By Pat Dray
The Garden Spot
Pat Dray
Every morning when I take my dogs outside, I look at my rhododendrons. They look nice and healthy and are budding nicely, but that’s only one reason I look at them. They are natural thermometers and will help you gauge the temperature.
Scientists call this process plant thermosensing. Although there are many things that plants do based on temperature changes, I think the themosensing of rhododendrons is one of the most fascinating since they can quickly change their appearance based on the air temperature.
Plants possess specific genes and proteins which are released based on temperature fluctuations and change the fluidity of the plant cell membranes (and therefore the leaves), making them harder or softer. At higher temperatures, membranes become more fluid, while at lower temperatures, they become more rigid. The end result is nature’s thermometer.
There are a couple of different changes to the leaves that occur at different temperature points to create nature’s thermometer. Start by looking at your rhododendrons and observe the following changes:
The first is leaf curling. One of the most obvious signs that rhododendrons exhibit in response to cold temperatures is leaf curling. As the temperature drops below freezing, the leaves of the rhododendron will begin to curl inward, forming tight rolls. This is a protective mechanism that reduces the surface area exposed to the cold, helping to minimize water loss and frost damage.
Think about it like a person putting their arms around themself to stay warm. The more pronounced the curling, the colder it is. If the leaves are flat and horizontal, it’s above 40 degrees. Right above freezing, the leaves will curl upward. If the leaves are only slightly curled inward, it tells you that the temperature is just below freezing. Tightly curled leaves tell you that it’s between 20-30 degrees.
The second thing to observe is leaf drooping in cold temperatures. When combined with tight leaf curling, drooping tells you that it’s really cold out – below 20 degrees. You’ll notice that the leaves hang limply, almost as if the plant is wilting. The degree of drooping can vary, with more severe drooping suggesting colder temperatures. With the recent single digit temperatures this was very pronounced.
By observing the physical changes in your rhododendron, you can get a good sense of the temperature in your garden. This natural thermometer can help you decide when to take additional measures to protect your plants from cold damage, such as covering them or moving them to a more sheltered location. I also think it’s a fun game to play with your family and friends and helps you notice the wonder that nature is.
Pat Dray is a past president of the Garden Club of Orange and a master gardener.