Thermonasty for Winter Protection

 

Rhododendron thermonasty

During particularly frigid days, the evergreen leaves of Rhododendron maximum display a unique trait known as thermonasty. In essence, the leaves are a type of natural thermometer. When temperatures dip below freezing, leaf margins curl inward causing the leaf to roll up and droop. Leaves are most tightly curled when temperatures are below 20 degrees.

Thermonasty is a useful adaptation that provides several important benefits. Rhododendron leaves are particularly susceptible to damage from desiccating winter winds—tightly curled leaves ensure surface area is reduced, thereby mitigating this threat. Additionally, during periods of extreme cold leaves are more prone to damage from the sun. Again, a reduced surface area helps to protect the leaves. 

Broad leaves positioned in a way to maximize sunlight collection is a liability when it comes to snow. Narrow, drooping, needle-like leaves help shed snow that could otherwise accumulate and break branches.

Even in late spring, some species of tulips close their flowers in response to cold temperatures at night and open them during warmer days. Similar to tulips, certain crocus species also exhibit thermonastic movements, closing their flowers in cold temperatures and opening them when it is warmer.