I seem to remember reading an RHS article about this. They dug out a hole big enough to take rootball, then put a cardboard box of roughly the same size into the hole. Box then filled with soil from another site, then the rose was planted in the box. I think the idea was that the box would degrade in time, thus giving the rose time to establish a healthy root system before the roots ventured out into the soil outside. A dollop of Rootgrow could, I think, only be beneficial.