- The larger the leaf, the more sunlight it can gather, but the more water it will lose to drying winds. Tropical rhododendrons that live in shady, humid forests need to gather as much sunlight as possible for photosynthesis and have developed very large leaves, 20 inches or more in length. Other types of rhododendron that grow in colder climates have smaller leaves, 7 or 8 inches long, that are still capable of using all light available in partly shady areas. Some rhododendrons, however, live in alpine areas in full sun and have small leaves, about an inch long, on low-growing plants that can survive snow and high winds.
- Many rhododendrons have evergreen leaves that last for three or four years rather than dropping each autumn. These have thick, sometimes waxy surfaces that protect the inner cells from drying out. They may also be slightly rolled downward to protect the undersides. Deciduous rhododendrons have delicate leaves without a tough protective surface. Since they are renewed every spring, there is no need to put extra energy into protecting the inner tissue.
- Rhododendrons often have hairs that cover the surface, providing even more protection from dry air. In some varieties, the hairs have been reduced to scales that cover the undersides of the leaves. In others, these hairs have developed into a felt-like covering called indumentum that may be white, tan or brown. The coating of indumentum is often thicker on the undersides of the leaves than the top, where it would interfere with photosynthesis.
- Deciduous species of rhododendrons have thin, soft leaves that are shed each fall. This adaptation allows the plant to survive the winter without the stress of water loss through the leaves; consequently, these species are often more hardy than evergreen types. Rhododendron schlippenbachii, for instance, is hardy to -30 degrees Fahrenheit.
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