Plants have been found to make sounds that are inaudible to the human ear, according to a new study. These noises become more pronounced when the plants are thirsty or under other forms of stress. This research challenges the long-held belief that plants are silent, suggesting instead that the natural world may be filled with the sounds of plants, as study coauthor Lilach Hadany explains.
Hadany, a professor at Tel Aviv University, has long suspected that plants weren’t entirely mute. “There are so many organisms that respond to sound, so I thought there was no good reason for plants to be deaf and mute,” she said.
Her interest in plant sounds began six years ago when she recorded a cactus in her lab using an ultrasonic microphone, although she couldn’t be sure the sound came from the plant itself. Previous studies had noted plant vibrations, but it was unclear if these vibrations became audible sound waves.
To determine whether plants truly made sound, Hadany and her team built soundproof acoustic boxes and placed tobacco and tomato plants inside. These boxes were equipped with ultrasonic microphones that could record sounds at frequencies between 20 and 250 kilohertz—far beyond the range of human hearing (which tops out at about 16 kilohertz). Some plants were stressed—either by having their stems cut or by going without water for five days—while others remained untouched.
The results revealed that the plants emitted sounds within a range of 40 to 80 kilohertz, which, when translated into frequencies humans could hear, resembled the popping of popcorn or the sound of bubble wrap being popped. Stressed plants produced around 30 to 50 sounds per hour at irregular intervals, while unstressed plants were much quieter, making only about one sound per hour.
“When tomatoes are not stressed, they are very quiet,” Hadany remarked.
Are the Plants Communicating?
While the discovery of these sounds is intriguing, the researchers are cautious about interpreting them as a form of communication. They believe that the sounds are likely produced through cavitation, a process in which air bubbles in the plant’s water column collapse under pressure, causing a popping or clicking noise. There is no evidence to suggest that these sounds are intentional or a form of communication between plants.
Richard Karban, a professor of entomology at the University of California, Davis, who was not involved in the study, emphasized that the sounds should not be interpreted as plant communication. “This result adds to our understanding of how plants respond to stress, but it doesn’t show that plants are actively communicating through sound,” he explained.
Despite this, other organisms may be able to use these sounds to their advantage. For instance, sensory ecologist Daniel Robert from the University of Bristol suggests that moths might interpret the sounds to determine if a stressed tomato plant is a suitable place to lay eggs or feed.
“Many sounds are not ‘intentional’ signals, but they can still be heard and used by other organisms for their benefit,” Robert noted. “The concept of communication is complex, and it raises the question of whether communication needs to be bi-directional.”
Who’s Listening?
In a follow-up experiment conducted in a noisier greenhouse, the team used machine learning to analyze the plant sounds. Their algorithm was able to distinguish between stressed and unstressed plants, as well as those that were thirsty or had been cut.
“This technique is exciting because it can rapidly identify patterns that are not immediately obvious to the human ear or eye,” Robert added.
While the study focused on tobacco and tomato plants due to their ease of cultivation, the team also recorded sounds from other plant species, including wheat, corn, cacti, and grapevines. They found that these plants also emitted more sounds when under stress.
In addition to insects and mammals that might hear and respond to these plant sounds, Hadany speculates that other plants might also be listening in. Previous research by her team has shown that plants can increase the sugar content in their nectar when they “hear” the sounds made by pollinators.
Hadany now sees plants in a new light. “There are many songs we can’t hear,” she said.