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Chapter 27: Plant Invaders!

In the summer of 1876, the United States threw itself a giant birthday party. It was the Centennial Exposition in Philadelphia: a world’s fair packed with brand-new inventions and exotic displays shipped in from around the globe. One of the showstoppers was a garden from Japan, full of a leafy climbing vine with sweet-smelling purple flowers. Visitors loved it. It was pretty, it grew fast, and it smelled wonderful. Gardeners took cuttings home and planted the vine on their porches for shade.

That vine was kudzu. And almost 150 years later, large stretches of the American South are buried under it.

Drive through parts of Georgia, Alabama, or Mississippi in late summer and you will see entire hillsides that look like someone draped a giant green blanket over the landscape. Trees, fences, old barns, road signs, even abandoned houses vanish under a smothering layer of leaves. People call kudzu “the vine that ate the South,” and while that nickname is a little dramatic (kudzu does not actually cover nearly as much land as the legends claim), the heart of the story is true. A plant that somebody brought over because it looked nice in a garden escaped, spread, and became one of the most stubborn problems in the region.

How does that happen? How does one harmless-looking vine go from a flower-show favorite to a landscape-swallowing menace? That is what this chapter is about. Welcome to the world of invasive plants, where some of the most beautiful, fast-growing, downright impressive plants on Earth are also some of the most destructive.

Wait, What Even Counts as “Invasive”?

Before we go any further, we need to clear up a word that gets thrown around constantly and almost always used wrong. People slap the label “invasive” on any plant they do not like. Their neighbor’s dandelions. The weeds in the sidewalk cracks. That one bush that got way too big. But “invasive” has a specific scientific meaning, and it is worth getting right. Let’s build it up in three steps.

First, every plant species has a home. There is a part of the world where it naturally grows. Botanists call a plant native when it is growing in the region it originally comes from. A sugar maple in a Vermont forest is native. A saguaro cactus in the Arizona desert is native. Back in Chapter 2, when we talked about classifying and naming plants, every species came with a natural home range built right into its story.

Second, sometimes a plant ends up growing somewhere it is not from, because a person moved it there, on purpose or by accident. A plant living outside its home range is called non-native, or introduced. Here is the part most people never realize: almost everything in your garden and on your dinner plate is non-native. Tomatoes come from South America. Wheat comes from the Middle East. Most of the flowers in a typical American front yard came from somewhere else entirely. Non-native is not a bad word. The vast majority of introduced plants are perfectly polite guests. They stay where they are planted and cause no trouble at all.

Third, every once in a while a non-native plant refuses to stay polite. It escapes the garden, spreads on its own into wild areas, and starts crowding out, smothering, or poisoning the plants that were already living there. It does real damage to the natural community around it. That, and only that, is what scientists mean by an invasive species: a non-native plant that spreads aggressively and causes harm.

So here is the cheat sheet:

• Native: growing where it is originally from. It belongs there.
• Non-native (introduced): growing somewhere it is not from. Usually harmless.
• Invasive: non-native, spreading aggressively, and causing harm. The troublemaker.

And one last distinction, because it trips everyone up. A weed is just any plant growing where a human does not want it. A dandelion in your lawn is a weed, even though it is harmless and, in a lot of places, perfectly native. “Weed” is about your opinion. “Invasive” is about actual damage to an ecosystem. A plant can be one, the other, both, or neither.

The Great Escape: How Plants End Up Where They Don’t Belong

Plants can’t walk. We hammered that point home back in Chapters 15 and 16, when we looked at all the clever ways seeds travel to new places: floating on the wind under little parachutes, hitchhiking in animal fur, drifting across oceans, even firing themselves out of exploding pods. Plants have designs that address solving the problem of how to move without legs.

Then humans came along and turned into the most powerful seed-spreading machine the planet has ever seen.

