David C. Zlesak
“Oh no, are my rhododendrons dying?” or something similar is a startling thought that comes to mind the first time many of us see this characteristic curling on leaves of our rhododendrons in winter. Fortunately, this is a normal response called thermonasty that actually helps our rhododendrons survive this difficult time of year. This curling and drooping of the foliage is in response to cold temperatures (thermo= heat or temperature and nasty= movement to a stimulus that is non-directional). As temperatures warm and cool during winter we can actually observe rhododendron leaves appearing less or more drooped and curled. As a broadleaf evergreen, the large surface area of rhododendron leaves makes them especially vulnerable to drying out during the winter. With the frozen soil this time of year, additional water cannot easily move up the plant and replace what evaporates from the foliage. Curling and drooping to prevent wind from reaching the undersides of the leaves, where stomates (openings for gas exchange) are typically more concentrated, can help prevent wind from drawing out as much moisture. In addition, many rhododendrons are native understory plants in deciduous forests. During the growing season the plants are shaded by the trees above, but during the winter when plants cannot utilize light well, leaves typically experience more intense sunlight capable of damaging exposed leaf tissue. Curling and drooping also aids the plant by reducing the overall amount of light intercepting a leaf. As we are excited for spring to come by this time of year in Minnesota, it might be fun for us as gardeners to look to our rhododendrons for a light hearted way to predict how much more winter we have left than groundhogs and how afraid they are of their shadows!