Well, I would use something medium coarse (1-2 cm diameter grains) and without any mineral content. That should keep the roots cool and moist, and gradually improve the soil structure, without measurably raising its height. Finer textured mulches, such as peat, can reduce the air flow to the roots, while coarser mulches provide too friendly an environment for pest bugs--including termites! Any organic mulch, minus lime, will gradually reduce the pH.

Tug , Good reply But do you see what i mean they say feed them every spring with Ericacious compost or whatever mulch! Bury the roots deeper?

most of the mulch material degrades and gets worked down into the soil. I have been mulching a cammelia now for 15 yrs and by 3months has passed the top of the roots are visible again. The soil quality is getting better but not really any deeper.

Japon unless your rhododendrons are planted in containers they do not need feeding every year the natural leaf fall and other detritus will do fine. There is a difference between planting the rhododendron too deeply in the ground and adding a light top dressing of, say, leaf mould, which will be absorbed into the soil over a period of a few months.

Thank you.

I just spread garden compost round my rhodos each spring. As MG says, it is absorbed into the soil over a period of a few months.