I used mostly mulch (finely shredded hardwood) with compost and a bit of top soil. As long as it's porous, it probably wouldn't matter much what you use. I set the Hydrangea under a tree until I noticed it was growing the flower. I also used bricks to hold other branches to the ground and those have all made roots also but not as quickly because it's been so dry. That works well over the winter, ready to separate in spring.
