Back when I was young and lacking enough sense to research facts before spouting Old Wives Tales, I used to tell everyone that plants grown around classical music would thrive while their twins, who had been exposed to heavy metal, would wither and die. It’s simply not true.
What is true is far more fascinating.
The sound of silence
We expect plants to be quiet. We’re not surprised when a seedling silently reaches for the sun. [Corn and bamboo can be exceptions to that quiet.] No matter how closely we listen, we cannot hear roots as they reach deep into the soil.
You can track down videos and products that translate these biological processes of plants into sounds and music, but that’s not what we’re talking about here. Today’s topic is about the perception and production of actual sounds.
Responding to sounds
Somehow, without brains, ears, eyes, noses, or nervous systems, plants can perceive, respond to, and communicate visual cues, aromas, gravity, and sounds to other plants. [Did you know that even bacteria communicate with one another through sound vibrations? I didn’t either.]
Many plants respond to the sound of caterpillars and other herbivores feeding by producing protective chemicals. Sound waves have also been shown to increase growth rates within some plants. One study found that sound wave treatments of 125 and 250 Hz increased drought tolerance in rice (Jeong, et al 2008). Detecting certain frequencies can also cause plants to produce growth hormones and protect themselves against microbial pathogens.
The technical term for a plant’s ability to perceive and produce sounds is ‘plant bioacoustics’. They can produce sounds in the 10–240 Hz range, as well as ultrasonic acoustic emissions (UAE) within 20–300 kHz. [We hear sounds in the 20 to 20,000 Hz range.] The sound frequencies plants produce vary, depending on the cause behind the need to communicate. These sounds are produced in the xylem, where water and bubbles of air are moved around. Scientists believe that plants produce sounds by manipulating these bubbles, something my fellow barbershop chorus friends will appreciate. In the plant world, this bubbling is called cavitation.
It ends up that plants really do “scream” when they are damaged. Did you know that tomato plants stressed by drought produce an average of 35 sounds each hour? The same plants produce 11 sounds an hour when stressed by stem damage. And those sounds are different, in both volume and frequency, depending on the cause and the species of the affected plant. Apparently, drought stress is grounds for louder communication.
We know that plants can detect neighboring plants by touch. We also know that they use chemical messaging to orchestrate exchanges with soil microorganisms, trading sugars produced through photosynthesis for specific nutrients. Trees and other plants recognize family members and create social networks.
If you had the proper equipment, and many mammals and insects do, you’d be able to hear those tomato plants from up to 16 feet away. It’s much noisier in my garden than I ever realized. And it’s happening underground, too.
It ends up that it takes very little energy to send sound waves through the soil. When root cells are damaged, plants generate sounds that can be detected by neighboring plants, warning them that danger may be at hand. Plants also produce sounds in the 100–300 Hz range that tell their neighbors where good food and water can be found. Your pea plant roots are actively listening for the sound of water in the soil.
Some insects use sound vibrations to make plants release their pollen. This is called buzz pollination or sonication. You can test this with a tuning fork. Depending on the plant species, sound waves between 40 to 1000 Hz cause flowers to expel small doses of pollen into the air. Other flowers detect the frequencies of certain bee wingbeats and respond by producing nectar that contains more sugar.
We still don’t know how plants perceive sound.
But researchers are currently exploring ways of using sound waves to help plants be more productive while protecting them from insect feeding. One study used sound to increase tomato crops by 13%. Wouldn’t it be nice if we could keep aphids, stink bugs, and weeds away while encouraging bigger, sweeter tomatoes? It just might happen. They are also exploring ways of use sound to reduce ethylene production so that produce is less likely to rot on the way to market.
Can plants hear? They sure can. Does talking to them make them grow better? It just might.
Kate Russell, writer, gardener, and so much more.