Rhododendrons are a classification of flowering shrubs that are very popular among gardeners in the United States. Rhododendrons include azaleas, and both types are known for their bright, showy spring flowers. The leaves of rhododendrons provide clues not only to their identification but also about their care and preferred habitat.
There are more than 800 species of rhododendron, so there are few rules about them that are without exception. According to the Alabama Cooperative Extension System, rhododendrons tend to have large, leathery leaves, while azaleas have smaller, thinner leaves. The difference in leaf predicts the shrubs' seasonal behavior.
Deciduous shrubs lose all of their leaves in the fall while evergreen shrubs retain them. During the winter, leaf loss prevents dehydration caused by evaporation of water from the leaves that cannot be replenished from the frozen ground. There are both deciduous and evergreen varieties of rhododendrons, and whether or not they keep their leaves tends to predict whether they can survive cold climates. According to the University of Missouri Extension, deciduous rhododendrons are best for cold climates. Most deciduous rhododendrons are azaleas.
The waxy coating found on evergreen rhododendrons and lacking on deciduous varieties has an important function for the survival of the plant. These coatings provide insulation to the leaf during low temperatures and also limits the amount of water evaporated from the leaves. During cold weather, a waxy coating helps a plant to survive, even if it keeps its leaves. Most azaleas lack this coating and will lose their leaves in the fall as a result.
In a study conducted by researchers at Iowa State University, rhododendron leaves were found to exhibit other traits that help them to adapt to harsh winter conditions. Leaves tend to curl during difficult conditions, protecting them from drying out and damage from the sun. All plant leaves contain small pores called stomata through which water and gases pass, but hardier rhododendrons tend to have fewer of pores, reducing water loss to evaporation.
Rhododendrons provide an example of how leaves serve plants and how leaf adaptations allow plants to survive in challenging environments. Leaves are required for the exchange of gases needed for photosynthesis, but they also result in a lot of water evaporated from the plant. Hardy rhododendrons survive because of leaf adaptations, whether through leaf drop, waxy cuticles or leaf curl. Less hardy species do not have these adaptations.