What size is this structure? Please tell us a little more about it? If you are overwintering (green)seedlings, will you will need to keep it above freezing. Will you be starting plants in early spring or carrying them over from fall? Cables will keep the roots warm but will they keep the air temp warm? After all, you are in zone 5 where soil freezes several feet deep. I think you can warm the air temp using a kerosene lantern. Use K-1 kerosene and keep your wick trimmed and clean. Before electricity, many people warmed cold frames by buring kerosene and before that with candles and whale oil.

I am planning to use my cold frame for starting seeds and to extend the growing season. We just built it so I don't know how well this will work, I got the plans online. We dug down 18 inches, added a mix of seasoned manture and staw. Covered that with 8 to 10 inches of soil. The cold frame is about 30X60" made from cedar with glass windows for the top. We plan to use styrofoam sheets which we just happen to have around the side to insulate the side when needed along with a covering of straw and a tarp for the top. We are planting this weekend so I will start monitoring temps then. Good luck with yours!

we're in zone 7 and have only ever used green manure for heating the soil in our cold frame. About 8" of green manure, 6-10" of top soil, and a pvc hoop frame with 6 mil plastic. We pull from the garden throughout the winter of cold season crops (lettuce, carrots, spinach, peas, etc)

We done it in other zones and know people that have successfully grown vegetables all winter in Maine this way.

The green manure heats the soil as it rots all winter long.

I am researching greenhouses so we don't have to bend over so much, but the heating/cooling requirements seem to offset the practicality for us, since we grow veggies, not flowers.

If the outside temp is getting below 35 I just lay a drop light in the back and cover it with a quilt that has worked great, I use about a 40-60 wat bulb.

I've never used it myself, but a friend with commercial GH's uses reflective mylar to line her cold frames and says it jump starts growth by at least a month.

But in our zone 5, I don't believe you could rely on it for heat in the deep freeze months, but heating cables might work. If you think the mylar may work for you, or even aid the cables, Google "Silver Mylar" and you will come up with a lot of info.

BTW: Green manure works for hot frames, but only if it IS fresh and the temps aren't too low, as in: Zone 7. So you need horses for a constant supply, and a milder climate than Zone 5 to use it as a winter heat source.

thx for the mylar tip! yes we have a steady supply of green manure every winter. Everywhere we have lived, country people with horses will gladly let you shovel it ;)