How plants communicate has long been a matter of debate.
In fact, it was initially very controversial: books like The Secret Life of Plants (1973) seemed to undermine the credibility of some of the early research in the field by suggesting that plants thrive if, say, you sing or play classical music.
While many of these claims were later debunked, researchers remain adamant that there is some kind of communication between plants—and between plants and animals.
Can plants communicate with each other?
Plant communication is a confusing topic. Perhaps because scientists are still learning about the process. What we do know is that plants have a wide range of mechanisms to communicate with each other and with their environment.
“It is quite clear that plants are not just unresponsive victims of their herbivores, but that they are very aware of all sorts of things in their environment. And they respond to reliable information,” says Richard Karban, a professor of entomology at the University of California, Davis.
“Yet it is not clear how important these different sources of information might be,” he continues.
Fortunately, experts are looking at the problem from different perspectives.
Read more: Can plants feel pain?
The Wood-Wide Web
A pillar of the plant communications debate takes its name from the Internet, the “wood-wide web.” This concept first emerged in the 1990s, when scientists began to hypothesize that forests are connected underground by complex networks of elongated fungi known as mycorrhizae.
Document from 1997 published in Nature serves as a cornerstone of this theory, suggesting that trees in forests do not compete with each other; they cooperate. In other words, trees use these fungal connections to share information, water, nutrients, and more.
Today, on wood-wide web theory is ubiquitous in both pop culture and educational materials, and researchers continue to search for how plant communication networks operate.
For example, one study showed that carbon moves between Japanese red pine tree seedlings. In the same way, the dye moves between them ponderosa pine seedlings.
Read more: Why do some plants close their leaves at night?
How do plants communicate?
The fact that the scientific jury is out on the existence of the tree network, however, does not preclude all communication. To get to the root of the problem, perhaps the question should be how plants communicate, not whether they communicate at all.
Plants use chemicals
When you smell the delicious smell of grass clippings, those blades of grass are actually calling for help with a warning of danger. A growing body of research shows that when plants are damaged by things that want to eat them, like insects, they release airborne chemicals.
Other sensitive leaves on the same plant, as well as the leaves of their neighbors, pick up on these chemical signals and subsequently increase their own defenses, according to Karban.
His own research, for example, shows this Wormwood plants alarm when attacked by pests, so others Wormwood plants respond growing faster and stronger. Even other species, such as tobacco, can sense and respond to the alarm.
“These volatile chemical cues are one potential source of reliable information about plants,” says Karban. “What is not yet clear is how general this is, how universal it is among plants and how important it is.”
Plants make sounds
Lilah Hadani, a professor at Tel Aviv University’s School of Plant Sciences and Food Security, wanted to better understand whether plants also use the acoustic realm of communication.
IN study published this yearthe team put a tomato on her and tobacco plants in an acoustically insulated box and then records any ultrasonic sounds produced between 20 and 150 kilohertz. They experimented with cutting stems or leaving them without water, as if to simulate drought.
The researchers found that the plants make popping and clicking sounds at about 60 decibels, about as loud as human chatter. However, these sounds were of an ultrasonic frequency that humans cannot hear naturally.
Hadani’s team even used artificial intelligence to match different sounds with the plants’ environment. Whether they’ve been pruned or water-deprived, each type of stress can be matched with a specific, recognizable sound.
It is not yet known how sound is produced, what the biological costs of sound production are, and whether they are of any benefit. In fact, researchers are still not sure if sounds are used for communication purposes at all.
“We don’t know if they use the sound, or if the sounds are emitted in a completely passive way due to physiological changes,” Hadani says. “But we know they are in the air and contain information.”
Plant communication through soil
Like the tops of plants, roots also produce volatile chemicals, hormones, and other signaling molecules that can be transmitted and detected by other roots through the soil.
And most importantly, there is a large proportion of microbes living underground. These microbes also respond to whatever the plants emit.
“We focus on the mycorrhizal fungal network [wood-wide web], I think because we can see it, we can visualize it in our head,” Field says. But, she continues, the role other microbes play is underappreciated and misunderstood.
If, for example, the root releases sugars or other carbohydrates, this can change the composition of the microbiome. The microbial communities that live in the soil can then influence what happens to another plant.
“So there’s also this indirect communication, [and] we don’t know how it works or what it does,” Field says.
Read more: Plants can help make your surroundings quieter
The role of fungi in plant communication
But while it’s true that most plant roots are colonized by fungi – fungi that give plants water and nutrients from the soil in exchange for sugars and carbon – evidence that these networks are used to communicate between plants is still scarce, according to published research in Nature last February.
Experiments show that most of the molecules “transferred” between the trees remain in the mushrooms. Furthermore, there is very little research on the other elements that might be involved in these processes, and scientists question why so little research has been done on what the fungi have to gain from the process.
“Fungi have their own agenda. Do your job. And we don’t really know what it does. We don’t know how he does it. And we don’t know why it does it,” says Katie Field, professor of plant-soil processes at the University of Sheffield in England. “I think we need to understand better that before we start assigning roles in communication.
Read more: If fungi could talk
Why plants communicate
A major takeaway from all this research is that plants may not actively communicate. Rather, according to Karban, they may be actively eavesdropping.
“If there is information in the environment that is reliable and that can increase their fitness, it’s reasonable to make an argument that plants will respond to that kind of information,” Karban says.
This may be why plants are sensitive to the quality and quantity of light – something that is important to them – and why they do not respond to caretakers talking to them or playing music. These things just don’t increase their fitness.
“We think that as long as these cues are reliable, plants will pay attention to them,” Karban says. “But that’s very different from the idea that there has been selection to actively emit these signals.”
Read more: 20 things you didn’t know about trees