I just found this at the American Rose Society.
https://www.rose.org/single-po...
• Concrete Blankets: This is a relatively new technique used in our District with great success. Concrete blankets are actually used in construction, typically for pouring concrete, to keep the ground or newly poured concrete from freezing. They are typically 1.5 inches thick and come in various sizes. The R values relate to insulation ability and the higher the R value, the more protection will be provided and the higher the cost. One member even had them custom made for her garden, but that is not necessary. Concrete blankets are easy to find at home improvement or building stores. An R value of 7.1 - 7.5 is ideal, however, I have used R value of 5 with success. Be sure to anchor with bricks or something similar and include plenty of mice poison under the blanket. You will need to cut down your bushes, so wait until temperatures are cold enough before laying down.
Not cheap but no more expensive than car covers I used and a lot tougher.