We move plants around in two main ways. The first is on purpose. People constantly carry plants across the world because they are useful or beautiful. Kudzu arrived as a garden ornamental and was later planted across the South on purpose, by the truckload, because the government wanted a fast-growing vine to hold soil in place and stop erosion. It did exactly that. It just didn’t stop. Purple loosestrife, a tall plant with gorgeous purple flower spikes, was brought to North America by gardeners who loved the look of it. Now it chokes wetlands across the continent.

The second way is by accident, and honestly that one is sneakier. Seeds are small, tough, and very good at stowing away. Remember from Chapter 16 how a seed coat can let a seed survive being frozen, baked, soaked, and even dragged through an animal’s gut? That same toughness lets a seed survive a trip in the mud caked on a boot, in a shipment of grain, tangled in the wool of imported sheep, or jammed into the tread of a truck tire. For centuries, ships dumped soil and rock called ballast in new harbors, and that ballast was loaded with foreign seeds that sprouted the moment they hit the ground. We have been accidentally planting gardens all over the world without ever meaning to.

Once a tough little seed or a snipped-off plant fragment lands somewhere new, the only question left is whether it can take over. Some plants are alarmingly good at it.

Why Some Plants Take Over (And Most Don’t)

Here is the puzzle. Back in its home country, kudzu is a normal, well-behaved plant. Nobody in Japan is panicking about kudzu eating their house. So why does the exact same plant turn into a monster the moment it crosses an ocean? The answer comes down to a handful of “superpowers,” and the biggest one is actually something missing.

Superpower 1: Nobody is eating them. In its home range, every plant has a long list of enemies. Insects chew its leaves. Fungi rot its roots. Animals munch it down to the ground. Think back to Chapter 25, where we saw how tangled up every plant is with the living things around it, including the ones trying to eat it. Those enemies act like a brake. They keep the plant’s numbers in check so it never spirals out of control. Now ship that plant to a brand-new continent and leave all its enemies behind. The bugs that would normally eat it are not there. The diseases that would normally weaken it are not there. The plant lands in a place where, for the first time ever, nothing is trying to hurt it. The brakes are gone. It is like a team that gets to play a whole season where the other side never shows up. It just runs, and runs, and runs.

Superpower 2: They grow ridiculously fast. Kudzu can grow up to a foot in a single day during the heat of summer. That is not a typo. You could practically watch it move. A vine that grows that fast can climb over a tree, blanket it in leaves, and block all of its sunlight before the tree has any chance to react. And remember from Chapter 1, and basically every chapter since, that without sunlight a plant cannot run photosynthesis, and without photosynthesis it starves. Invasive vines do not really attack other plants. They just steal all the light.

Superpower 3: They make an absurd number of seeds. A single purple loosestrife plant can release more than two million seeds in one year. Two million. From one plant. Every seed is a fresh chance to start a new colony somewhere downstream. When a plant floods an area with that many seeds, yanking up the parent barely matters, because the soil is already loaded with the next generation waiting for its turn.

Superpower 4: They don’t even need seeds. This is the truly frustrating one. Plenty of invasive plants can grow an entire new plant from a chopped-off piece of themselves. Remember the stem superpowers from Chapters 8 and 9, the underground runners called rhizomes and the creeping stems that sprout roots wherever they touch down? Invasive plants are masters of this. Japanese knotweed is the nightmare example. Snap off a fragment of its root the size of your fingernail, and that single scrap can grow into a whole new plant.

So chopping the plant up, which feels like a great idea, can actually spread it farther. Every piece you scatter is a new plant. We even met a gentler version of this trick back in Chapter 11, with that plant (mother of thousands) whose leaves sprout tiny baby plantlets right along their edges. It is impressive in a houseplant. It is terrifying in a plant that is trying to take over Florida.

Here is a video about a similar invasive plant: Mother of millions.

