The Great Rhododendrons (Rhododendron maximum) are easily spotted in the winter by their evergreen leaves. Rhododendron comes from the from Ancient Greek rhódon "rose" and déndron "tree". Now when you walk through the forest in the winter you can look around and say “Look at all the Rose Trees!” The waxy, dark green leaves are alternate, 4-10 inches long and 1-3 inches wide. They are smooth above and wooly/fine-textured beneath. R. maximum are our largest Rhododendrons in the Joyce Kilmer Slickrock Wilderness. They are in the Heath family and love acid soil. The early pioneers called them “Laurels”. They are also called Rosebay and Great Laurels.
It has been my experience in the midst of winter that the leaves of the Great Rhododendron can clue me in on the temperature outside. The colder it is the droopier the leaves become, until instead of being horizontal, they appear almost vertical and rolled up like green cigars. Why do they do that?
This question has been the subject of debate for many years. First it was thought that the Rhododendrons were attempting to protect themselves from winter dehydration by curling their leaves, allowing them to hang down. When cold, dry winds blow across leaf surfaces they take a lot of extra water with them. This theory sounded good until it was discovered that the stomata, the small holes in the surface of the leaves, close up tight during the winter months. If the stomata are closed, no transfer of water can occur. The leaves have a thick, waxy surface that also helps prevent water from evaporating. Plus, the leaves droop and curl a long time before the ground freezes.
A current theory is that the drooping and curling of the leaves are connected with the increased sun to which the leaves are exposed. Evergreens like the Rosebay Rhododendron get more sun in the winter when the overstory is not there to shade them. Once all the leaves fall, their protection is gone and they are exposed to much more sun than they are used to and they can get sun burned!
Another theory is that when living tissues freeze, ice crystals form that can rupture cell membranes. If the frozen leaf thaws too quickly the leaf looks like a piece of wet, wilted lettuce and dies. The best way the plant can prevent this is to (1) freeze the leaf as quickly as it can and 2) thaw out slowly. Researchers found that Rhododendron leaves freeze completely at temperatures below 18 degrees Fahrenheit. The researchers also discovered that flat leaves thaw much more quickly than curled leaves. Since the curled leaves don’t thaw out as fast, they avoid a lot of the damage that the leaf would experience on a daily basis, freezing and thawing.
At this point, what is known is that leaf drooping and leaf curling are two separate behaviors responding to different environmental pressures. There is a lot more research that is going to be needed to learn exactly why the Rhododendron exhibit these really interesting characteristics. In the meantime, it’s really fun to match the outside temperatures with the degree of leaf drooping and leaf curling!
Much of this information can be found in an article which originally appeared in the internationally-recognized podcast and blog, In Defense of Plants, by the notable plant expert Matt Candeias, Ph.D. His book — In Defense of Plants: An Exploration into the Wonder of Plants, is a great reference.