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Soil Heating Cables

Posted by 7000feet (My Page) on
Wed, Dec 15, 10 at 11:49

We are new users of a Greehouse. Last summer we built a 15 X 28' Greenhouse with tripple wall polycarbonate walls and roof. We decide to look at alturnative options (other than stoves/solar/gas) to heat our Greenhouse in the winter because of our cold climate (32 to -30 degrees during the winter) and the large cubic foot area of our Greehnouse (the ridge beams is 16' high). It would be hard for us to justify the cost of heating this Greenhouse for the amount of produce we want to grow for our own use over the winter months.

We came up with the idea of creating a hot box within our Greenhouse to grow vegtables over the winter. We have raised beds in our Greenhouse (36" high from the ground) 30"s deep. If we bury soil heat cables (with inline theramotates) in the raised beds about 6-8"s deep permantly and hinge a polycarbonate lid with sides over the section of the raised bed with soil heat cables - will the soil/air temperatures remain warm enough to grow cold weather vegtable crops like lettuce, carrots, radishes, spinach etc during worst winter months? We planned to have about 10" from the soil to the polycarbonate lid. We thought we would hang grow lights over the section of the soil heat cables for the short and dark winter days. We would also use the beds to start summer seedlings (like tomatoes) and then in the spring simply lift and leave the lids open during the growing months and use the beds as intented for summer growth.

We need the advice of experienced Greenhouse users in cold climates! Will this idea work? Has anyone tried this idea or do you have other ideas that may work for us? We would like to draw on your experience before trying this method. Any advice or ideas you can offer would be appreciated!

Follow-Up Postings:

RE: Soil Heating Cables

Sounds like you have done your homework and yes, I'd expect that to work very well. During the coldest times you could also throw a nice thick frost blanket over the top for some added insulation.

As you probably already know, it doesn't take much warmth just to keep the cold-weather crops alive. Getting active growth takes more warmth and light and so you should have good results with your plan.

I have all of those same plants growing in my unheated GH right now and they are doing fine, albeit not growing much.

Let us know how it works out!


RE: Soil Heating Cables

Thanks for your reply - are you using buried soil heat cables? If so - how deep did you bury them? Do you leave them in the soil permanently? Do you have your plants in a box with a lid or only covered with a frost blanket? We currently have the vegetables growing now too - without soil heat cables and only cover them with a frost blanket - but as you mentioned - the are growing very slowly depending on how much sun they get and night time temperatures here in Wyoming. Is your Greenhouse of similar size and material?
7000feet and snowing

RE: Soil Heating Cables

Congratulations on your greenhouse!

There is a discussion about this on the Tropical Fruit forum you might want to check out. I have used soil warming cables in potted fruit trees but not in beds. If you do use cables, try plugging them into a Kill-o-Watt to find out how much electricity it's using.

I am not sure if you would be interested in this, but I have heard that a solar hot water panel system works really well with radiant heat. -Very efficient. We are planning on having a solar hot water system connected to pipes buried in raised beds when we build our greenhouse because keeping the plants' roots warm will allow us to keep the air temperatures relatively low.

RE: Soil Heating Cables

It is interesting how one looks at options differently at different stages of construction. We had a budget and over spent as it was - and - we never thought we would be interested in growing in our GH during the winter at our elevation and winter temperatures! I think your hot water system is probably the best solution but difficult for us to now incorporate.

I did read the Tropical Fruit forum you mentioned which actually gave me the idea. I had hoped that someone may have tried it in zone 4 to test it with vegetables. I guess I will go ahead and try a small bed and see how it works. Thanks for you comments.
7000feet, -4 degrees tonight but the vegetables have survived -20 so far in the GH without heat - just not growing much

RE: Soil Heating Cables

Hi 7000 feet,

I have not tried soil heating cables. I use the heat mats as a stop gap, which works fine, but only with the plants that are on the shelves. I am pretty well resigned to not have much growth from now until late Feb. But if I wanted to go that direction I would try something along the lines of what you are thinking. I especially like the idea of a frame and a lid - just another way to capture some of the warmth from the ground.

