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Chapter 25: Frenemies, Freeloaders, and Backstabbers: Plant Relationships

Quick question. How many friends does a plant have?

If you said zero, that’s a totally reasonable guess. A plant doesn’t text anybody. It doesn’t have a lunch table. It can’t even wave. It just stands there in the dirt, silent and motionless, looking like the least social thing on the entire planet.

And you would be completely, hilariously wrong.

The truth is that a single plant is wrapped up in more relationships than you are. It has business partners it depends on for survival. It has roommates living rent-free on its branches. It has neighbors it sends warning messages to. It has bodyguards it pays in sugar. And it has enemies that sneak up, latch on, and slowly drain the life out of it like something from a horror movie.

Plants are not loners. Plants are absolutely tangled up in a web of deals, alliances, freeloaders, and outright crimes. They just do all of it quietly, underground and out of sight, where you’d never think to look.

Scientists have a word for two different living things sharing a close, long-term relationship. They call it symbiosis, which literally means “living together.” And once you understand the three main flavors of symbiosis, you’ll start seeing them everywhere.

Here’s the cheat sheet. In any relationship between two living things, somebody is winning, somebody is losing, or somebody is just hanging around not really affecting anyone. That gives us three categories:

• Mutualism: both sides win. A genuine partnership.
• Commensalism: one side wins, the other side doesn’t care either way.
• Parasitism: one side wins by hurting the other.

Let’s meet the cast of characters.

The Best Deal in the Dirt

We’re going to start with the relationship so important that about four out of every five plant species on Earth have it. And almost nobody knows it exists, because it happens entirely underground.

It’s a partnership between plants and fungi.

Now, when you hear “fungus,” you’re probably picturing a mushroom, or maybe that questionable thing growing in the back of the fridge. But mushrooms are just the tiny visible tip of something enormous. Most of a fungus is a vast network of microscopic threads spreading silently through the soil, thinner than the finest hair, reaching into spaces far too small for any plant root to fit.

These threads team up with plant roots in a partnership called mycorrhizae (my-kuh-RYE-zee), a word that literally means “fungus root.” And the deal they have is genuinely brilliant.

Here’s the problem the plant has. Roots are great, but they’re kind of chunky. They can only reach so far and squeeze into so many gaps. There are nutrients sitting in the soil, especially one called phosphorus, that are locked away in tiny pockets the roots simply cannot get to. The plant is basically standing on a buffet it can’t quite reach.

The fungus, on the other hand, has those superthin threads that snake into every microscopic crevice. It can grab water and phosphorus and other nutrients from places a root could never dream of touching. The fungus is a master scavenger.

But the fungus has its own problem. It can’t make food. It has no leaves, no chlorophyll, no way to turn sunlight into sugar. It’s starving in a world full of soil.

So, they work together! The fungus wraps around and even reaches inside the plant’s roots and starts delivering water and hard-to-reach nutrients straight to the plant. In exchange, the plant pumps sugar (the food it makes up in its leaves) down to the fungus. Some plants hand over a shockingly large chunk of all the sugar they produce, just to keep their fungal partner happy and well fed.

Think of it like this. The plant is a chef who can cook incredible meals but is terrible at grocery shopping. The fungus is an expert shopper who can find anything in the store but can’t cook to save its life. So the shopper brings the chef ingredients, the chef cooks for both of them, and everybody eats. Neither one would survive nearly as well alone.

This isn’t some rare oddity. This is the rule, not the exception. The grass in your yard is doing it. The trees in the forest are doing it. The vast majority of plants you have ever seen in your life are secretly hooked up to a fungal partner under the soil, running this exact exchange every single day. You’ve just never seen the handshake.

The Air Factory

Here’s a frustrating fact about Earth. The air around you is about 78 percent nitrogen. Nitrogen is everywhere. It’s in every breath you take. And plants are absolutely desperate for nitrogen, because they need it to build proteins and DNA and basically grow at all.