Superpower 5: Chemical warfare. Some invaders do not just outgrow the competition, they poison it. We spent real time on this in Chapters 19 and 25, where we learned about allelopathy, the trick of releasing chemicals into the soil that stop other plants from growing nearby. Garlic mustard, a nasty invader of North American forests, has an especially sneaky version. Instead of poisoning the other plants directly, it releases chemicals that kill the helpful soil fungi, the mycorrhizae, that native plants secretly depend on.

Think back to Chapter 25 and that underground partnership, where the fungus feeds the plant water and hard-to-reach nutrients in exchange for sugar. Garlic mustard sabotages that partnership. The native plants lose their underground business partners and slowly starve, while garlic mustard, which does not need the fungi at all, sails right past them. It is one of the meanest moves in the entire plant world.

Put even two or three of these superpowers together in a place with no natural enemies, and you get a takeover.

Most Wanted: A Lineup of Notorious Invaders

Every region has its own villains, but a few invasive plants are famous, or infamous, all over the world. Here is a quick rogues’ gallery.

Plant	Where It’s From	Its Trick	The Damage
Kudzu	East Asia	Grows up to a foot a day and smothers everything it climbs	Buries trees, fields, and buildings across the American South
Water hyacinth	South America	Floats on water and can roughly double its numbers in a couple of weeks	Forms mats thick enough to block boats, sunlight, and oxygen in lakes and rivers worldwide
Purple loosestrife	Europe and Asia	One plant makes over two million seeds a year	Takes over wetlands and crowds out native plants that ducks and other wildlife depend on
Japanese knotweed	East Asia	Regrows from a tiny root fragment and pushes up through cracks	Damages pavement, drains, and building foundations, and is nearly impossible to remove
Garlic mustard	Europe	Poisons the soil fungi that native plants rely on	Wipes out native wildflowers in forests across North America
Cheatgrass	Europe and Asia	Dries out early in the year and turns into tinder	Fuels faster, more frequent wildfires across the western United States

Notice the pattern, or really the lack of one. There is no single way to be an invader. One floats, one climbs, one regrows from scraps, one poisons the dirt, and one literally turns the landscape into kindling. Different tools, same result: they shove out whatever was living there before.

Okay, But Why Does It Actually Matter?

It is a fair question. So a hillside is covered in kudzu instead of native trees. So a lake has a mat of water hyacinth floating on it. Green is green, right? Who cares which plants win?

It turns out a lot of things care, because of an idea we keep circling back to in this book: nothing in nature lives alone.

Go back to Chapter 1, where we saw that you, and basically every animal, ultimately depend on plants for food and oxygen. Then layer on Chapter 25, where we saw that specific plants are wrapped up in specific relationships with specific insects, fungi, and animals. A native caterpillar might only be able to eat one particular native plant. A native bird might depend on that caterpillar. A native flower might depend on one kind of bee, and that bee might depend on that flower’s pollen. It is a web, and every strand is tied to the others.

When an invasive plant moves in and turns a rich, tangled community into a single-species blanket, all of those connections snap. Biologists call a giant patch of just one kind of plant a monoculture, and a monoculture is a catastrophe for everything that used to live there. The caterpillar cannot eat the new plant. The bird loses the caterpillar. The bee loses the flower. A meadow that once buzzed with dozens of species becomes a silent stand of a single invader. The land is still green, but it is nearly empty of the life it used to hold.

And that is only the wildlife. Invasive plants drain enormous amounts of money out of people’s pockets too. In the United States alone, invasive species are estimated to cause tens of billions of dollars in damage and control costs every single year. Water hyacinth clogs rivers so badly that boats cannot pass and water cannot flow. Knotweed cracks foundations and can drag down the value of a house. Cheatgrass turns grasslands into fire hazards that burn homes. This is not just a nature problem. It is a problem that shows up in harbors, in backyards, and on bills.

Fighting Back

So what do you do about a plant that has no natural enemies, grows a foot a day, makes millions of seeds, regrows from scraps, and poisons the soil? Honestly, it is hard. But people have a few strategies.

