I noticed that my rhododendrons leaves are drooping and curled. Is this normal? Is my plant dying? What should I do? Thanks for your help.
All is well. Much like we shiver as a reaction to cold temperatures, Rhododendron species have a reaction to cold temperatures called a thermotropic response. The rhododendrons at Laurelwood Arboretum are exhibiting the same plant behavior.
Here’s what is happening to plants in the winter:
Deciduous plants drop leaves in response to short days and in anticipation of a “drought” situation. When water freezes in the soil, it is unavailable to the plant and produces, technically, drought conditions. Since plants cannot suck up ice cubes, the plants go dormant. Broadleaved evergreens (rhododendrons, azaleas, hollies, Mahonia and Leucothoe, for example) have to find a way to survive freezing temperatures, lack of water, wind, and intense sun. For Rhododendron species, the leaves curl and droop. In fact, this plant reaction indicates cold hardiness in Rhododendron species.
According to Erik Tallak Nelson, Associate Professor of Biology at VPI, Blacksburg, VA, in an article in “Arnoldia” (a publication of Arnold Arboretum, Harvard University), “Rhododendron is one of the few evergreen genera on the East Coast that has the capacity to tolerate frequent freeze and thaw cycles.” Think about it. Most of our temperate, broadleaved evergreens are shade-tolerant and comfortably found under the canopy of other trees. In the winter, when the deciduous trees loss their leaves, there is an increase in the light intensity. The highest radiation of the year is during the coldest weather. How cruel! The evergreens are dormant. The stomata, or pores on the underside of the leaf, are closed so that there is no loss of water in or out of the plant. In addition, water in the soil is frozen. The increased sunlight would stress a plant that had its leaves fully exposed to the sunlight. Leaf edges would then exhibit sunscald, or burning. So leaf drooping and curling act to decrease the quantity of light impinging on the leaf during the coldest temperatures, thereby preventing, or at least limiting photoinhibition.
Leaf cell membranes are susceptible to damage by intense radiation when they are cold. Chloroplasts, the green cells responsible for photosynthesis in plants, have no way of dissipating the chemical energy (oxygen production) because the stomata are closed. Cold temperatures could be lethal if leaves did not curl and droop.
To best preserve tissues at freezing temperatures, the tissue must freeze quickly and thaw out slowly. Researchers found that rhododendron leaves freeze completely at temperatures below 17.6⁰F. The leaves freeze most evenings and thaw in the early morning. Flat leaves thaw out more rapidly than curled leaves because flat leave have more surface area exposed to the warming sun. Curled leaves do not thaw out as fast and are able to avoid much of the damaging effects of daily freeze-thaw cycles. You get the same effect with leafy vegetables and herbs placed in the freezer for preservation. A quick thaw makes the food mushy.
You may wonder if antitranspirants are helpful. These spray-on materials leave a colorless film on the leaf surface that allows the transfer of gases by not water vapor. A hardy rhododendron species or cultivar should not need antitranspirants, but it may be helpful to borderline hardy rhododendrons. Really, the plant knows what it is doing.
The best way to prepare your rhododendron for winter is to keep it as healthy as possible with adequate soil moisture, fertilization, and insect and disease control. For more information on rhododendrons, refer to Rutgers Fact Sheet 1146, “Rhododendrons and Azaleas: Injuries, Diseases and Insect Damage” by Elaine Fogerty and Dr. Edith Wallace.
Don’t miss The Laurelwood Arboretum Rhododendron Festival this coming May 19 and 20. There will be tours led by our Rhododendron Study Team and Dr. Al Fitzburg, noted hybridizer and Past President of Friends of Laurelwood, to the most unusual of our Rhododendron Collection. On Saturday, I will be doing a demonstration about how to best maintain your rhododendrons for a long, healthy life and on Sunday there will be a special presentation by the head of our Rhododendron Study Team, Joe DiGiacomo, about the history, legacy and significance of the amazing Knippenberg and Laurelwood Arboretum rhododendrons.
Enjoy the winter scenery at Laurelwood Arboretum.
Elaine Fogerty, Executive Director