Very interesting approach Dino.

I have a low overhead "A frame" greenhouse, about 1000 square feet. I heat predominantly with four 50 gallon drums of water - two at each end. I also have a string of pop-bottles [3/4 full] running along the peak. When the overnight temps average below 0°Celsius I cover my seedlings with plastic.

With this system I have managed a 10° difference [from outside to inside] not including the added difference the overnight blankets give - probably another couple of degrees. Everything is dependant on whether we had good sun during the day.

Your approach strikes me as being more inclined to keeping the area around the seedlings warm - as opposed to the whole greenhouse, as in my case. Could be, that it would be more efficient, for all intents and purposes.

The only thing I see, that would be a problem for me, is the amount of space a square would utilize, leaving less room for plants. Have you thought about lining the entire row, with the seedlings "sandwiched" between?

Anyway, I think your system would be well worth a try. Good luck with it, and keep us informed.

Axel2

Well... space is relative... tomatoes are a crawling vine, cukes are a vine et cetera. And what if the square of milk jugs were set in the ground.. insulated around the sides to reduce contact loss on the sides. Depending on the spacing one could add lettuce, scallions, and if one lifed the top a little, some peppers perhaps....two possible problems besides cold... getting too hot in the day and humidity.

The thing about the water containers is that they temper the ambient air temperature - draw the cold while releasing their heat, and vice versa, cool while drawing heat through the day. You won't see a huge difference, but they may help in preventing over heating.

I've not sunk any containers, but I imagine, if at least half of each container were above ground, it would essentially heat the surrounding soil as well, to whatever depth they were. That would be a really good way to get bottom heat for germination if placed strategically. I might play with this one myself...

The humidity is easily taken care of with air circulation - a small amount of air exchange during the hottest part of the day, probably the best scenario - and containers with small top openings would cut down on humidity as well. Air exchange will - but, by how much? - affect the rate of heat absorption in your bottles.

This system might be too warm for lettuce....

You realize, I'm just giving you my best guess, based on my experience with my own system. We are looking at vastly different spaces. Mine is a ten foot high centre.

Axel2

I'm with you on this. I've got 30 2.5 gallon water jugs (cat litter containers) that I sprayed black to absorb the sun. I am in the process of lining them up under black plastic along the planting rows to help warm the soil. I plan on using them in my future greenhouse, under ag fleece row covers, and in my cold frame. Anything will help. Provide a little shelter for the plants and they will thrive. Good exercise moving them around, too!

If you look at Mother earth news I believe you will find greenhouses heated this way. Some folks paint 55 gal drums this way.

I just use the milkjug greenhouses and use the winter seed sowing. The seedlings are hardened off before you plant them. I put the greenhouses close to the foundation of the house so they absorb some of the heat from the concrete/brick.