The evidence is mounting that as a species we are uniquely horrible listeners. But by being self-absorbed, we may be missing a lot of what’s going on around us including—it seems—plant chatter. Susan Dudley, an evolutionary ecologist from Ontario’s McMaster University has been studying the unique behaviour of flora for nearly three decades, but even she hasn’t heard all there is to hear on the subject just yet.
‘The more we learn, the more questions we find left to ask,’ says Dudley. ‘Communication is defined as needing a message sender and receiver who both desire this signal to be conveyed. So far we know plants release coercive chemicals to pollinators, attractants to species that eat their pests, exudate substances from their roots, and eavesdrop on others. But to what end, or whether this is truly intentional [communication]—that’s another question.’
This is what researchers currently know about the confusing world of talking plants.
Plants can communicate with each other
In 1983, two studies showed plants had communication-based defence systems. Damaged poplar and maple trees were shown to send out chemical cues to nearby of their kind, inducing them to also ramp up their defences, and Sitka willow were shown to actually lower the levels of nutrients in their leaves when under attack by caterpillars and webworms. Wounded willows also sent the message to their neighbours to make their leaves less healthy too, stunting the bug’s growth.
In the 1990s interspecies communication was also proven a thing—when insect-chewed sagebrush caused tomato plants to increase their production of compounds that upset insect digestion. Tobacco plants have also been shown to display the same response to sagebrush’s alarm calls, faring better against devouring predators when next to bug-munched sage brush.
And plants can recognise themselves too, and their kin. In 2007 Dudley showed searockets, prone to living in clusters of directly related individuals, acted more politely in the presence of their genetic family. In other words, the searockets invested less in their roots when it would impede on their family members’ fitness.
Dudley says this makes sense, given the extraordinarily close-quarters most plants live in, often with their kin. ‘A plant can increase its fitness by helping itself, but also by helping others that share its genes. Competition is incredibly expensive, so sacrifices can also be seen as a bit selfish.’
Plants have also been shown to emit sounds other plants can hear. In one study, chilies grew quicker in the presence of sweet fennel, which elicits a chemical that slows the hot-plant’s growth, but not because they caught onto chemical cues. In a box allowing only sound to be transmitted, the chilies in some way heard the fennel, and quickened their development in anticipation of being slowed down.
In 2010, the intent of inter-plant communication altogether was questioned. It was shown that lima plants defence-chemicals mostly fell on its own leaves, not on other plants—leading to a declaration by some researchers that what we thought of as communal chatter was more likely a soliloquy, a signal only meant for one organism but hijacked by others to their own advantage.
However the idea that communication is a completely passive process seems a little deterministic. Especially so, given the context of recent discoveries, like vast underground information networks connecting plants beneath the forest floor via the thin, soil-bound threads of fungi called mycelium—an information superhighway just under our feet.
It remains a contentious issue, and the verdict is still out as to what constitutes active and passive communication between plants. But for now many scientists choose to refer to flora that respond to neighbour’s cries, be them inner-species or not, as eavesdroppers—listening in on a signal not meant for them.
Plants can communicate with animals
There’s plenty of proof that plants and animals chat too. Plants have been shown able to recognise their predators when under attack, and some even enlist non-plant life for help.
Lima beans under the hold of spider mites have been found to release chemicals enlisting another type of mite that dines on the spider mite, and hornworms chowing down on tobacco leaves cause the plant to spew out a luring scent to hornworm predators.
Tobacco plants can also potentially switch up their whole routines when things go sour with their usual pollinator, the hawk moth. These big moths lay their eggs on tobacco stems, but if caterpillar infestations become too intense, in a matter of eight days some plants were witnessed moving their flowering time from night to morning, attracting hummingbirds.
Some rare plant types are also known to draw in pollinators using their shape, reflecting encouraging directions to echolocating animals like bats. Just this June researchers found a carnivorous pitcher plant in Borneo whose specific structure allows native bats to pinpoint their location even in cluttered underbrush. But these bats aren’t brought in to pollinate the pitcher plants, but rather to fertilise them. Bats are offered a roost—the plants get their droppings.
Plant language may be more evolved than we’d like to think
So why haven’t humans figured out what plans are talking about yet? Well, for one thing humans are evolutionary infants compared to plants. Fungi have been land-colonisers for around 1,300 million years—plants roughly 700 million. Humans have only been around for the last 200,000 years or so.
And during all this time, most plants have racked up some pretty massive genomes from all the adaptions they’ve tried out. Onions have about 12 times the DNA humans do.
Matthew Gilliham, an associate professor from the University of Adelaide’s School of Agriculture, Food and Wine, says plants face a unique set of problems compared to most other forms of life, and hence, have had to learn equally unique skill-sets to cope.
‘Plants can’t run away from a lurking predator, seek out heat when it’s cold or shelter from the sun—essentially they must take on every challenge, sense and respond, from where they stand. They adapt ways to meet these needs or don’t survive.’
Gilliham just published a study showing that plants have their own receptors for GABA, neurotransmitters animals release when stressed out. While GABA is an important metabolite for carbon and nitrogen cycling in plants, it also accumulates in plant tissues under stress. Gilliham wanted to know if GABA was acting as a signal like it does in animals, sending the message to calm down.
‘Plants don’t have nervous systems, nor the axons and neurons to facilitate true consciousness, but they do use electrical and chemical signals and we found GABA seems to calm down plants too.’
Gilliham says he’s on a mission to use this kind of information to help boost crop yields, a huge pressure as our global population continues to climb. He says firming the link between plants and humans helps compel more people to explore green-life forms, or reconsider our bond, but sometimes efforts go too far—and without good reason.
‘Plants are fascinating as plants. We don’t need to compare them to anything else—it probably doesn’t even make sense to,’ he says. ‘We can’t talk plant yet, but when we learn how to, we’ll probably find they use a whole different language than we suspected, which could logically be more complex than animals like ourselves.’