What if you ran a little plastic pipe under the ground, with some of the black portion above ground, all sealed, and let the sun slowly move warmer water through the pipes, just a sealed, passive system.
That may work but the cost would be considerable.
With the amount of mulch (1 foot) and two feet of snow most of this winter, I reckon it never got below freezing in the root zone.
Next year we plan to measure temperature in the root zone to see how warm it stays.
Regards
Lurking’