Do Plants Have Consciousness?

Cleve Backster was a lie-detection expert who trained CIA interrogators. He spent his career reading the signals of human deception.

One February night in 1966, out of curiosity, he clipped the electrodes of a polygraph machine to a plant and watered it. To his surprise, the plant responded.

The electrical signature was similar to what he had seen countless times before, resembling how a person would feel when they are happy or excited. The plant was apparently happy after being watered.

Backster considered burning a leaf to see the plant’s reaction. Even before he struck a match, however, the polygraph needle jolted across the page, tracing a line similar to the human response to fear or stress.

Backster wanted to run out into the street yelling, “Plants can think!”

The experiments were recorded in his book, “Primary Perception: Biocommunication with Plants, Living Foods and Human Cells.” Backster called the phenomenon he observed “primary perception”—the idea that awareness extends beyond the nervous system and that some form of awareness is present even at the cellular level. He believed that all plants and animals exhibit the “presence of a yet undefined perception” at microscopic levels.

The prevailing view among scientists contradicted his claim that plants are conscious. Backster faced intense backlash for his work. Experiments failed to yield reproducible results in subsequent testing.

However, the rejection didn’t conclude that plants were inert. Rather, science continued to focus on more modest phenomena, something that it could recognize, verify, and reproduce.

Communicating Without Words

While plants lack neurons, they still readily communicate, including relaying danger cues.

When a caterpillar bites a plant, it releases a molecule known as glutamate—the same neurotransmitter used in humans’ nervous system. Glutamate triggers wound-related signaling throughout the plant. Its release primes undamaged parts for defense against pests.

Plants also transmit external alerts. When the caterpillar begins to chew on a leaf, the plant actively emits chemicals into the air. Neighboring plants catch these volatile compounds and begin preparing their defenses.

In 1983, David Rhoades was the first researcher to document plants’ airborne signaling. However, he faced criticism and opposition, largely because it was an unrecognized and unverified phenomenon, and partly because it sounded fantastical. However, over time, his theory was tested, verified, and replicated in more than thirty species.

In 2023, a Japanese team filmed a healthy plant detecting the airborne chemicals of a damaged neighbor. Within seconds, the plant lit up internally, emitting volatile chemicals. The plant was sending alarms from one leaf to the rest of its body, while nearby plants detected messenger molecules emitted during the process.

Additional experiments have demonstrated that plants can communicate not only via chemicals but also through electrical signals. In short, plants communicate and “talk” to each other more than we realize.

Making Decisions and Emitting Sounds

Some plants also appear to have cognition. The Venus flytrap doesn’t snap the second you touch it. It waits, analyzes, and counts before shutting down the lid.

Inside the trap are tiny trigger hairs. When touched once, nothing happens. A second touch within 15 to 20 seconds causes it to snap shut.

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Even with the trap closed, the plant doesn’t start digesting immediately—it waits to confirm if it caught something alive. The plant tracks movement inside the trap as the insect struggles, repeatedly activating the trigger hairs. Once these hairs are stimulated about five times, the plant decides to activate enzymes to digest its prey.

The Venus flytrap effectively keeps a count and uses that information to make a real decision, despite having no brain to perform this task.

What else can you do once you know how to count? Gamble.

In one experiment, researchers divided a single pea plant’s roots between two pots. One pot had a constant supply of nutrients. The other had periods of higher and lower nutrient availability, though it averaged to the same amount.

Researchers tested whether peas preferred a stable or a less predictable resource by observing how the plant invested in its root growth. When the pot with a constant nutrient supply offered enough for survival, the plants played it safe and directed more root growth toward that dependable source. But when that reliable supply was too low for survival, the plant took a risk and shifted toward the pot with an unpredictable nutrient supply—effectively favoring a risky possibility of obtaining enough resources over the certainty of scarcity.

Researchers concluded that plants have “risk sensitivity”—this is logic similar to that an investor uses.

Research has found that plants can also be vocal and whine when stressed. So a woodland quiet to us may be loud to many other life forms.

A 2023 study documented that a dehydrated or injured plant can emit clicking sounds. These clicks are ultrasonic, travel through air, and can be recorded using microphones. For instance, the researchers found that a poorly watered tomato plant made clicking sounds about 35 times an hour. While a happy one was basically silent.

The emitted sounds were specific enough that a machine-learning model could use them to determine whether a plant was dried out or cut.

