[[quoteright:350:[[Film/WarOfTheWorlds http://static.tvtropes.org/pmwiki/pub/images/WOTW_redweeds_5314.jpg]]]]
[[caption-width-right:350:At least it prevents soil erosion.]]

->"''Next day, the dawn was a brilliant fiery red, and I wandered through the weird and lurid landscape of another planet, for the vegetation that gives Mars its red appearance had taken root on Earth. As man had succumbed to the Martians, so our land now succumbed to the red weed. Wherever there was a stream, the red weed clung and grew with frightening voraciousness, its clawlike fronds choking the movement of the water. And then it began to creep like a slimy red animal across the land, covering field, and ditch, and tree, and hedgerow, with living scarlet feelers. Crawling. Crawling.''"
-->-- '''The Journalist''', ''Music/JeffWaynesMusicalVersionOfTheWarOfTheWorlds''

One of the possible side-effects of an AlienInvasion.

In essence, Alien Kudzu is a [[FantasticFlora plant-like]], fungus-like, or otherwise predominantly sessile alien lifeform that infests a planet's environment and begins reproducing itself rapidly. It is almost always harmful to the local habitat, whether directly (such as giving off toxic gases) or simply by outcompeting and choking out the local vegetation. It is also usually very difficult to kill, making the act of reclaiming these infestations take up great deals of time and resources (if not outright impossible).

The aliens may release it deliberately as part of their invasion in order to {{Terraform}} the Earth [[HostileTerraforming into a planet more suited to their own environment]], or it may be just the result of the alien plants naturally being able to outproduce the local competition.

Alien Kudzu can sometimes be used for a MeatMoss effect as a sort of visual shorthand to show that things are ''really, really bad''.

Note that neither the plant nor the invaders need to actually come from another planet; "alien" in its original sense simply refers to something foreign and/or not native. (Indeed, in RealLife this is common enough that we have a word for it -- invasive species -- and the kudzu plant is one of the most notorious examples, as [[http://www.netfunny.com/rhf/jokes/90q1/kudzu.1018.html any Southerner can rightly attest]].) They usually are, though, especially in ScienceFiction settings.

Compare ExplosiveBreeder, which is a similar concept applied to animals. Named after the same plant as KudzuPlot, but [[JustForFun/IThoughtItMeant the two tropes are unrelated]]. It's also [[JustForFun/IThoughtItMeant not to be confused with]] the NewspaperComic of [[ComicStrip/{{Kudzu}} the same name]].


[[folder: Anime and Manga]]
* The Radam plants in ''Anime/TekkamanBlade'' are basically this.

[[folder: Comic Books]]
* ''Comicbook/TheLeagueOfExtraordinaryGentlemen'' Volume 2 deals will Wells' Martian invasion (see Literature below), and so features the red weed. In ''League'''s case it is introduced to choke off the Thames river, and so stop the movements of Captain Nemo's Nautilus, the only weapon that was having much success at all against their tripods.

* In ''FanFic/AeonEntelechyEvangelion'' Leng is a mythical plateau from the Lovecraftian AnotherDimension slowly establishing its hold in our reality, starting with its ecology spreading through-out China and most of south-east Asia.

