Dying plants are ‘screaming’ at you

While plants can’t chat like people, they don’t just sit in restful silence. Under certain conditions—such as a lack of water or physical damage—plants vibrate and emit sound waves. Typically, those waves are too high-pitched for the human ear and go unnoticed.

But biologists can now hear those sound waves from a distance. Lilach Hadany, a biologist at Tel Aviv University in Israel, and her colleagues even managed to record them. They published their work in the journal Cell today.

Hadany and colleagues’ work is part of a niche but budding field called “plant bioacoustics.” While scientists know plants aren’t just inert decorations in the ecological backdrop— they interact with their surroundings, like releasing chemicals as a defense mechanism—researchers don’t exactly know how plants respond to and produce sounds. Not only could solving this mystery give farmers a new way of tending to their plants, but it might also unlock something wondrous: Plants have senses in a way we never realized.

It’s established that “the sounds emitted by plants are much more prominent after some kind of stress,” says František Baluška, a plant bioacoustics researcher at Bonn University in Germany who wasn’t a part of the new study. But past plant bioacoustics experiments had to listen to plants at a very close distance to measure vibrations. Meanwhile, Hadany and her colleagues managed to pick up plant sounds from across a room.

[Related on PopSci+: Biohacked cyborg plants may help prevent environmental disaster]

The study team first tested out their ideas on tomato and tobacco plants. Some plants were watered regularly, while others were neglected for days—a process that simulated drought-like conditions. Finally, the most unfortunate plants were severed from their roots.

Plants under idyllic conditions seemed to thrive. But the damaged and dehydrated plants did something peculiar: They emitted clicking sounds once every few minutes. 

Of course, if you were to walk through a drought-stricken tomato grove with a machete, chopping every vine you see, you wouldn’t hear a chorus of distressed plants. The plants emit sounds in ultrasound: frequencies too high for the human ear to hear. That’s part of why researchers have only now perceived these clicks.
“Not everybody has the equipment to do ultrasound [or] has the mind to look into these broader frequencies,” says ecologist Daniel Robert, a professor at the University of Bristol in the United Kingdom who wasn’t an author of the paper.

The researchers were able to record similar sounds in other plants deprived of water, including wheat, maize, wine grapes, pincushion cactus, and henbit (a common spring weed in the Northern Hemisphere). 

Biologists think the clicks might come from xylem, the “piping” that transports water and nutrients through a plant. Pressure differences cause air bubbles to enter the fluid. The bubbles grow until they pop—and the burst is the noise picked up by scientists. This process is called cavitation. 

Most people who study cavitation aren’t biologists; they’re typically physicists and engineers. For them, cavitation is often a nuisance. Bursting bubbles can damage pumps, propellers, hydraulic turbines, and other devices that do their work underwater. But, on the other hand, we can put cavitation to work for us: for instance, in ultrasound jewelry cleaners.

Although it’s known cavitation occurs in plants under certain conditions, like when they’re dehydrated, scientists aren’t sure that this process can entirely explain the plant sounds they hear. “There might not be only one mechanism,” says Robert.

The authors speculate that their work could eventually help plant growers, who could listen from a distance and monitor the plants in their greenhouse. To support this potential future,  Hadany and her colleagues trained a machine learning model to break down the sound waves and discern what stress caused a particular sound. Instead of being surprised by wilted greens, this type of tech could give horticulturists a heads-up.

[Related: How to water your plants less but still keep them happy]

Robert suspects that—unlike people—animals might already be able to hear plant sounds. Insects searching for landing spots or places to lay their eggs, for instance, might pick and choose plants by listening in and selecting a plant based on their health.

If there is an observable quality like sound (or light or electric fields) in the wild, then some organisms will evolve to use it, explains Robert. “This is why we have ears,” he says

If that’s the case, perhaps it can work the other way—plants may also respond to sounds. Scientists like Baluška have already shown that plants can “hear” external sounds. For example, research suggests some leaf trichomes react to vibrations from worms chewing on them. And in the laboratory, researchers have seen some plants’ root tips grow through the soil in the direction of incoming sounds.

If that’s the case, some biologists think plants may have more sophisticated “senses” than we perhaps believed.

“Plants definitely must be aware of what is around because they must react every second because the environment is changing all the time,” says Baluška. “They must be able to, somehow, understand the environment.”