Plant Communication via Ultrasound

For centuries, biology textbooks have described plants as passive, silent organisms. While we knew they responded to light and touch, we assumed they suffered in silence. Recent research has shattered this assumption. Groundbreaking studies have revealed that plants actually emit airborne sounds—specifically ultrasonic “clicks” or “screams”—when they undergo physical stress.

This discovery changes our fundamental understanding of botany. It suggests that a field of corn or a tomato patch is actually a noisy place, filled with information that humans simply cannot hear without technology.

The Discovery at Tel Aviv University

The most significant breakthrough in this field was published in the journal Cell in 2023 by a team of researchers at Tel Aviv University, led by evolutionary biologist Lilach Hadany. The team set out to answer a simple question: Are plants truly silent?

To test this, they placed tomato and tobacco plants in soundproof acoustic boxes and set up ultrasonic microphones capable of recording frequencies between 20 and 150 kilohertz. For context, humans generally hear up to about 20 kilohertz.

The researchers subjected the plants to two specific types of stress:

  • Dehydration: They stopped watering the plants for several days.
  • Physical Injury: They cut the stems of the plants.

The results were immediate and distinct. The stressed plants emitted short, ultrasonic sounds resembling clicks or pops. While a healthy, watered plant remained quiet (emitting less than one sound per hour), the stressed plants were vocal. A dehydrated tomato plant, for example, emitted an average of 35 distinct clicks per hour. When cut, the plants emitted up to 50 clicks per hour.

Decoding the "Screams"

These sounds contain surprisingly specific information. They are not just random noise; they vary based on the plant species and the type of stress the plant is experiencing.

Using machine learning algorithms (AI), the researchers analyzed the recordings. The computer program learned to identify the state of the plant just by the sound it made. The AI could distinguish between a dry plant and a cut plant with approximately 70% accuracy.

Here is what the data revealed about the nature of these sounds:

  • Volume: The pops are surprisingly loud. At a distance of about four inches, the sound pressure is comparable to human conversation volume, though the duration is extremely short.
  • Frequency: The sounds range from 40 to 80 kilohertz. This is high-pitched enough to escape human ears but falls squarely within the hearing range of many animals, including mice, bats, and moths.
  • Timing: The sounds increase as stress increases. For dehydrated plants, the clicking starts slowly, peaks as the plant dries out, and eventually stops when the plant dies.

The Mechanism: How Do Plants Make Sound?

Plants do not have vocal cords, lungs, or a larynx. They are not “screaming” in the way a mammal does to express pain emotionally. The leading theory explains this phenomenon through a physical process called cavitation.

Cavitation occurs inside the xylem, which are the tube-like tissues that transport water and nutrients from the roots to the leaves. Consider the following process:

  1. Tension: When a plant is dehydrated, the water column inside the xylem is under immense tension.
  2. Bubble Formation: This tension can cause air bubbles to form and expand within the water tubes.
  3. Collapse: When these air bubbles expand or collapse violently, they create vibrations.

These vibrations reverberate through the stem and into the air as acoustic waves. While this is a passive physical process, the result is an auditory signal that accurately communicates the plant’s physical state to the surrounding environment.

Who Is Listening?

If humans cannot hear these ultrasonic bursts, does the sound matter? Biologists argue that it matters immensely because other organisms can hear them. The research suggests that these sounds play a role in the wider ecological system.

Animals and Insects

Many creatures that interact with plants have hearing ranges that overlap with these frequencies.

  • Moths: A moth looking for a place to lay its eggs might hear a tomato plant emitting “drought sounds.” The moth might choose to avoid that plant, knowing the leaves will not provide enough water for its larvae.
  • Predators: Mice and bats hunting in dense vegetation might use these sounds to navigate or identify activity.

Other Plants

There is also debate regarding whether other plants can “hear” these signals. Previous studies have shown that plants can respond to the sound of chewing caterpillars by increasing their chemical defenses. It is scientifically plausible that a healthy plant could detect the vibrations from a neighboring “screaming” plant and close its stomata (pores) to conserve water in anticipation of a drought, though this specific reaction requires further testing.

Future Applications in Agriculture

This discovery is not just a biological curiosity; it has massive potential for the farming industry. Currently, farmers rely on visual cues to determine if crops need water. By the time a plant looks drooped or brown, it has already suffered damage and its growth has slowed.

Integrating acoustic sensors into greenhouses or fields could allow for:

  • Precision Irrigation: Sensors could alert an automated watering system the moment plants start “clicking,” ensuring they get water exactly when they need it, but not before.
  • Disease Detection: Since different stresses produce different sound signatures, future sensors might identify specific diseases or pest infestations based on the acoustic patterns.
  • Resource Efficiency: This could save varying amounts of water and fertilizer, optimizing yield by treating plants based on their actual physiological state rather than a set schedule.

Frequently Asked Questions

Do plants feel pain when they make these sounds? While plants emit sounds in response to injury, this does not equate to “pain” in the human or animal sense. Pain requires a nervous system and a brain to process subjective suffering. Plants have neither. The sounds are a physical reaction to stress (like cavitation), not an emotional cry.

Can I hear my houseplant crying if I don’t water it? No. The frequency is too high for the human ear. However, the researchers noted that if you pitched the recording down into the human range, it would sound like rapid popcorn popping or the snapping of bubble wrap.

Did they test plants other than tomatoes and tobacco? Yes. While the primary study focused on tomatoes and tobacco because they are easy to grow in labs, the team also recorded sounds from corn, wheat, grapes, and cacti. They found that these plants also emit ultrasonic sounds when stressed.

Is this the first time plants have been found to make noise? Scientists have used vibration sensors directly on leaves to detect vibrations before. However, this study is distinct because it proved the sounds travel through the air and can be detected by microphones at a distance.