I live in lower MI and purchased 3 Inv. Ruby hydrangea's - upon planting around a new patio built on a hillside I found that there is no soil where I had hoped to plant one of them. There is 2" to 3" of mulch and under that is what looks to be a limestone smaller rock base. I need to know if I pull the mulch back, and place soil on top of it, how deep the soil would need to be to support this type of hydrangea? I would then re mulch the area after the soil application. Is it worth the effort to put soit there in the first place? I cannot find how deep the roots will grow, nor if the lime stone base will affect growth / bloom. If anyone has suggestions / information I would really appreciate the input.

Hydrangea root growth is tiny and shallow, can be variable so plan for roots around 2 feet deep or less. Consider using raised beds in caliche soil or in the various sedimentary rock formations prevalent in the western and central parts of the Lower Peninsula. You could also try growing them in half wine barrels with well draining, ericaceous potting mix. In spring, monitor for suckers and cut them off if the original planting starts to get "too wide".