragging the blankets on and off.

And yes, you'll want ventilation. Big doors or windows oriented to the breeze will do the job. I'm zone 6, and my greenhouse can get 50 degrees above the outside temp on a sunny February day.

Thanks for the reply. I've been doing some reading and getting confused by some contradictions. The Winter Harvest Handbook says things like spinach and carrots can be grown year-round in Maine with nothing but cold frames inside a typical greenhouse. Other things I've been reading have been much, much more discouraging.

Could I grow cold-hardy plants year-round, including December, with cold frames in a greenhouse? Or just beds with frost blankets suspended over them in a greenhouse? Should the greenhouse have glazing on all sides, or only the south? Would it be usable at all in summer?

Thanks.

A year-round greenhouse for northern climates tends to be a different design. I put a link below to show you some examples. You want a roof steep enough so that the snow slides off. They often have an insulated north wall.

The natural heat of the ground is the cheapest way to keep your plants from freezing. Frost blankets and multiple layers of plastic act as insulators to keep that heat around the plants. Cold-tolerant plants like kale will actually tolerate a little freezing; the edges freeze and die, but the plant grows back. Root crops like carrots are similar.

With good ventilation, a northern-style greenhouse can be used in the summer. Depending upon how hot your summer gets, you may even use it as a shade house to grow cool-weather crops in the hot summer. With rigid poly panels, you can buy temporary shade paint, or just use mud. With the cheaper greenhouse plastic, you can replace it with shade cloth. I throw mud on mine when the outside summer temps get into the 90s.

Here is a link that might be useful: Ideas from northerngreenhouse.com

We have a greenhouse with many passive features, but not totally passive. There are greenhouses in northern environments that are totally passive, and still keep winter inside temperatures above freezing. Cheyenne has a noteworthy one in Lions park, and Shane Smith, who runs/ran it has a web site with lots of good info on the design. I would recommend the Greenhouse Gardeners Companion by Smith as a good introduction. In our greenhouse we supplement passive heat storage with some backup gas heating. We opted to not go totally passive because we liked the looks of a more conventional greenhouse, especially having glazing on more than the south wall. Water storage inside the greenhouse as part of the plant supports and surrounding beds contributes a great amount to our heating. By using a little suplemental heating we are able to grow things like peppers and tomatoes through the winter. If you cannot tell, I am very enthused about using at least some passive features; they really do not impact our enjoyment of the greenhouse, and they save money while allowing us a wider variety of crops we can eat through the winter.
Renais

I really wanted an "active solar" design like the one described in the link below, but I didn't have the funds.

Here is a link that might be useful: How To Build A Solar Greenhouse

For best results, only the south wall should be glazed. As Cole suggested, use the earth as your heat sink and maximize its advantages. Growth slows in Nov, Dec and Jan due to lack of light, so plan on growing your fall greens to near-maturity by Halloween, so that there is enough for weekly harvests until February with no additional growth occurring. In zone 5, I think you will be fine with a greenhouse and some heavy row cover. For best heat retention, have the row cover as close to the ground as possible. Some freezing is fine. The hardiest crops are mache, claytonia, tatsoi, kale and spinach. Good luck!

Thanks much for the replies! This is all really helpful information. The spot I have in mind is on the north side of my back yard, but there are a few trees that will obscure some of the sun. All the leaves will fall off by fall, but I'm wondering how much of a hindrance the branches will be. If there's a little bit of shade throughout the day on a greenhouse in winter, does that make it pointless to build one there?

Last winter I followed advice in Elliot Coleman's book (Winter harvest handbook) and had great luck in *harvesting* fresh vegetables all winter. They did most of their growing however in the fall (I started them all in late September or later) and then later in the very early spring as in the winter months they barely grew (although they did a tiny bit).

My set up:

Hoop house - It was an existing (built by me and my husband a handful of years ago for season extention) 12x16 plastic covered hoop house. We made it out of plastic conduit and the only thing we modified for the winter is that we swapped out the flat 4 way fittings at the roof peak with adjustable angle fittings. I found that 45 or 90 degree fittings didn't work (too sharp an angle) and I didn't want to not have an angle for the snow load so I went with manually setting the angle as steep as I could with the adjustable ones. (You can find them by googling "PVC 1" Adjustable Ell Fitting 3 Way" or "4 way"). We never removed snow off the top of it - By design or by luck the snow would shed off the side. (Although I hear brooming it off is good practice.)

I sealed the window and door with weatherstripping.

I also added ~ 800 gals of water - some large tanks (which were a royal pain to get in) and lots of single gallon containers. against the north wall. I also added 1" foam insulation along the north wall - including covering 1/2 the "ceiling" on the north side (taped to the conduit frame in some areas). I dug the foam insulation ~ 6" deep around the perimeter.

I then planted in two 18" rows separated by a small 1ft step path - all under a 4ft wide floating row cover (the row cover hovered on smaller plastic conduit hoops). Under the row cover in the small 1ft step path I put as many gallon jugs as fit leaving me with step spaces. I also put a few 1L and 2L bottles on the outside edges under the row cover. I also put one strand of old-fashioned (ie not LED) christmas lights suspended from the conduit as my "emergency" heat source.

