I just don't know, Sandy. One factor, and this is only a guess on my part, may be the fans that run from 9:00 AM till 4:00 PM (winter run-times). The solar blanket material is supposed to increase the heat within an enclosure, not decrease it. This material transmits plenty of light as evidenced by the growth of all the different tropical plants in the greenhouses.

You're definitely going to have trouble using passive solar when your greenhouses only get 4hr. of sun per day, Ken. Your electric bills must be huge. Can you thin the oak tree branches so you get more sun?

DYZXY thanks for posting pics of your tall zippered GH. It gives me an idea. We have three large plants in big pots on the west facing patio and no room in the 6 x8 GH. So we are running out every night when it freezes and covering them. I have a nice corner on that patio where they get protection from two sides. I may have enough left over from my solar pool cover to build a shorter but wider GH. That patio gets afternoon sun from the west and I could easy move the water jugs to the edge of the patio every afternoon to let them warm - as I like how you harness natural heat! (Even if it means schlepping water jugs every day!). And it would be easy to throw a blanket over the entire entourage when needed. Might build it with PVC that can be easily disabled at end of season. Well, when I say "I" that usually includes my husband Ed who is the "master implementer". My friends always say, "well, you have Ed"!

That idea will work very well. Here is what I built this fall to over-winter my potted plumeria and some of my hanging staghorn ferns. I built the frame out of 2x2" and used 16 mil solar blanket material for the covering. It is heated by a "base-board" electric heater, set at the lowest W possible. That heater kept the inside of this temporary greenhouse a toasty 70 F even when the garage itself was 40 F this morning. I used Velcro strips along the sides so that I can get into the greenhouse. That "door" just rolls up and Velcro strips were placed at the top to hold that rolled-up door open. There is one window that looks west, and that gives a good amount of light. If the garage door is open, light comes in from the south as well. I am not trying to keep my plumeria growing but wanted them to go dormant without disturbing their roots. I take the staghorns out of this greenhouse every week, and submerge them in the laundry-room sink filled with very diluted fertilizer.

I only have a small growcamp greenhouse, which I use for my orchids mostly. Like what others have said, it gets same temps as outside during winter and as hot as it can during summer, so I have the plastic foil flaps removed during summer, just the screen is in place.

For this winter, since reading this thread, I added some bubblewrap too and some mylar, but I have decided to just pull in my more sensitive orchids and in their place I put my succulents that can take the cold but not the rain, since our winter is oftentimes more with rain. I have now accepted the fact that when temps of 20's come in winter, I am just deluding myself my orchids will stand it. They get cold damage even while covered by my growcamp, so I just pulled most of them in. Thankfully my collection is not that much and can go to the our rooms facing the south window. Even in our mild winter conditions, I am just asking too much of my orchids, I think if I continue to keep them outdoors. The only orchids left inside the growcamp right now is my Zygopetalum and this noid Den that really likes being cold but dry. My tillandsias will remain inside the growcamp too, they handled the winter temps well as long as inside the growcamp. My Vanda coerulea, pleurothallids and cymbidiums, they can take the winter temps nicely here, so they stay out as well, or if I change my mind, there is still space inside the growcamp for them.

But I still want to research more of other ways of passive heating..thanks for the discussion about the water jugs..makes sense..I just need more sun! The city trees and our community trees are still fully leafed so sun is blocked a bit, it gets only a couple of hours south sun.

Anyways, I saw some youtube videos where they were using inverted clay pots with some metal inside and heating it up with tealights. I have not experimented with it yet. But it looks interesting to me. Maybe I can use sterno instead of tealights. Just for those nights that we do have a forecast in the 20's which often happens in January-February. For now we have only gotten down to 39F a few nights ago, so it is still okay.

I like the tealight idea a lot, Tarev. Don't know about you, but I have a small 'collection' of old candle stubs and more or less used up candles. I keep them around as hurricane supplies just in case. But a candle gives quite a bit of heat for a long time, and an inverted clay pot would also act as a bit of a heat sink. If I put a couple of warm water jugs beside the pot, I wonder how warm it would stay over night in one of my little shelf units. . . . An experiment is imminent

And we have yet another night forecast to drop into the 40's this week, unbelievable! Usually these cold fronts stay north of us until at least January.

