You may have noticed on a particularly cold winter hike that rhododendron (Rhododendron maximum) also seems to huddle up for warmth. The waxy, evergreen leaves are flat and held horizontally for most of the year, but begin to droop at around 32 degrees Fahrenheit. As temperatures drop further, the leaves curl inward, reaching maximum tightness at 20 degrees Fahrenheit.

The explanation for this thermonasty (non-directional plant movement in response to temperature) is a bit complicated, and in fact two phenomena are likely at play. Leaf drooping is counterintuitively a result of the sun as much as the cold! This understory species gets the most sun during winter months when the canopy has shed its leaves. But at cold temperatures chemical processes within chloroplasts (the part of a plant cell where photosynthesis takes place) are not as effective, so a spike in light absorbed would damage cellular tissue that is specialized for conveying electrons rather than storing them. Drooping leaves receive less direct sunlight, reducing cellular damage.

The likely explanation for the curling is also tissue protection, but a more physical one. Frozen leaf tissue can safely thaw for cold-hardy plants, but only if the warming happens slowly. A frigid Appalachian morning can quickly turn balmy in the sun of the winter understory, so rhododendron leaves would benefit from a way to slow down the thawing process;curling up into tight cigar-shaped tubes does this effectively.

Rhododendron is a quintessential Appalachian species. Found predominantly on mesic slopes of mountains, the tough plant relentlessly garnishes our landscape with a deep green all year.

If you see something blooming, leafing out, ripening, or otherwise changing in your woods, send us photos (forestsforthebay@gmail.com) to include in next month’s Forests for the Bay newsletter for more phenological fun!