The small brown hairs on your rhododendron are called indumentum, and they're incredibly helpful
Q: What is the brown, hairy coating that I am seeing on the leaves of my rhododendrons? Does this indicate a problem, and should I be doing something to get rid of it?
What you are seeing is perfectly normal – and, you’ll be interested to hear, it’s a characteristic of rhododendrons that is much sought after by collectors. Called indumentum, it appears on the undersides of the leaves, is usually cinnamon brown in colour and resembles felt. Indumentum is, in fact, a covering of fine hairs or sometimes scales that help to store water and protect the plant from cold and heat. One of the most popular indumentum-rich rhododendrons is the Japanese species R. yakushimanum and its hybrid ‘Ken Janeck’.