On a day when the temperature dropped below zero, I noticed that the leaves on a rhododendron bush in our front yard were curled up tight, appearing as green pencils hanging from the bush. After noticing this, I began to check and found that the colder the temperature becomes, the tighter the leathery leaves curl.
This process is called thermotropism, which comes from two Greek words: thermo, meaning measuring temperatures, and tropism, meaning a turning.
Just as the sunflowers in the summer turn their heads toward the sun for warmth, the rhododendron's leaves curl because of the cold. The leathery leaf becomes slightly brittle and a bluish-green color.
of the rhododendron curl up in
for protection, right, but unfurl
when it’s warm.
leaves are curled.
Above, a rhododendron blooms in June.
Note its unfurled
and mild green leaves.
The old mountain people reckoned there was no need for a clock if they had a crowing rooster, and only plumb fools would squander money on store-bought thermometers when they could look at a leaf on a rhododendron bush.
The leaf curls to its own temperature and not to the ambient air temperature. Of course, if the air is cold, the leaves are cold; however, a snow covering will help keep the leaves warm and less curl will be evident.
The curled leaf always will expose the waxy surface of the top of the leaf, while the softer underside receives the protection. The colder the temperature becomes, the tighter the curl of the leaves, and as the temperature warms, the leaves uncurl. The process is the rhododendron's way of survival during the winter's harsh weather.
I asked my forester friend, Jim Lacek, who lives in Towanda, if all green plants photosynthesize during the winter months.
Here is his answer: "I reviewed many books in my library and could not find any direct answer to your question. However, I can state that all plants with green surfaces do photosynthesize whenever there is enough sunlight, air and water to enable the process to occur. Even during the so-called dormant period caused by drought, cold and flooding, plants (with linear leaves); trees (pine, spruce, fir, yew); herbaceous plants (moss and fern); broad leaf plants (laurel, rhododendron and boxwood); and hardwood trees (with young green bark such as boxwood maple and striped maple) do have photosynthetic periods during the so called dormant or winter months."
To paraphrase from Alex Shigo's book, "A New Tree Biology," the cell locked within the wood requires a constant supply of food and water and other essential elements. The cells also must have ways to exhaust waste and gasses. The movement is powered by energy trapped in the process called photosynthesis.
Even when the tree is not growing (dormant) or when the leaves in summer are trapping energy, the living cells still require the constant supply of food, water and energy.
Evergreen trees initially were found in cold climates in the northern hemisphere. Trees in more southern areas enjoyed an extended growing season, with plenty of sunshine for photosynthesis; however, evergreens in the north had a much shorter growing period of warmth.
These trees and bushes had to find ways to continue collecting sunlight all year long in order to survive. They used the chlorophyll found in their leaves (the reason for their green color) in order to absorb and convert sunshine into food, allowing the evergreen plants to remain green all year long, which was necessary to survive.
This also is the reason why leaves (needles) found on conifers, such as spruce and pines, actually are tightly rolled leaves. Although the rhododendron leaves are broader, they are leathery and do curl up during extreme cold weather.
During the winter months, an evergreen tree could be seen with brown needle tips either on one side of the plant, only one or two branches or on the whole tree. The injury, which is found on the outer portion of the branches, is most severe on the side of the tree facing the wind. It is called winter desiccation, a common type of winter injury to spruce and pine trees that occurs during the winter months when the photosynthetic process is slowed.
The evergreen tree continues to lose water at a higher rate (through its needles) than a deciduous tree which has lost its leaves. A warm, sunny or windy day increases the amount of water loss from the needles.
If the soil is frozen or soil moisture is low due to dry conditions, the tree's roots are unable to pick up enough water to meet its needs; therefore, the needles dry out and die. However, they could hold their green color until spring when warmer temperatures arrive, delaying the browning of the needles.
The closer we look at nature, the more we are amazed.
Bower retired after 34 years as a wildlife conservation officer for the state Game Commission. He has published several books about his experiences. Questions and comments may be sent to him at 153 Redington Ave., Troy PA 16947.