For now I just have things planted in the ground beds and covered with the frost blanket. Whoops -- but tonight I forgot the frost blanket which is too bad because it's going to be near zero. I have maybe 2 or 3 salad's worth of various greens that have grown since it got cold. Of course I don't want to harvest them because that will be the end of it. :)

I agree about using a kill-a-watt meter. I use that all over the house to measure the electricity draw. Very interesting results! But I imagine the soil heating cables don't use too much energy, and certainly a lot less than a space heater. Besides, there in WY you have plenty of coal to make electricity with. :)

I would recommend taking out the soil heaters in the spring. Seems like they would last a lot longer with minimal chance of you hitting them with a shovel accidentally.

Your greenhouse sounds really nice. We'll look forward to seeing some pictures. Sorry that I wasn't able to be more helpful with instructions. (I was on my way to go skiing, after all!)

RE: Soil Heating Cables

The Temperature dropped to -24 Friday night. The soil in the GH raised bed was froze everywhere except where I had covered the plants with a frost blanket. I was amazed that the lettuce & spinach survived. I am going to try the soil heat in a small area with a polycarbonate lid and see how it performs. I may come to the same conclusion (resignation to growing Dec-Feb) but we do want to start tomato plants early and heat mats/soil cables appear to be the best option in our climate.
I was able to post photo's in the practice forum but haven't figured out how to post on this forum yet. Thanks for you help. Your GH looks great -by the way!
7000feet - 100% chance of snow the next two days but at least the temp will be above 20 degrees.

RE: Soil Heating Cables

I think I am getting a handle on your instructions for posting photos - here are a few.
We built the GH from plans my wife designed - Here I am filling the raised bed with top soil.
Then we installed the polycarbonate on the framing. We used all redwood then put on two coats of semi-transparent stain for longer life.
We put a storm door and windows on both ends of the GH for venting and view. We also have an auto matic gable fan with vents on a thermostat.
We built raised beds outside of the GH for Cold weather corps that grow well in Wyoming (strawberries, asparagus, beets, carrots etc.).
We grow wide rows of peas - beans - squash - cucumbers etc. and cover them with a frost blanket in early spring & fall. We get a great crop as long as we watch the weather report and get the plants covered with blankets.
Now here we are in winter! This photo was taken a couple of weeks ago. The GH is almost finished except for the electrical and additional raised beds I plan to complete over the winter.
Here is our small patch of experimental lettuce & spinach as it looks today, Dec 18th. They just sit there unless the sun comes out and it warms up a bit. Hoped soil heat cables with a polycarbonate lid may change all that and we could actually grow something over the winter?

RE: Soil Heating Cables

Awesome photos - you have a really nice place there. You guys don't mess around either; it looks like you do things at full tilt!

The greenhouse is beautiful. It's so nice and big, which is something I would be quite happy with. Looks like an impressive design and very well built. The outdoor raised beds are lovely too. I like the flat board on top so you can sit there are weed. (Do you have weeds there in Wyoming?)

How cold was it in there while it was 24 below outside?

My spinach and lettuce are often frozen fairly solid in the morning. Looks like they're goners but then they thaw out and perk up. Kind of miraculous!

In addition to winter growing, your soil heat idea will definitely give you a jump start in the spring. Tomatoes will love that. I start mine in March and they grow so fast I have to keep potting them up. By May they are in gallon pots and are like little trees. The ones that I intend to keep in the GH all summer get planted in the ground bed in early May. The outdoor ones go in in early June.

Speaking of which, it is dinner time and I have just cooked up the last of our Super San Marzanos and made a fresh batch of pasta. YUM!

Thanks for the pictures - really impressive!

RE: Soil Heating Cables

It's about that time again... Thanks for the building description, karin. Which soil heating cables are you guys using? Built-in thermostat or heavy-duty cabling where the thermostat is connected in series? Thanks.

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