The most basic is pulling and cutting, simply removing the plants by hand or machine. This works for small patches, but it is exhausting, it has to be repeated for years, and with plants like knotweed you have to be careful not to scatter fragments that just start new plants.

The next tool is carefully applied weed-killing chemicals, which can knock back huge infestations that hand-pulling could never touch. The downside is obvious. You have to be extremely careful not to poison the native plants, water, and animals you are trying to protect in the first place.

The cleverest strategy is called biological control, and it is basically the problem run in reverse. Remember Superpower 1, the idea that invasive plants run wild because they left all their natural enemies back home? Biological control says, fine, let’s go get those enemies and bring them here too. Scientists travel to the plant’s home country, track down the specific insect or fungus that keeps it in check there, and, after years of careful testing, release it to do the same job in the new place. Special leaf-eating beetles have been used this way against purple loosestrife with real success, chewing the invader back down to a manageable size.

But, and this is an enormous but, biological control has to be done with extreme care. You are introducing yet another non-native creature, and if you get it wrong, the cure can become a brand-new invasion all its own. History is full of cautionary tales where someone released a “helper” species that ended up causing even more damage than the problem it was supposed to fix. So before any new biological control is released today, it gets tested for years to make absolutely certain it will attack only the target and will not turn around and devour the crops or native plants. It is a powerful tool, but a risky one, and it demands real patience.

A True Story From Our Own Garden

Want to watch everything in this chapter happen in one spot? Take a walk through our garden here at Guest Hollow.

Every summer, without anybody planting it, a plant with cheerful yellow flowers pops up wild around our property. It’s St. John’s wort, a plant some people dry and use as a supplement for mild depression.

It’s a beautiful, cheerful flower with bright five-petaled blooms and little leaves that, if you hold one up to the light, look like they are dotted with tiny pinpricks. Those “holes” are actually clear glands packed with the plant’s chemicals, and they are where the perforatum in its scientific name, Hypericum perforatum, comes from.

Here’s the twist, though: St. John’s wort is not originally from here. It came over from Europe, escaped, and turned into a serious invasive weed across the western United States. And it turns out that it’s poisonous to cattle and sheep! A chemical in the plant called hypericin makes animals that eat too much of it break out in nasty, sunburn-like blisters. So, this innocent-looking yellow flower in our garden is actually one of the invasive troublemakers we have been talking about this whole time.

Every year, like clockwork, its enemy shows up.

Look closely at our St. John’s wort in early summer and you will start to spot them: small, oval beetles that shine metallic green, bronze, or blue in the sunlight, like tiny living jewels crawling over the leaves. These are Klamathweed beetles, and they have exactly one job in life. They eat St. John’s wort. That is pretty much all they eat. Their plump little larvae chew on the leaves too, until the some of our St. John’s wort plants look ragged, stripped, and gnawed down to almost nothing.

Those beetles are not from here either! Their ancestors were carried over from Europe back in the 1940s and set loose on purpose, as biological control, to chase down the runaway St. John’s wort that was swallowing rangeland across the West. And the plan worked spectacularly. In the western United States the weed had overwhelmed more than 400,000 acres of open range before the beetles arrived. Afterward, the beetles largely wiped it out across several hundred thousand acres, saving ranchers millions of dollars a year in grazing land that would otherwise have been lost or poisoned. The beetles spread across state after state, including right here in Idaho, hunting their one favorite food wherever it grew. Decades later, nobody has to release anything anymore. The beetles just find the plant on their own and get to work, summer after summer.

There is also a chemistry twist to this story. Remember hypericin, the chemical that blisters cattle out in the sun? When the beetle larvae eat the plant, that same chemical builds up inside their bodies and makes them sensitive to sunlight too. So the larvae cannot feed in the bright midday glare. They do most of their eating in the dim hours around dawn, then hide in the shade to wait out the day. The plant’s own weapon ends up setting the dinner schedule of the very bug that is eating it.