So plants are surrounded by an ocean of the exact thing they need most.

And they can’t use a single bit of it.

The nitrogen in the air comes as a gas called N2, where two nitrogen atoms are glued together by one of the strongest bonds in all of chemistry. Plants have no way to break it apart. It’s like being starving in a locked pantry and not having the key. The nitrogen is right there, taunting them, completely out of reach.

Enter the bacteria.

Certain tiny bacteria called rhizobia have a special ability. They can rip apart that ultra-tough nitrogen gas and turn it into a form plants can actually use.

It’s a chemical magic trick almost nothing else on the planet can pull off. We call it nitrogen fixing.

Note: The following video doesn’t have any sound, but explains the process:

A group of plants called legumes (think beans, peas, clover, peanuts, and soybeans) know how to hire these bacteria. The plant builds little lumpy structures on its roots called nodules, and it invites the rhizobia bacteria to move in. The bacteria set up shop inside the nodules and start cranking out usable nitrogen, handing it straight to the plant. In return, the plant feeds the bacteria sugar.

The plant even builds the bacteria a custom workspace. The nitrogen-fixing machinery is wrecked by oxygen, so the plant produces a special pinkish substance inside the nodules that grabs onto oxygen and keeps it under control, like a personal climate system for its bacterial employees. If you ever slice open a healthy bean root nodule and see a pink center, that’s the proof the factory is running.

This deal is so powerful that farmers have used it for thousands of years without fully understanding it. Plant beans or clover in a field, and the soil ends up richer in nitrogen afterward, because the plants and their bacteria were quietly fertilizing the ground the whole time. It’s a partnership that helps feed the entire world, and it all comes down to a bean handing snacks to a microbe.

It’s also the reason why we (at Guest Hollow) plant cover crops that contain peas and other nitrogen fixing plants in the fall – to nourish the soil for the next year!

Paying the Bodyguards

So far the plant’s partners have been microscopic. Let’s go bigger. Let’s talk about the plant that hires an actual army.

In Central America there’s a tree called the bullhorn acacia, and it has a serious problem. Lots of things want to eat it. Hungry insects, browsing animals, and pushy neighboring plants that try to grow over it and steal its sunlight. The acacia can’t run and it can’t fight. So instead, it does something incredible.

It hires mercenaries.

The acacia grows big hollow thorns, swollen and rounded, that make perfect little apartments for ants. Then it produces nectar from special glands, basically free sugar water, to feed those ants. And then, as if that weren’t generous enough, it grows tiny nutrient-packed nuggets right on the tips of its leaves, loaded with protein and fat, like prepackaged ant snacks set out on a tray.

So now you’ve got an ant colony that has free housing, free sugar, and free protein, all provided by the tree. And these ants do not take their job lightly.

The moment anything lands on that tree, the ants swarm it. A caterpillar starts munching a leaf? The ants attack and haul it off. A big animal brushes against the tree? The ants come pouring out and bite and sting it until it leaves. Another plant tries to grow too close and creep over the acacia? The ants march over and chew the intruder to pieces, clearing the area so their tree gets all the light.

The acacia has essentially built itself a private security force, paid in room and board. The tree gets a 24-hour bodyguard squad that will fight, bite, and landscape on its behalf. The ants get a home and free meals. Both sides come out ahead. That’s mutualism in its purest, most over-the-top form.

Note to Christians: The following video briefly mentions evolution. You can think of this relationship as God’s design. 🙂

The Bribe at the Heart of Every Flower

There’s one more mutualism you have absolutely seen, probably today, and never recognized for the clever trade it really is.

Flowers.

A flower is not just a pretty decoration. A flower is a billboard, a vending machine, and a delivery service all rolled into one, designed to solve a serious problem. Plants need to get their pollen from one flower to another flower to make seeds. But plants can’t move. So how does the pollen get there?

The plant pays for a ride.

