Great Rhododendron

Rhododendron leaves are thermonastic, which means that they curl and droop in direct response to temperature. Above 40 degrees (F), they are flat and horizontal, but once the temperature dips below freezing, they start to curl up. Isn't that cool? 

Scientists believe this might be a photo-protective strategy, because when leaves are really cold, they are worse at photosynthesis, and by curling the leaves, the rhododendrons are preventing a surplus of solar radiation. This curl response happens when cells on the upper and lower surfaces of the leaf expand and contract at different rates. 

I love rhododendron. They are beautiful all year, but they are also an aggressive competitor to many other species across Appalachia, and have contributed (alongside us humans) to accelerating the loss of the American chestnut in the early 20th century, and the Eastern hemlock more recently. This one in the watercolor is planted in Shadyside, PA, outside its typical range, but because it is so hardy, it is doing just fine. In the summer when it flowers (i.e. soon, since I am writing about this watercolor from Jan 1 now in April, and summer is nearly upon us), it is really very beautiful. Perhaps I'll paint it in that condition, too so you can see the difference in the leaf curling and how it looks when it is in bloom. Keep an eye out!