A rhododendron bush near Deering Pond shows thermonastic leaf response to winter’s cold.
Photo: Kevin McKeon
By Kevin McKeon, Maine Master Naturalist
Many of Sanford’s trails lead us along areas where bushes and ground plants peek above the snow cover, showing that they’re still green. Some of these evergreens have evolved interesting adaptations to survive the cold. One of these adaptations is called thermonasty, and rhododendrons display this effect boldly. When winter’s temperature drops, especially below 20º F, the leaves of these plants will curl in from their margins or edges. The petioles, or stems, will often do the same, causing leaves to droop.
Our native rhododendron, rhododendron maximum — also called great laurel, great rhododendron, rosebay rhododendron, or American rhododendron — has evolved thermonasty to provide survival benefits in this area, close to its most northerly range. These evergreen leaves can be susceptible to damage from various winter conditions, and curling up reduces their surface area, making them less likely to be damaged.
Rhododendrons live in the understory of forests, preferring an acidic, damp, dapple-shaded habitat, but winter brings with it leaf drops from the forest’s hardwood trees, subjecting the understory to winter’s bright sun. The drooping and curling also helps protect rhododendron leaves from burning under winter’s direct sun. Another benefit is that by drooping, the bush is protected from snow accumulating on its dense leaf cover, lowering the risk for branch snapping under the snow’s weight. And since rhodies love dampness — and the cold, winter air holds less water — thermonasty was thought to mitigate water loss, but this theory has come under scrutiny.
Various chemical and physical actions happen within plants to prepare for winter; water maintenance is one of these. To protect plants’ cells from freezing, a type of anti-freeze protein (AFP) is manufactured within the plant. ATF displaces water in the cells, mitigating freeze damage to the cells. The water forced out of the cells into the area around the cells, however, is subject to freezing. Since the cells are more numerous on the top of the leaf than the bottom, the expanding, freezing water causes the leaf to expand more on the top than the bottom, causing the curling. ATF also is the seasonal “flu shot” for the plant: It contains anti-bacterial properties to protect from cold-tolerant fungi. Another specialization in leaves is the stomata — “pores” on the leaf’s surface — that allow the leaf to breathe, exchanging gasses and water. When the plant needs to hold its water, the stomata close, which is what happens in winter.
So, the stomata’s response to the dry air along with the rhododendron’s thick, waxy leaf coating are now thought to serve more of a water maintenance function. This theory prompted the Iowa Agriculture and Home Economics Experiment Station to study thermonastic reactions to cold more closely. It demonstrated that thermonastic plant behavior may be an adaptation to increased light rather than protection from drying. Evergreen plants experience the highest light intensities of the year in the winter, when they aren’t shaded by the forest overstory. The study showed that rhododendron leaves that were not allowed to droop during the winter experienced permanent sun damage. Upon thawing, this resulted in considerably lowered photosynthesis than in leaves that were allowed to droop and curl. One theory is that by drooping and curling, the plants are reducing the amount of direct light hitting them throughout winter, thereby mitigating light damage.
Many folks in Sanford have planted rhododendron along their house foundations, so a walk along Main Street while checking out the many historic sites along The Sanford and Springvale Historic Walks is an easy way to see this effect. There’s also a pair of rhodies at the McKeon Environmental Reserve Trailhead entrance on Blanchard Road. Another awesome sight is a visit to the Native Plant Trust’s five acres of rhododendrons at the Harvey Butler Rhododendron Sanctuary, a 56-acre parcel where a trail-building effort has recently been announced.
Sanford Historic Walk: https://sanfordme.myrec.com/forms/5789_sanford_historic_walk.png
Springvale Historic Walk: https://sanfordme.myrec.com/forms/5788_springvale_historic_walk.png
Rhododendron Sanctuary: https://www.nativeplanttrust.org/visit/native-plant-sanctuaries/
New Trail Building: https://sanfordspringvalenews.com/funding-awarded-for-a-new-rhododendron-trail/
Editor’s note: Did you see something unusual last time you were out in the woods? Were you puzzled or surprised by something you saw? Ask our “Out in the Woods” columnist Kevin McKeon. He’ll be happy to investigate and try to answer your questions. Email him directly at: kpm@metrocast.net
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