I think it would make the most sense to heat the concrete slab floor just like it were any other type of building. Then you can grow in raised beds or containers. Heating the slab will help it maintain a constant air temperature as nighttime temps drop, which is what you want.

babyboomer: Then go with a small electric heater and small fan to keep the air moving and prevent hot and cold spots. Since you're there all day, you can keep checking the temps and open the windows manually to regulate the temps. An indoor-outdoor thermometer would be helpful. How did your plants burn up with all the windows open? You kind of lost me there. Wont it happen again unless someone gets out there to open the windows in the future or have you rigged up auto-vents? I'd hate to see you lose your beloved plants again.

"My view - and I'm doing garden trials now - is that hot water underground in tubes is cost-effective. " I am not sure how this is to be executed. Will the hot water be in a container and insulated? Otherwise most of the heat will be lost to the earth. "A solar-powered pump is inexpensive" Solar cell generates not much power. Depending on many hydrostatic factors, I think a water pump about at least 100 watts will be needed, A 100 watt solar panel is very expensive with other electrical and electronic to go with it. When you need it the most when the sun is not out, you donÂt have the power to operate the pump. " I also see the high-end method of taking the hot air at the top of the GH and putting it underground to heat the soil. " Not easy to make this practical at all. Geothermal, where there is no hot spring type of underground heat source, I believe is a concept works best with a heat pump system to extract the small amount of BTUs in the soil. "I also use dark-colored water in containers to absorb solar radiation to help things along." Water does not have a lot of heat storage capacity (One BTU per lb per degree F) unless it goes thru phase change (144 BTU/lb of ice). Whatever heat it gives up in the cold nights, it will take away in the day time in your greenhouse. dcarch

This greenhouse project was born when I met my future wife. Her mother gave me a curry leaf tree sucker from a tree she smuggled inside her carry-on when she flew from Kerala to Austin. At the time (and still in effect I believe) there was a customs ban on importing curry leaf trees to the US. In the beginning, I would move the tree into the bathroom during the winter. Over time, I re-potted suckers and even propagated some seeds. The bathroom was simply not big enough. Anyway, that little tree I received has turned into more than 20 trees, all in various stages of growth. The original tree is over 8 feet tall now and it should be mature enough to survive Texas winters (light freezes, Zone 8b) in the ground. I built this 10x10 hoophouse/greenhouse made from 1" EMT conduit. I bought the hardware bits and plastic sheeting from a greenhouse supply here in Texas and I bought the EMT from a local retailer (McCoy's). Originally (2014) I used a scissor-type door on each end. While this was inexpensive and functional, it did not work as well as I had hoped. It was cumbersome for venting purposes and it was virtually impossible to seal the bottom of the door, which caused a billowing effect when the wind was blowing. I also added a 2 foot duct extension from the dryer vent on the house and slipped it under the plastic to warm the greenhouse with the dryer on cold nights. It worked pretty good. The whole design did work overall, just not to my satisfaction. This fall (2015/16) I removed the scissor doors and built end wall door frames supported by a 2x6 center beam at the top and a 2x6 base 'apron'. The doors are quite large and have no trouble venting humidity in winter. (we have fairly mild winters here with cold nights and relatively warm days). I also installed a dryer vent on the inside of the greenhouse so I could quickly connect and disconnect the duct for night time winter heating. For Summer I can keep the doors open as well as roll up the sides. I also have a 12x12 shade cloth to be installed overhead. (it's not uncommon to have 60-90 days over 100 degrees in Summer here) The original metal frame is secured in the ground with twelve 18" rebar stakes. Here are the pics: This is an 8x4 table I made so I would have a large surface to bend the 10 foot EMT into the proper shape. I made the table with EMT and plywood (2014): This is me building the original EMT frame (2014): This is basically it. I Rolled up the excess plastic on the sides and trimmed some excess from the doors, but that's it. If you look in the window on the house you can see the dryer and the dryer duct coming out of it. The vent is located just around the corner of the house out of view.(2014) Here you can see the scissor door. The hinge is at the top. To secure it you clamp the two poles together. The bottom will move back and forth perpendicular to the way the door opens. (2014) This is the new wood frame I built to support the new end wall doors. The doors are made from 2x4's so they are quite heavy. They are also about 38 inches wide. The hinges are secured with hex bolts. (2015/16) Here it is again. (2015/16) Here it is finished and trimmed. I used plain wood clothes pins to secure excess plastic from the hoop where it meets the end wall. It's quite secure like this. Later I trimmed some of the excess plastic you see on the ground facing the beehives. (2015/16) Here I installed a dryer vent inside the greenhouse. It makes for a quick connect/disconnect for those times I want to add some heat on cold nights. It is very close to the actual dryer inside the house so it doesn't degrade the dryer performance at all. By the way, the wall facing the house also faces North by Northwest.(2015/16)