I put my thermostat for my lights outside my row cover and set it to turn on at 28*.

I also grew a few earthboxes full of veggies but I found that anything in a box didn't grow nearly as well as things in the ground. I think the roots get too cold and they stop growing much, much earlier (but they didn't die - just didn't grow as much) even though they were under the same row cover but they didn't die.

The last thing I did was put a small muffin fan attached to a tiny solar panel under the row cover for air circulation.

I had my window/door always closed (ie I disconnected my autovents) for the winter months (Dec-Mar). I also never went in to harvest except during the day (almost always a sunny day) when opening my row covers wouldn't release any trapped heat that was vital for the night. I weighed down one side of the row cover with cylindrical bags of sand that I could easily remove and then the other side with soil/rocks piled on top.

Watering was mostly a non-issue, but I'd carry in my water in jugs on sunny days and water by hand. I think I had to do this maybe 3-5 times all winter (ie monthly).

With this set up I successfully grew: kale, leaf lettuce, carrots, beats, chard, mache VIT, bok choi, spinach, etc. For most plants I would harvest outter leaves and leave the inner leaves to continue growing. With that set-up each plant continued to provide until the late spring heat caused them to bolt (which was fine since I needed the space for the next round of crops).

The temperature inside the row cover was much more stable than in the hoop house in general. The temp in the row cover never dropped under I think 25 - even after multiple cloudy/stormy days.

In addition my hoop house is drafty so perhaps with a more sealed-tight environment no extra energy would be needed and/or maybe more ventilation.

Oh and my hoop house is on the north side of my property and has mature trees ~ 50 feet away from it on the east side which did shade some in the morning (but not so bad because they didn't have leaves) and then we have evergreen trees that would shade the late day sun as well. I still think it's worthwhile but it depends on how much sun you get.

I'm planning on building a "real" greenhouse (twinwall, etc) and am spending this winter scouting for the right location (best sun). I plan to use an earthen floor due to my experience of growing in pots vs in the ground, but am still doing my research for that as well. I've read that wherever possible you want to insulate in the ground to keep from losing heat that way. I plan to do that with my final greenhouse. Also wherever you are realize that both snow and a kneewall (if you have one) will shade part of your greenhouse floor in the winter due to the low sun angle so you may lose "significant" growing space that way.

Here's a picture of my winter garden in late December. You can see the small muffin fan on the right (the solar panel was elsewhere in the hoophouse) and the christmas lights strung inside the floating row cover. Again I started all my plants late september or later (I was planting every week) and could/should have started them a couple of weeks earlier.

That's all really helpful, thanks! The Christmas lights are a cool idea. I also got a lot of ideas from the Coleman book. Here's a photo of my greenhouse so far (it's 10'6" by 6'). I'm thinking of insulating the north wall with standard fiberglass insulation and putting a space heater on a 35/45F thermostat, but I'm also debating water tanks and stuff like that. I remember Coleman's book said plants grew at twice the speed if kept above freezing. Someone on this site said water tanks WITH a space heater wouldn't make sense, so I'm leaning toward a space heater alone.

If I double-glaze the wall with plastic film, do you think row covers would still be a good idea? Thanks again for all the great information.

As long as you aren't expecting to garden in 3D (ie hanging plants vs just plants in the ground) I would use the floating row covers since I believe it adds extra temperature stability for your plants and it also helps protect them from drafts - including as you enter your winter greenhouse (the door presumably is closed before you take the floating row covers off). In addition it does have some insulating value and depending on what part of your greenhouse you have your heater in it will save some cost in heating.

I believe that those that say water tanks and a space heater don't make that much sense together are pointing out the fact that you are wasting energy to heat up the water once your heater kicks in. I haven't done the math but depending on your sunlight situation the water can diminish how many hours your heater is working and if your heater is like mine - it is either off or on and it doesn't really matter that it is also heating the water since mine at least wouldn't turn off once on until the sun kicked in (it didn't have enough juice perhaps to overcome the cold outside temps at night). It also can depend on where you put your thermostat/heater vs where you put your water tanks (ie all in the floating row cover, or all outside, etc). I put my heat INSIDE my floating row cover and most of my water OUTSIDE it. However they took tons of space which I didn't like (but I had plenty of salad greens for my family so it wasn't that big a deal to lose the space).

Here is a link that might be useful: Calculating Cost of Heating Greenhouse

I forgot to add that you probably want to dig in some foam board insulation into the earth to help from losing heat that way. I've read from multiple sites that this is significant loss of heat for the winter greenhouse.

On my hoophouse I also put insulation up to the center beam on half the hoophouse (the other half only until the cross bar at ~ 3.5ft off the ground). I don't feel like I lost much light this way since I had my water tanks in the back and the sun is so low in the sky in our winters.

Also whatever you plan to do for doors - make sure it works once you have snow on the ground. I also had to latch mine from the outside so that for our nor'easter storms it didn't move/swing with the strong wings