Plants and animals that live in New England have various winter adaptations to aid in survival. Some animals stay warm in underground burrows, while others migrate south to warmer temperatures and more plentiful food. Plants, on the other hand, are rooted in place, and are not afforded the opportunity to find a warmer place to spend the winter. The frigidly cold temperatures of the last couple weeks have allowed me to observe one of the Rhododendron’s adaptations to cold.
This evergreen shrub has thick waxy leaves that are normally open flat and extended horizontally, to maximize photosynthetic potential. As the temperature drops, however, the leaves curl into tight rolls and droop downward into a near-vertical orientation. The colder it is, the more they droop and curl. Based on my observations, the Rhododendrons in my yard generally respond as follows:
- Above 40°F: Leaves are open and horizontal to catch the sun’s rays
- 40 – 34°F: Leaves start to droop, but don’t really start to curl yet
- 33 – 25°F: Leaves droop all the way down and begin to curl
- Below 25°F: Leaves curl up into tight rolls
When plant parts move in response to temperature, these movements are termed thermotropic. The most frequently observed thermotropic movements in plants are in response to hot, dry conditions. This makes Rhododendrons fairly unique because their thermotropic movements are in response to severe cold. Many suggest that drooped, curled leaves can help prevent desiccation during cold periods. As the soil begins to freeze up and water uptake becomes more difficult for roots, cold air above ground can begin to sap moisture from plants. Once rolled, the inner part of the leaf, which lacks the waxy coating of the upper leaf surface, is hidden from the wind. Others suggest that curled leaves protect cell membranes from damage that could result from thawing too fast after freezing. Leaves that are curled tightly and are hanging down vertically will thaw much slower than flat leaves held horizontally. Slower thawing would protects the cells from this freeze damage. Perhaps it’s a combination of the two.
Recent Rhododendron drawing from my perpetual nature journal.