When Plants Kill (the article that gave birth to “Semiosis”)

This article appeared in the Spring 1997 issue of Terra Incognita magazine. It was inspired by my own houseplants, which had attacked each other in my living room. Plants cooperate, but they compete, too, viciously. Plants are active, not passive. As I researched the article, I realized that we live in the midst of a photosynthetic war. And I began to wonder: What if…

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Imagine alien beings totally dependent on light for nourishment. Could such beings live peaceably together? No. Sooner or later, they would compete mercilessly for light. They would kill for light — starving, maiming, poisoning, strangling, crushing, and burning any rival.

Here on Earth, we call these vicious beings “plants.”

In 1820, botanist and taxonomy pioneer Augustine Pyrame de Candolle said, “All plants of a given place are in a state of war with respect to each other.” If we humans moved slower, we’d notice the carnage in our ecosystem’s green kingdom — and we’d be in peril ourselves.

It’s a jungle out there, so let’s visit one. In a tropical rain forest, the darkness may surprise you, so let your eyes adjust a moment. Now look up. You can see a green ceiling, the forest canopy. That’s why it’s so dark. The leaves up there hog all the sunlight.

Look down. On the forest floor, you can see a few ferns, struggling saplings, gaunt seedlings, some moss, and a lot of roots. Nothing else.

Look around. The space between the floor and canopy is filled with sapling stalks, tree trunks covered by clambering vines, and 60‑foot-long roots lunging from the canopy so taut we can pluck them like guitar strings — in all, an impenetrable tangle between the floor and the canopy. But few leaves grow beneath the canopy. It’s too dark. Everything aims for the canopy to grab the sunlight.

Trees use brute force, investing in thick trunks to carry their branches upward. Saplings stand here and there in the jungle, but young trees flourish only after an old tree crashes down, ripping a hole in the ceiling. Light pours onto a waiting sapling that will rise like a titan.

Trees joust with each other. Softwood trees grow faster and outrace the hardwoods to reach the top first for their day in the sun. Hardwoods follow, slowly sawing their way up: hardwood branches, stirred by winds, grind against softwood branches and carve through, amputating softwood branches one by one.

On many trees you can see thick, woody vine-like stems, called lianas, climbing up the trunk. Climbing pays off, Charles Darwin said in 1893, because climbers can reach the sun “with wonderfully little expenditure of organized matter, in comparison with trees, which have to support a load of heavy branches with a massive trunk.” A climber three inches in diameter may have as many leaves as a tree with a trunk two feet across.

Lianas weigh down trees and spread their leaves over their hosts, stealing the light. Trees starve. Saplings snap under the sheer weight.

But trees fight back. They try to grow faster than lianas, or shed bark to make them fall, or grow large shady leaves to starve them, or sway in the wind to knock off their attackers.

Lianas and vines continue up, twining around stems, impaling trunks with thorns and hooks, pushing roots and twigs into cracks, even gluing roots onto smooth bark. If a liana falls, or when the poor tree finally dies and collapses, the liana just starts climbing again.

One huge vine with yard-wide leaves is an ordinary philodendron, like the one you might have in your living room, but mature. Even the baby in your own home could attack another plant — a would-be murderer might be perched on your bookcase. Watch out.

In the jungle, watch out for the rattan, a palm used in wicker furniture. It climbs, and to do that, it grows spines on long, whiplike extensions of its leaf stalks: claws to stab into anything that can offer support. The spines hang like in the jungle loose barbed wire and can slash through your skin. Roses have thorns to climb the same way.

Many climbers send out tendrils to grasp for handholds. Darwin wrote, “It has often been vaguely asserted that plants are distinguished from animals by not having the power of movement. It should rather be said that plants acquire and display this power only when it is of some advantage to them.”

A passionflower has its tendrils ready for action, poised, according to Darwin, “as a polypus places its tentacula.” Slowly across the days, the tendrils will revolve like the feeler of a monster, searching for a victim.

Touching something, in 30 seconds the tendril will start to coil around it. Within hours, the tentacle will firmly clutch its new brace in a life-or-death race to the forest top.

One member of the Pea family can grow a vine with a two-foot trunk. With tiny leaflets, it can drown an acre of the canopy — 64 unlucky trees.

In the jungle, on some trees, you can see roots thick as branches clamped all around a trunk. They come from another tree, a strangler fig, growing on its host tree from a crotch of a branch about halfway up. As a seed, the fig germinated there, getting its water from the rain an as it sent down its roots.

You can see the victim tree’s trunk bulging out between the roots of the strangler fig. The host is being strangled. Its trunk can’t grow wider and add a ring of new channels to carry water and nutrients to the leaves. It will eventually choke, die, and rot away, leaving the fig standing alone as a mature tree. The fig’s trunk will resemble a long, deadly cage.

Some plants employ an even more cunning scheme to live. They become parasites. Mistletoe sinks its roots into the living flesh of the tree to find water. As tropical mistletoe grows, it can drain its host dry.

Parasites can grow without bounds since they don’t have to work to support themselves. No surprise, then, that the Earth’s largest flower is the Rafflesia, a parasite of lianas in Southeast Asian jungles. Its brown and purple 3-foot flowers can weigh 15 pounds. They are called “stinking corpse lilies” for their stench, which attracts hundreds of flies to pollinate them. Let’s not look for one.

Outside the steamy jungle, it’s still a dog-eat-dog world in the plant kingdom.

Dodder, also called “devil’s sewing thread,” is an orange, creeping parasite of temperate zones. Within hours of germination, it reaches out with a tendril-like stem that circles until it touches a victim. Dodder attacks, wrapping around its host and sinking roots into its host’s stalk. Dodder forms dense mats of stems and flowers up to a mile long around its victim, sucking out every drop of sap.

Plants also poison each other. In 1832, de Candolle suspected plants release toxic materials into the soil to weaken the growth of competitors, but he couldn’t prove it. He was right.

Walnut trees leaves excrete a poison from their leaves that is washed by rainfall into the soil. Barley releases water-borne toxins to destroy weeds. The rubber-producing guayule of the American Southwest excretes toxic cinnamic acid from its roots. Cattails even poison their own young. Their seeds won’t germinate in water contaminated by cattail leaves.

Fire is the weapon of mass destruction in the vegetable kingdom. Some plants can survive fire more successfully than others, and they use that ability to their advantage.

Ponderosa pines in the western United States drop flammable dead needles. When lightning or humans spark a fire, it blazes through the dead needles, incinerating less fire-resistant species. Until human intervention, such fires were common, giving Western forests a different look in the past.

In the hills around Los Angeles, certain shrubs such as chamiso do the same by dropping oily leaves as tinder — and humans pay the price.

Plants will even sneak up on each other. Fields of British heath can be invaded by the roots of bracken ferns. The roots will send up leaves that shade and ultimately kill the heath.

Plants need light to live. They also need carbon dioxide, water, and certain minerals. Our atmosphere contains only about three-hundredths of a percent of carbon dioxide, which limits most plants. Given more CO₂, plants will grow faster until they hit the next limit, which is usually light.

In perfect conditions, we could have a photosynthetic riot.

So watch where your shadow falls. You could annoy a heartless killer whose long, green tentacles would wrap themselves around your body. Slowly, unceasingly, those tentacles would tighten with unexpected strength until you slump dead to the floor.

Your potted philodendron might want you out of the way. Beware.