I got this in my sandbox thread:

but have not personally tested it...looks good idea...especially if really in dire cold and power goes off..and clay pots..gardeners like us have that a lot!

I hate watching videos, so I did a little looking around about the tea light idea and found this. What is interesting is the idea one of the commenters came up with. http://www.instructables.com/i...
Scroll down to the comment by ewbray on that page. His idea is to use an empty tuna can filled with solid shortening (melted, then re-solidified to remove air) and using birthday candles for wicks, instead of using tea lights. Says it works for a longer period of time.

I notice that plans for cold frames almost always have the north wall, or usually three walls, made of an insulator like wood.

If almost all the solar rays come from the south, the other walls probably lose more heat through the day and night than they can absorb during the brief period when the sun shines on them.

Maybe a greenhouse would benefit from heat blankets on North, East and West walls during the winter.

Some book about winter gardening for markets and restaurants in Maine spoke well of "layers" for keeping winter crops warm enough.

The outer greenhouse walls (or outer hoop tunnel) were one layer.

small hoop tunnel inside the main greenhouse was a second layer.

Spun-woven row covers were a third layer.

Cloches under the row cover were a fourth layer (for example, for germination or hardening off).

I recall they got something like 5-15 degrees temperature difference from each layer. The differences were cumulative, and the inner layers protected the plants against drafts.

(Any draft or air leak was a huge heat loss.)

That's more or less what I've been doing, in a slightly less organized fashion. Getting rid of the drafts was a major improvement, too! My GH has a raised floor, so I finally lined it with 2" styrofoam sheets underneath. I have bubble wrap inside, and a solar pool cover outside. And when it gets really cold, I cover the flats (which are on heat mats) with bubble wrap covered with a blanket or towel. This has been an ongoing learning process, and each little addition, as needed, has made a major difference, between 5 to 10 degrees for each layer. Why, yes, I do have an impressive array of thermometers.

Just today I finally wrapped the east, west, and roof of one of my greenhouses with a second layer of solar blanket material. A single piece, 12'x22' covered those areas. It will be interesting to see if I can tell the difference.

If I were building a new greenhouse I would definitely insulate at least the north wall. According to Eliot Coleman ("Four Seasons Gardening," among others), each layer of cover raises the environment to the next zone up... I'm not sure that's strictly true, but it's probably close.

Interestingly, in my hoophouse, which is covered with a layer of 3-mil plastic, I have lettuce, chard, spinach, broccoli, turnips, kohlrabi, mache, chives and parsley, covered with Reemay over fiberglass hoops inside the HH, and even though we've had 2 nights in a row with a temp of 8F, and many nights that went down to about 20F, everything is still doing fine. (We also haven't had a heck of a lot of sunshine to warm things up recently) So, while I definitely didn't get some of the things started nearly early enough, I consider this to be a total success... my goal was simply to be able to harvest a few things for a salad by Christmas, and I'm feeling pretty confident now that I'll at least come close to that. Then there will be a couple of months off, and sometime in February I'll be able to start planting a few things again. (at least this way I feel like I'm still gardening... )

Hi all! I have had great success keeping my greenhouse plants alive here in Michigan cold springs using a method an elderly Mennonite lady told me. I take a large metal pot, mine is an old canner, put two or three three wick candles in it and pour water in it to go half way up the candles. When I wake up in the morning and check my greenhouse the candles are still burning and the house is warm and steamy. I have done this for the last seven years with success.

This is an old thread from 2014

kcolegio4 said:Hi all! I have had great success keeping my greenhouse plants alive here in Michigan cold springs using a method an elderly Mennonite lady told me. I take a large metal pot, mine is an old canner, put two or three three wick candles in it and pour water in it to go half way up the candles. When I wake up in the morning and check my greenhouse the candles are still burning and the house is warm and steamy. I have done this for the last seven years with success.

That's a great idea! Love the fact that it is very inexpensive and quite safe.

I too have used the candle method mine are sitting in a large pot of sand, it does work very well

when I don't have any spare candles I have used an oil lamp ,we have several around for power outages

You can very easily heat small greenhouse structures with an oscillating electric space heater that has a thermostat and forced air

Well that works IF you have electricity
my small hobby house has neither electricity or water
mostly just extends the spring and fall seasons.

Ahhh. I used to have to run an extension cord out there but it worked well for me.