I have an area that is now hard packed gravel/dirt and I want to plant Rhododendrons there. Do I have to remove the gravel area or can it stay with good soil on top? What is the minimum soil depth I can get away with for rhodies? I don't want this to be a deep bed. I'm in zone 5.
I would first loosen and amend the soil in this area so that it is healthy enough to support plants. This article will help with that: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/improving-compacted-soil.htm
This article will help with growing and caring for rhododendrons: https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/rhododendron-care.htm