The authors suggested that the sound may come from cavitation, in which bubbles form and burst within the plant’s water-transport system. Even though humans can’t hear these sounds, they are loud enough for insects and some other animals. For example, early evidence suggests that a moth deciding where to lay her eggs listens, in order to choose a healthy plant over a stressed one.

To us humans, a forest might sound silent, but to other ears, it may be quite loud.

Memory Without a Brain

The sensitive plant Mimosa pudica is known for its tendency to fold its leaves when touched. However, in a 2014 study, researcher Monica Gagliano and her team found that the plant could learn not to react.

Her team dropped a potted Mimosa a short distance onto a cushion, producing a mild but harmless disturbance. At first, the Mimosa folded its leaves after each drop. After several drops, however, the response diminished, and the plant stopped reacting, suggesting that it had learned to stop expending energy on a harmless stimulus.

However, when exposed to a different, unfamiliar stimulus, the plant folded again. Moreover, when the plant was tested weeks later, the effect persisted, suggesting a form of retained memory lasting nearly a month.

The authors conclude that: “the process of remembering may not require the conventional neural networks and pathways of animals; brains and neurons are just one possible, undeniably sophisticated, solution, but they may not be a necessary requirement for learning.”

Gagliano’s team conducted another experiment in 2016, modeled on Pavlov’s classic conditioning studies, in which dogs learned to associate a bell with food. The researchers attempted to train pea plants by pairing a fan with a light—the breeze signaled the direction from which light was expected. The outcome was that the plants learned this association and grew toward the breeze in anticipation of light that had not yet appeared.

The findings on memory have been more robust than those on associative learning. However, the failure to reproduce such results does not establish that plants are inert or lack consciousness. It simply means that some of these findings are not yet conclusive evidence and require a fundamentally different approach to understand what Backster sensed in 1966.

What If ‘Mind’ Is Too Small a Word?

Disagreement in this field is partly empirical but also conceptual.

Few researchers would dispute that plants sense their surroundings, communicate chemically, transmit electrical signals, and adapt and develop according to changing conditions. The dispute begins when those capacities are described as “intelligence” or “consciousness.”

In 2006, a group of researchers tried to launch a field called “plant neurobiology,” and the backlash was so intense that they ended up changing the name, because the objection was hard to argue with: plants have no neurons—and by inference, some botanists argue, can have no intelligence or consciousness.

Because of the slippery definitions, some scientists argue that the field of plant intelligence shouldn’t even be considered a scientific study. “As scientists, we must only deal with that which is testable and falsifiable, and intelligence, with a subjective definition, is neither,” Daniel Chamovitz, a geneticist, wrote in an essay.

Stefano Mancuso suggests a contrasting interpretation. He argues intelligence need not be centralized in a brain. He views the plant root system as a decentralized network, with many growing tips sensing chemicals, moisture, gravity, and obstacles in a coordinated way to make decisions and solve problems. Mancuso said that plants have a different physical setup, but the underlying strategy may be the same.

The skeptics somewhat argue in a circle. If consciousness is defined as something that brains produce, then concluding that brainless organisms don’t possess it simply restates the definition. Perhaps we don’t need to define the term so narrowly nor define it from scratch. Let’s look at the past.

Between Machine and Mind

Historical ideas may help clarify, if not settle, the modern debate.

Around 2,400 years ago, Aristotle didn’t view plants as conscious creatures, but he also didn’t treat them as inert, lifeless objects. He called them “threptikē psychē”—a vegetative soul—neither mind nor feelings, but an inner principle of growth and survival.

Classical Chinese thought offers a parallel in ziran (自然), often translated into English as “nature,” but really meaning something closer to “self-so”: the way something unfolds from a being’s own nature without requiring a conscious agent behind it. A root finding water or a leaf reaching for light—the action is its own, the agency is there in the unfolding.

We gradually lost that middle category between “mechanical object” and “conscious mind,” and the debate over plants continues.

It’s not necessarily that ancient philosophers proved plants are conscious. As humans, we have an intuitive gut sense that we need a brain in order to have an inner sense of our own belief and not something that’s ever been tested.

Backster spent the rest of his life convinced that plants have a secret inner life. The science that followed complicated some of his findings. Yet he was right to be astonished. He looked at the houseplant nobody noticed and asked whether anyone was home.

The honest answer is that we still don’t fully know. Plants may have no minds, or they may possess the kind that remains beyond our understanding.

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