I tried heating cables for the first time this year and my plants have been in the garden for almost 2 weeks now. I have 2 plants (one with jumpstart cables) and one without. For the plant without cables in the ground I decided to try something else and had a couple seedling heat mats I wasn't using so I placed one on each side of the plant on top of the ground. Anyway ... 12 days later the one with the heat mats is doing better than the one with the cables. I took soil temps and both at 8" down were 75F. The difference is with the heat mats it was warmer at the surface and the air temp in the hoop house was kept warmer at night. I think people assume heat rises so you can't get heat to go DOWN in the soil. Heat rises in air but in a solid or semi-solid (soil) it radiates in all directions. I'm no scientist, but that is my conclusion. Take from it what you want, but if you have a seedling heat mat laying around it's not too late to heat up the soil, even if your plant is already in the ground. They maybe don't heat up as much of an area as cables but they are definitely easier to use and by the time the roots grow out of the warm zone you likely don't need soil heating anyway.
John won the 150 square foot contest this year with a 1175.6 lb. pumpkin. The first one grown over 1000 lbs. in this contest. Congrats John!