Since rhododendrons (Rhododendron spp.) have fine, silky roots that grow shallow in the soil, they can get fussy about water and nutrients if there is competition from other plants. Nearby trees and groundcovers take up nutrients and available moisture. Perhaps this is a reason for some yellowing leaves.  Remove groundcovers and apply an organic mulch such as pine needles around your shrub, but away from the main trunk--this will help conserve moisture.

For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service