When the soil reaches the optimum temperature the heat is turned off until it is needed again. Trapping the heat under the bench keeps the air temperature much lower than the soil temperature. Therefore, rooting activity can take place while the top of the cuttings remain dormant. Professional growers also use very complex systems that circulate hot water through plastic lines buried in the soil. This works well, but a second heating system is usually required to keep the air around the top of the cuttings from getting too cold.
Nurserymen are innovative people.
After all, we're only interested in propagating a few cuttings or maybe a few hundred, not fifty or sixty thousand. When they build their frame for their hardwood cuttings they dig a hole the size of the frame, about two feet deep, exactly where they are going to set the frame. They pack this hole full of manure all the way up to ground level. They then set the frame over this bed of manure and fill the frame with course sand just as described earlier. Through the natural process of decomposition the manure heats up as it decomposes.