There is one catch, though. The beetle was supposed to eat only the invasive St. John’s wort, but it turns out it will also nibble on related ornamental and native Hypericum species. That is exactly the kind of “the helper doesn’t always stay on target” surprise that makes scientists test biological control agents so carefully before letting them loose.

So… those beetles munching our garden weeds are living history. Every single year, our own backyard quietly re-runs one of the most famous biological control success stories in the world (to my chagrin, because I happen to love the St. John’s wort flowers)!

What You Can Actually Do

You, personally, are one of the ways invasive plants spread, which means you can also be one of the ways they get stopped. It doesn’t take much!

• Clean the mud off your boots, your bike, and your gear before hiking somewhere new, so you are not carrying hitchhiking seeds from place to place.
• If your family has a boat, rinse it off and let it dry between lakes, since plant fragments and seeds love to grab a ride on trailers and propellers.
• Never dump an aquarium or a backyard pond into a wild stream or lake. Water hyacinth and a long list of other water invaders got loose in exactly that way.
• If you garden, ask which plants are native to your area and which ones are known troublemakers. Choosing the right plant on purpose is the easiest win there is. Also, be careful when buying wildflower seed mixes. Check and see if some of the seeds are invasive in your area like oxeye daisies (which are invasive in our area).

None of these actions are dramatic. But invasive plants spread one stuck seed and one careless dump at a time, which means they also get stopped one careful choice at a time.

The Big Picture

Here is the strange twist at the heart of this whole chapter. The plants we have been calling villains are not actually bad plants. Kudzu is not evil. Water hyacinth is not plotting against us. Every one of these species is simply doing the exact thing that made it so impressive in the first place: growing fast, spreading wide, making seeds, grabbing sunlight, defending its turf. Back home, those same talents make them ordinary members of a balanced community.

The problem was never the plant. The problem was dropping an extremely talented plant into a place with no rules to hold it back. An invasive species is really just a story about what happens when you pull a plant out of the web of relationships that kept it in check and turn it loose somewhere those relationships do not exist.

Which, when you think about it, is a pretty good reminder of the big idea we keep coming back to in this book. Plants are never truly alone, and neither is any other living thing. Every plant is part of a community, and when you yank one out of its community and drop it into another, the whole system feels it. So next time you spot a hillside swallowed in green vines, or a pond choked with floating leaves, you will know you are not just looking at a plant. You are looking at what happens when a plant escapes its own story and writes itself into someone else’s.

Note: The following video briefly mentions the word “evolve.”

Explore It Yourself: Map an Invader

Everything in this chapter is happening somewhere near you right now, and you can pull up a map and see it. Scientists, park rangers, and ordinary people log invasive plant sightings into giant online databases, partly so that a new invader can be caught while the patch is still small enough to stop. Remember, an invasion almost always starts tiny. Here are a few places to look.

EDDMapS (eddmaps.org) is the big one. It maps invasive species all across the United States and Canada, and you can click a plant and see exactly which counties it has been reported in. It even has a free phone app for reporting what you find.

iNaturalist (inaturalist.org) lets you snap a photo of a plant or bug, get help identifying it, and see a live map of everywhere else people have spotted that same species.

Your state’s weed list is worth a look too. Idaho’s, for example, lives at invasivespecies.idaho.gov and has fact sheets with photos for official troublemakers like oxeye daisy. For the native-or-not question on almost any plant, the USDA PLANTS Database (plants.usda.gov) has range maps.

Try this:

1. Look up your state (or country area) + invasive plants.
2. Look it up on EDDMapS or iNaturalist and zoom the map in toward where you live.
3. Find the closest reported sighting to your home. How near is it? Is the plant already in your county, your town, maybe your own yard?

If you find one growing wild near you, you have just done real science. You have spotted a plant living far from home, in a place where its story is still being written.