Bright colors and sweet smells are advertisements, screaming “fresh food here” to bees, butterflies, moths, beetles, hummingbirds, and even bats. Deep inside the flower is the reward, a sugary liquid called nectar. An insect shows up, shoves its face in to drink the nectar, and accidentally gets dusted with pollen all over its body. Then it flies off to the next flower, where some of that pollen rubs off, and boom, the plant just shipped its genetic material across town using a courier who has no idea it’s even working a job.

The pollinator thinks it’s just getting a free drink. The plant is getting its entire reproduction handled. About three out of every four kinds of flowering plants depend on animal pollinators like this. A huge slice of the food on your dinner plate exists only because some insect went looking for a sugary snack and got tricked into being a delivery driver. Next time you see a bee, remember it’s basically a helper for a flower.

Roommates Who Pay No Rent

Now we shift gears. We’ve covered mutualism, where everybody wins. Let’s talk about the relationships where one side wins and the other side just… shrugs.

That’s commensalism, and the easiest place to see it is up in the branches of a tree, where you’ll find plants growing on other plants.

Walk through a rainforest, or even parts of the southern United States, and you’ll spot plants living on tree branches without any soil at all. Orchids perched high on bark. Ferns sprouting from the sides of trunks. Stringy gray Spanish moss draping off the branches like nature’s tinsel. These are called epiphytes, which means “upon plants.”

Here’s the clever part. An epiphyte isn’t a thief. It doesn’t tap into the tree or steal the tree’s food. It just uses the tree as a place to sit. And the reason it wants to sit way up there is simple. Down on the rainforest floor it’s dark, because the giant trees above hog all the sunlight. So instead of fighting for scraps of light at the bottom, the epiphyte skips the climb entirely. Its seeds are so tiny and light that the wind carries them straight up into the branches, or a bird drops them there after a meal. The seed sprouts right where it lands, high up in the sunny penthouse level, without ever touching the ground. From up there it soaks up rainwater and grabs nutrients from the air and bits of debris that collect around its roots.

The tree, meanwhile, basically doesn’t notice. It’s not being helped, but it’s not really being harmed either. The epiphyte is the ultimate freeloading roommate, the one who lives in your house, uses your address, and contributes absolutely nothing, but is also so quiet and harmless that you can’t even be mad about it.

The Backstabbers

Alright. We’ve done the partners and the freeloaders. Now it’s time for the villains.

Parasitism is when one organism survives by hurting another, and the plant world has some genuinely sinister parasites that would make a great horror movie if anyone bothered to film them. These are plants that steal from other plants. Some of them have given up making their own food entirely and just rob their victims instead.

Meet the worst offenders.

Mistletoe looks innocent. People hang it up at the holidays and kiss under it. How dangerous could it be? Well, mistletoe grows right on the branches of trees, and it sinks special root-like structures straight into the host’s plumbing, tapping the pipes that carry water and minerals. It steals the host’s water supply for itself. Mistletoe does still have green leaves and makes some of its own food, so it’s only a part-time thief, what scientists call a half parasite. But make no mistake, that romantic holiday decoration is a freeloader stealing drinks from a tree, possibly for years.

Dodder is far creepier. It looks like someone sprayed orange silly string all over a bush.

Dodder has almost no green in it at all, because it has nearly given up on making its own food. Instead, a young dodder sprout actually senses nearby plants (it can detect chemicals in the air given off by its potential victims) and grows toward them like a heat-seeking missile. When it reaches a host, it wraps around and around the stem and jabs in tiny suckers that plug directly into the host’s veins. Then it just drinks. It sucks out water, sugar, and nutrients, growing thicker and spreading to more victims while its host slowly weakens. It’s a plant vampire, and that is not an exaggeration.

