Can you tell how cold it is in winter by looking outdoors at your rhododendron? Locals in New Hampshire tell me that a quick glance out the window will indicate whether the temperature has dropped to 32° or not. When the temperatures drop to freezing, the normally horizontal rhododendron leaves begin to droop and curl.
The amount of droop and curl does correlate to the severity of winter temperature. The lower the temperature, the tighter the curl. At 20° they are curled as tight as they can possibly get. Our rhododendron leaves are drooped and tightly curled right now and that’s a clue to the frigid outdoor temperatures…. a -8° at daybreak and currently a -3°.
But why do the rhododendron leaves droop and curl in the first place? Theories and debates abound. Some say it is to prevent branch damage from the snow load. Others theorize it helps prevent or reduce water loss in the leaves, although horticulturists and scientists dismiss this theory because the openings on the underside of the leaf are closed during the winter.
A likely reason is drooping and curling prevents rapid freezing and thawing of the leaves. If the leaves are horizontal as they are in warm months, thawing may occur on a sunny day in winter, then the leaves may quickly freeze again overnight. This quick freezing and thawing could destroy leaf cells. So possibly, the drooping and curling would be nature’s way to protect leaves from the thawing solar rays during the day. They are better off staying frozen until they can thaw slowly.
More study is needed to answer all the rhododendron leaf questions but I’m just happy to know I can rely on these magnificent shrubs to let me know when the thermometer hits 32°.