How rhododendrons can tell the temperature

Rhododendrons at reporter Monique Singh-Roy's East Quogue home. (Credit: Monique Singh-Roy)

Rhododendrons at reporter Monique Singh-Roy’s East Quogue home. (Credit: Monique Singh-Roy)

Editor’s note: This post was published on January 12, 2016. The post originally appeared on the writer’s blog, YourPotLuck.com.

What’s the temperature outside? That’s a question all of us have asked our friends or family members at some point these past frigid weeks. It’s the first thing we want to know when we wake up and probably the last thing we want to know before heading to bed.

With so many outlets to access current temperatures at our fingertips, I had to do a double take the other day when my husband wanted to know the outdoor temperature and asked me to check the rhododendrons.

I was gazing out the window one particularly frigid morning when he asked me, “What are the rhododendrons doing?”

“What are the what doing?” I asked.

“What are the rhododendrons doing?” he repeated himself. “You can tell how cold it is by how tightly the rhododendrons are curled up.”

Can a plant really reveal the temperature? My husband, who majored in horticulture at SUNY Farmingdale, says their leaves are a fairly accurate way to predict how cold it is without having to step outside.

I’m not particularly fond of the plant myself. I find the stark bush, with its stringy leaves, unattractive year round, and the flowering period during early summer is woefully short.

But my husband likes them and our East Quogue property has several rhododendron plants. One large bush sits right outside our kitchen windows, so I immediately looked down at it.

The leaves were curled up so tightly into themselves, they looked almost like long, green cigars. “They’re pretty tightly closed,” I answered.

Rhododendron leaves.

Rhododendron leaves. (Credit: Monique Singh-Roy)

My husband joined me at the window and examined the plant.

“I would say it’s about 26 degrees outside,” he estimated.

I was now interested in testing his theory, so I turned on the television turning to our local 24-hour news channel which always shows the temperature in the lower right screen. It said 24 degrees — two degrees off from the rhododendron’s reading.

I was impressed. As a former city dweller, the closest thing I had to an outdoor barometer was looking out my apartment window to see what people were wearing as they passed by on the street.

The process that causes rhododendrons to curl up is called “thermotropism” and is believed to give the plant certain survival advantages under harsh conditions. If the leaves are open and laying flat, that usually means above-freezing temperatures.

If the leaves are curled in, then it’s below freezing. If the leaves are really curled tighter, then the temperature is dropping even more.

Now, results may not be as accurate if the plant sits in the sun or away from the wind.

So far, I haven’t learned of another plant that can reveal the temperature as closely as the rhododendron. If I do, maybe we’ll re-plant.

A rhododendron in bloom. (Credit: Monique Singh-Roy)

A rhododendron in bloom. (Credit: Monique Singh-Roy)

One Comment

  • Glad to see that the Farmingdale State College horticulture education still pays off. Please ask your husband to visit the campus if he hasn’t done so lately. The gardens look great and we have beautiful new buildings – a Campus Center and School of Business building.
    Happy New Year.