East Lansing Rhododendrons: “Baby, It’s Cold Outside”

Wednesday, January 2, 2019, 6:25 am
Aron Sousa

Above: Rhododendron with curling leaves in the author’s yard at -4°C (25° F) yesterday.

Wandering wintertime East Lansing residents are likely to come upon the curled leaves of rhododendrons (tribe: Rhodoreae; genus: Rhododendron) in subfreezing local neighborhoods. When temperatures drop to freezing, the evergreen broadleaves of Rhododendrons begin to curl along their length in response to temperature.

All East Lansing life has to deal with the subfreezing temperatures right now. Some local animals are hibernating, some are submerged in water and breathing through their skin, while others are settled in by a fire with a hot beverage, bingeing on Buffy the Vampire Slayer. We don’t judge; ELi is famously non-partisan.

To withstand winter as species, some East Lansing plants and animals die and leave behind their seeds and eggs. But most East Lansing trees and shrubs are deciduous and deal with winter by losing their leaves and becoming dormant.

While they may look dead, deciduous plants are alive while dormant, as their cells are using sugar to power their systems and keep their roots, trunks, and limbs in good shape for the return of warm weather and partly cloudy skies.

For the evergreen plants of wintertime East Lansing, there is no escaping water-freezing weather, but many use antifreeze-like chemicals to keep their wood and leaves from freezing.

The sugars that plants and animals use to power their cellular structures drops the freezing point of water, just as salt or calcium chloride drops the freezing point of water and melts icy driveways and sidewalks. Overwintering plants and some animals increase the concentration of sugars and some amino acids in their cells and surrounding fluid in order to prevent their water from crystalizing into ice at cold temperatures.

When it does get very cold, even sugar-saturated water will freeze, breaking open cells and the plant’s structures, potentially causing death. Rhododendrons and other evergreens resist freezing while tender annuals like lettuce will freeze solid at 0°C (32° F).

The following time-lapse video shows three sets of leaves:

  • On the far right are rhododendron leaves at room temperature.
  • In the middle is a rhododendron leaf right out of the freezer.
  • On the far left is a green leaf lettuce right out of the freezer.

The thermometer is also right out of the freezer and shows the temperature increasing in degrees Celsius. (Budding scientists and scientists with a thinning crown will note that the mass, specific heat, and thermal conductivity of the glass thermometer and the plants are different, and so the leaves are probably warming faster than the thermometer. Leave me be.)

It is worth mentioning that the lettuce leaf was brittle and completely frozen after only 5 minutes in the freezer at about -10°C (+14° Fahrenheit, but no wind chill). Lettuce has no systems to preventing freezing, so it freezes solid at temperatures very close to 0°C. When the lettuce freezes, the ice crystals rupture the cells that form the stems and leaves and the lettuce wilts when it warmed.

You can watch the lettuce wilt as it warms in the video. Meanwhile the rhododendron leaf was still soft and not frozen out of the freezer. The only real effect on the rhododendron leaf was curling. The leaf then unfurls as its temperature rises to room temperature. The video stops when the leaf has completely unfurled at about 15°C (59°F).

You will notice that the rhododendron leaf begins to unfurl before 0°C. The picture below shows the same leaf on the plant in the yard at -1°C and the leaf is just beginning to droop and curl. Intrepid readers, and middle schoolers looking for a science fair project, can observe their East Lansing rhododendron response to the outside temperature.

It is not completely clear what if any advantage curling in the cold provides to rhododendrons. While it would make sense that curling might reduce leaf dehydration in winter, the literature claims that rhododendrons close their stomata (water pores) in cold weather and have very thick waxy coatings, so desiccation and water stress do not explain leaf curling in rhododendrons in the winter.

Rhododendrons also curl their leaves sometimes in the summer, but that’s because of dehydration, not because of temperature. The curling in summer helps minimize water loss (and signals that you should probably water your rhododendron).

One of the preferred explanations among scientists for the curling of wintery rhododendrons is that the leaf-uncurling may slow the warming of the rhododendron leaves, thereby helping prevent cell fracture as the leaves warm. That theory has not been well proven experimentally. But it is interesting that it changes the question from “Why would curling help cooling rhododendrons?” to “Why would uncurling help warming rhododendrons?”

Another interesting theory for the winter curling of rhododendron leaves is that too much photosynthesis in cold weather can produce more energy (and toxic byproducts) than cold leaves can manage. Curling might limit the amount of light shining on the leaf and thereby limit photosynthesis.

Regardless of the reason for the curling, eagle-eyed readers should see East Lansing rhododendron leaves reliably begin to curl at about the freezing point of water and reach their maximum curl at about -10°C (14°F). That makes rhododendrons a handy way to look out the window for a sense of how cold it is outside.