I use soil heating cables in my GH exactly as you describe. What are you growing that you need soil 36" deep? Are the roots going to actually penetrate that far? I put the cables down about 15-18". They barely get warm to the touch so they won't hurt roots. I'd worry more about roots or cultivation damaging the cable. If you plug the cable in before it gets really cool in your GH it won't have to supply enough heat to bring the soil temp up, which would take "forever" just maintain what is already there. Soil temp stays around 68 all winter. Additionally when it is really cold outside and the GH air temp is lower than normal I cover the raised beds with clear plastic to help hold in the heat.Hope this helps you somewhat.

PQ

Citrus are what I plan to grow. It might not be 36" deep-but I want a basically permanent setup. Maybe more like 24" or so.