Rhododendrons: Rhododendren leaves on bottom turning yellow

I live in Charlotte, NC area and have miniature Rhodo's in area surrounded by carport walls and house. It gets 5 or 6 hrs direct sun. Lower leaves turning yellow. New growth remains green. Is this from lack of water, too much sun, or combination of both???

 

Rhododendrons and other broadleaf evergreens shed old leaves every year, just as deciduous trees and shrubs do. However, if the amount of leaves lost appears excessive then there may be a problem such as dry or wet soil. The amount of foliage density maintained through winter does vary with the cultivar, compare yours with other specimens of same kind to help you evaluate them.

 

Ah,

Reason why I always have a lot of leaves under the Rhodi then, thanks Ron! There is a defiency beginning with C that can cause leaf yellowing.

I have just bought a new white mini Rhodi.