I have some native rhododendrons that have tan or reddish leaves every spring. The red leaves are new leaves, and they are red for about two weeks. They are under some 100 year old Douglas fir trees, and get some shade and some southern light. They were moved to my garden from and altitude of 4000 feet to my altitude of about 1850 feet. What causes the red leaves?
New red shoots and bronze leaves are not uncommon in Rhododendrons. In fact, some are grown purposely for their colorful new growth. The creator of Paghat’s Garden, a local website writes enthusiastically of a Rhododendron ‘Hill’s Bright Red’:
“When it is first finishing with its bright red blossoms, it begins producing new leaves, that are at first bright red themselves! …The new red leaves mature to green..”
She also has written specifically about the Pacific rhododendron, Rhododendron macrophyllum.
As to why this coloration sometimes happens, it is possible that the reddish-brown pigment seen in young [Rhododendron] leaves
affords tender foliage some protection from sunburn. This is important because too much ultraviolet radiation can destroy chlorophyll in plant
leaves. Plants avoid getting too much UV radiation by producing red pigments called anthocyanins, which can absorb UV radiation and thus prevent it from damaging other parts of the leaf. Because
the pigment “accumulates in the cell sap, and is usually located in the
epidermal and subepiderma cells,” its presence vividly changes the color of the
leaves (Levitt, p. 294). This, and more information about anthocyanins is available in a book available at the Miller Library called Responses of Plants to Environmental Stresses, volume 2, by J. Levitt (New York:
Academic Press, 1980). In effect, your plants are producing their own sunscreen for the tender new
growth!
You can also find more information about anthocyanin in the article “Nature’s Swiss Army Knife: The diverse protective roles of anthocyanins in leaves,” by Kevin S.
Gould, hosted by PubMed Central.