The rhodies are alright.

Don’t fret if your rhododendrons seem to be struggling. Those dark, sagging, curled leaves are not a sign of distress. Rather, they are a sign of success.

This evergreen shrub reacts and responds to the wily weather of winter in a way that assures the plant’s survival, and its foliage’s form can be used as a low-temperature thermometer. 

Normally, rhododendron leaves are flat and extend horizontally. When temperatures plummet and range between 20 and 25 degrees, those leaves begin to curl toward their centerline. Once temperatures go below 20 degrees, the leaves are at their maximum curl, becoming vertical, hollow greenery tubes that resemble dark, fat green beans. The plant appears to be on its last legs.

It is quite the opposite. This curling is a brilliant adaptation with a cool designation. The word thermonasty describes nastic, or non-directional, movement of plants that results from changes in temperature. Call these talented plants living thermometers and include in their midst tulips and crocuses, whose flowers also close with cooler temperatures.

In the case of those rhodies, there are a few hypotheses on why the leaves behave as they do. Consider that many of these shade-loving shrubs are enveloped by other trees and woodland species. During most seasons, deciduous trees shadow the rhododendrons and protect them from sun, wind and desiccation.

Once those deciduous neighbors lose their leaves, the rhodies are on their own and become very susceptible to sun scald and photooxidative foliage damage. Their leaf-curling condition protects their greens from the sun’s damaging rays, which now shines on them directly without the cover of a seasonal tree canopy. 

Another benefit is the slowing of freeze and thaw cycles, which damages leaves if occurring too rapidly. And, finally, the curled condition reduces photosynthesis, which is important in winter because these shrubs are in their dormant state.

The botanical world is full of “nasties.” Besides temperature-dependent thermonasty, leaves can also exhibit hydronasty, which is curling due to drought. Photonasty is the movement of plants in response to light and causes flowers to close at night and open in the morning. Dandelions exhibit photonasty. Contrast that to nyctinasty, in which flowers open at night and close in the morning, which describe some varieties of leguminous species like peas and other flowers, including morning glories.

Additional stimulus-inspired movements include chemonasty, thigmonsasty, and epinasty. Sundew plants exhibit chemonasty and close in response to nitrogenous compounds; the Venus fly trap and mimosa are great examples of thigmonasty, being inspired by physical touch of another organism. Epinasty occurs when the heaviness or weight of flowers or buds pulls the top of the plant downward.

In the plant world, the opposite of nastic movements are tropic movements. These are travels in the direction of the stimulus. Plants reaching toward water is hydrotropic movement; toward light, phototropic. And geotropism is when roots grow with gravity.

It is a botanic gym out there with plants stretching, moving and exerting their not-so-free will — and us exercising our observations, hypotheses and understanding to keep up.

Suzan Bellincampi is Islands director for Felix Neck Wildlife Sanctuary in Edgartown and the Nantucket Wildlife Sanctuaries. She is also the author of Martha’s Vineyard: A Field Guide to Island Nature and The Nature of Martha’s Vineyard.