Rafflesia might be the strangest plant on the entire planet. We’ve already mentioned it in earlier chapters, but we’ll quickly review it here. For most of its life, the rafflesia isn’t even a plant you can see. It lives as a network of threads hidden completely inside the tissue of a jungle vine, no leaves, no stem, no roots of its own, silently feeding off its host like an infection. Then, when it’s ready, it bursts out and blooms into the largest single flower in the world, a monster that can grow three feet across and weigh more than a backpack. And the flower smells like rotting meat. On purpose. The stench attracts flies, which the flower uses to spread its pollen. Imagine a plant that lives as a hidden parasite for years, then explodes into a giant flower that smells like roadkill.

And then there’s the sneakiest crime of all. There’s a ghostly white plant called Indian pipe that has no green color anywhere, because it makes no food at all. But here’s the twist. It doesn’t tap into a tree directly. Instead, it plugs into those helpful underground fungi we talked about way back at the start of this chapter, the ones partnered with trees. The Indian pipe siphons sugar out of the fungus, sugar that the fungus originally got from a tree. So, the tree sends food to its fungal partner, expecting a fair trade, and the Indian pipe is hiding in the middle of that pipeline, skimming off the top. It’s stealing through a middleman. It’s the kind of move that’s almost impressive in how shady it is. 😉

Chemical Warfare

Before we wrap up, there’s one last relationship worth knowing about, and it’s pure aggression. Some plants don’t make friends or steal from neighbors. They poison the ground so nothing else can grow near them.

It’s called allelopathy, and the classic example is the black walnut tree as we discussed in chapter 24. The black walnut releases a chemical into the soil that’s toxic to many other plants. Tomatoes and lots of other species planted too close to a black walnut will wilt, struggle, and sometimes die. The walnut isn’t eating them or touching them. It’s just chemically clearing out the competition, so it gets all the water, light, and space for itself. It’s a plant fighting a turf war with invisible weapons, and it’s been winning that war in backyards for as long as people have been confused about why their garden died next to that one tree.

Here’s a chart that shows some allelopathic plants:

Plant	Chemical Weapon	What It Hurts	Garden Notes
Black walnut	Juglone	Tomatoes, peppers, eggplant, potatoes, many others	The classic offender. Its toxic zone reaches well past the trunk, since roots spread wide.
Tree of heaven	Ailanthone	A broad range of nearby plants	Aggressive invasive. Poisons competitors as it spreads.
Sunflower	Several compounds in roots, leaves, and hulls	Many small plants and weeds	Why grass and weeds often struggle right under bird feeders full of sunflower seeds.
Fennel	Various allelochemicals	Most common vegetables and herbs	Gardeners are told to plant it off by itself. It stunts almost everything around it.
Wormwood (Artemisia)	Absinthin	Seedlings and many nearby plants	Even rain dripping off its leaves can carry the chemical into the soil.
Garlic mustard	Compounds that attack soil fungi	Native plants that rely on mycorrhizae	Invasive. Wins by poisoning the helpful fungi its rivals depend on.
Knapweed	Catechin (from the roots)	Native grasses and wildflowers	This plant is very aggressive. It clears out competition and forms dense stands.
Mustard and other brassicas	Glucosinolates	Weeds, fungi, soil pests	Chopped into soil on purpose in a trick called biofumigation.
Eucalyptus	Oils in leaves and bark	Understory plants	Dropped leaves leave bare ground beneath many eucalyptus trees.

So, How Many Friends Does a Plant Have?

Way more than you guessed at the start of this chapter.

A plant has fungal business partners trading nutrients for sugar beneath the soil. It has bacterial employees pulling nitrogen out of thin air inside its own roots. It has bodyguards it pays in nectar and snacks. It has couriers it bribes with sweet drinks to carry its pollen across town. It has freeloading roommates camped out in its branches. It has vampires and ghosts and giant stinking flowers trying to drain it dry.

The motionless thing standing silently in the dirt is actually the center of a busy, crowded, dramatic social world. It’s making deals, hiring help, feeding allies, and fighting off thieves, every second of every day.