Stressed Plants Click in Frequencies We Can't Hear

Lilach Hadany's lab at Tel Aviv University put tomato and tobacco plants in soundproof boxes, pointed ultrasonic microphones at them, and waited. The plants clicked. A well-watered tomato emitted fewer than one click per hour. A plant going five days without water cranked that up to 30–50 clicks an hour, all in the 40–80 kHz band — well above the 20 kHz ceiling of human hearing.

The sounds also showed up before any wilting was visible. By the time the plant looked sick to a person, it had been broadcasting that fact for days.

A trained machine-learning model could tell drought-stressed plants from cut-stemmed plants by the click pattern alone, and could even tell tomato from tobacco. The work, published in Cell in March 2023, was the first to record airborne sound from intact plants in open air rather than through contact sensors taped to the stem.

The likely physical cause is cavitation: under drought stress, the columns of water inside a plant's xylem vessels snap, and the collapsing air gap radiates a tiny shock. Plants weren't evolved to make these sounds on purpose. They're a side effect of plumbing under tension.

Which is the part that opens the question. Many moths, mice and bats hear well into the 40–80 kHz range. A moth choosing where to lay eggs, or a mouse choosing what to chew on, would be walking through a forest of audible distress signals it has presumably been hearing the whole time. Whether anything is actually listening is the next experiment.