The best way to deal with clay and alkaline soil is to plant on top of it, not in it. Digging any kind of hole is really a receipe for disaster as it will hold far too much moisture and far too little air.

You will need a depth of 8 to 10" on top of the native soil. The dry powdery peat sold in bales is pretty much useless. What you need is a coarse, highly organic planting mix. Partially decomposed softwood bark is excellent, but check to see what's available in your local area.

Thx mainegrower.

"Digging any kind of hole is really a receipe for disaster as it will hold far too much moisture and far too little air."
That is exactly my thought/fear, especially with that clay layer at the bottom.
Raising some 8"-10" inches though would expose the root system to more frost, Jan & Feb overnight lows can get to -40, we are Zone 3 after all. How deep does the root system go & how far does it spread at a plant size of about 5-6ft growth & 5ft spread?

Rhododendron root are very shallow. Mulch well to protect the roots from temperature changes in the winter. The cold is OK, but rapid changes are not.

The raised be should be larger than you expect the plant to get. For a PJM that might be 2 to 3' out from the plant. The mulch should be about 2" deep.