Rhododendron Leaves: Living Thermometers

 January 2015

You know it’s cold when…the leaves of the rhododendrons hang down and curl inward. 

Dr. Jennifer Frick-Ruppert, our neighbor at Balsam Grove and the chair of the Division of Science and Mathematics at Brevard College, refers to rhododendron leaves as “living thermometers.” When the temperature is above freezing, rhododendron leaves stick out from the stems of the shrub at just lower than right angles. At just below freezing, about 28 degrees, the leaves begin to droop noticeably but are still flat. At about 23 – 25 degrees, rhododendron leaves begin to curl in, and by 20 degrees, they are pointed straight down and curled as tightly as cigars. 

Here is a photograph of rhododendron leaves I saw on a walk during the first week in January. When I saw that the rhododendron leaves were hanging down and curling in, but not completely tight, I knew it was probably around 23 – 25 degrees, and when I checked at home, it was indeed 23 degrees.

The curled leaves prevent water loss from the underside of the leaves, both during extreme cold and during a drought. Only some rhododendrons keep their leaves in the winter; the species that we have at Richland Ridge does: the rosebay or great laurel (Rhododendron maximum). Rosebay rhododendrons are quite common throughout the Southern Appalachian mountains, and are usually found on moist shady hillsides and along woodland streams. It is one of the most hardy, and most adaptable, rhododendron in its species, Elepidote, and one of only two in this species found in the Eastern United States. Note that there is a bud at the end of one plant’s stem — this bud will easily survive through the winter and bloom in the late spring.  

Rhododendrons are part of the Ericaceae or Heath Family along with mountain laurel, azaleas, blueberries, huckleberries, dog hobble, sourwood, trailing arbutus, wintergreen, and Indian pipe. This family of plants grows on acidic and infertile soil and is generally stress tolerant. Usually mycorrhiza, the symbiotic association of the mycelium of a fungus with a plant, supports the shrubs in the Heath Family by helping the roots absorb water and minerals in return for essential carbohydrates from the plant. Native rhododendrons often grow with oaks in what is called an oak-heath forest. Both rosebay rhododendrons and their cousin, mountain laurels (same family, different genus) grow profusely in the acidic and previously logged soil of Richland Ridge. It is easy to distinguish rhododendrons from mountain laurel in the winter, because only rhododendrons  drop and curl their leaves tightly in subfreezing temperatures.

The following photograph is of both a mountain laurel and a rhododendron; the rhododendron leaves are curled inward, but the mountain laurel leaves are not.

Rhododendrons, as well as mountain laurel and dog hobble, can form tall, dense thickets, sometimes called “laurel hells,” which make hiking through them challenging.  They can grow as tall as trees in the Southern Appalachians, and the wood is frequently used for fences, porch railings, and rustic furniture. Plants in the Heath Family are often among the first to grow in an area that has been burned and/or extensively logged, but the dense shade from mature rhododendron and mountain laurel thickets reduces woodland diversity beneath them and can inhibit forest regeneration. Rhododendrons are not very nutritious plants for browsing animals, and the woody parts of the plants are dry, so they can present a fire hazard during droughts.

On the plus side, as well as being a “living thermometer” in cold weather, both rhododendrons and mountain laurels provide lovely splashes of green in the winter and exquisite blossoms in the spring.

    by Linda Martinson   January 10, 2015  

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