* In a segment of the 1982 film ''Film/{{Creepshow}}'' entitled "The Lonesome Death of Jordy Verril", the titular character (played by Creator/StephenKing, who wrote the script) discovers a newly fallen metorite. After he is contaminated with the oozing interior "meteor shit!" while trying to retrieve the object, he starts to sprout a plant-like fungus which quickly spreads to much of his body. The substance also causes him to itch unbearably and even after being warned by the ghost of his father not to take a bath, he finally can no longer resist and sinks into the filthy tub. The next morning we see that the water has accelerated the growth of the fungus/alien kudzu and it now covers Jordy, farmhouse and surrounding land completely. Jordy ends his horrible ordeal with a shotgun. The segment ends with a radio weather forcast calling for extended heavy rains.
* In Disney's ''Disney/SleepingBeauty'', Maleficent causes a wall of thorns to spring up instantly around her fortress when the Prince is trying to hack his way in.
* ''Film/InvasionOfTheBodySnatchers'' embodies this trope. That, and [[TheVirus assimilation]].
* ''Film/{{Dreamcatcher}}'' features a red fungus that spreads like a rash but also grows on the scenery.
* In ''Film/SuperMarioBros'', the former king of Dinohattan has been de-evolved into a fungus, which got his revenge by taking over the whole city. It helps the protagonists in many ways, such as breaking their falls or providing them items.
-->Great, a building with athlete's foot!
* The movie version of ''Film/TheWarOfTheWorlds'' shows the Martian tripods seeding the red vines across the landscape (apparently created by converting human biomass into a fine red mist and possibly contributing to the Martians' inevitable defeat by exposing themselves directly to the microbes inside human beings).

* Creator/HPLovecraft's ''Literature/TheColourOutOfSpace'' in which a meteorite lands in a small, backwards New England farm. The meteorite contains an alien colour which infects the land and water around the farm. Ammi Pirece, the narrator telling the story to a guy from a land development agency, states that [[spoiler: the area of infection/contamination is growing slowly, year by year...]]
* H.G. Wells's 1898 novel ''Literature/TheWarOfTheWorlds'' is the story of a [[AlienInvasion Martian invasion]] of Earth. As an UrExample of this trope, the invasion sees the Red Weed spread from Mars to Earth via the Martian's ships. Growing explosively, the Red Weed annihilates other plants and chokes rivers. It increases the sense of the Earth being overwhelmed, acting as a parallel for the Martians crushing human civilization and its military. The image was used symbolically on the 2005 movie [[http://www.impawards.com/2005/war_of_the_worlds.html poster]].
* The Triffids from ''Literature/TheDayOfTheTriffids'' may count. Depending on the character, interpretation and adaptation they are either an invasive alien species or genetically-modified organisms. The truth is never known for sure.
* In David Gerrold's ''Literature/TheWarAgainstTheChtorr series'', Red Kudzu grows extremely quickly and can cover an entire town in weeks, and it shelters predatory Chtorran life forms. Attempts to control its spread through napalm strikes are negated in days by further growth, but massive amounts of human remains have been found before it reclaims the scorched ground. All attempts to permanently destroy it have failed, and it is resistant to all known poisons.
* The Croach in the ''Literature/CodexAlera'' is basically an {{expy}} of the [[VideoGame/StarCraft Zerg Creep]], but it resembles a pale green wax rather than a giant spleen. Air pockets within the surface recreate the ideal Vord atmosphere, and any natural life that gets caught in the Croach is gradually broken down into nutrients for the Vord creatures to sustain their biomass. There are specialized creatures that spread it, much like the red plants in ''Literature/TheWarOfTheWorlds''.
* Played with in Creator/DeanKoontz' ''The Taking''. Alien fungi overrun the Earth in what appears to be a massive attempt at rapid terraforming, but then the fungi promptly vanish and everything returns to normal. [[spoiler: The "aliens" prove to be demons, whose transmission turns out to be a simple time-reversed message: "My name is Legion, is Abaddon, is Lucifer, is Satan, eater of souls."]]
* In ''Literature/StarshipTroopers'', the alien kudzu is from Earth; on Sanctuary, where there is almost no radiation, there is a [[ArtMajorBiology very slow rate of mutation]], so the native flora and fauna are simply overrun by Earth [[GoalOrientedEvolution "more advanced"]] plants and animals.
* The grey mould in ''Expedition Venus'' by Hugh Walters. In this case it's not associated with an alien invasion; it was inadvertently brought back by an unmanned space probe.
* ''The Bongleweed'' by Helen Cresswell, a children's book about an incredibly fast-growing escapee from a botanical garden, that overruns the countryside during a heatwave. [[spoiler:It's defeated when the semi-tropical heatwave (conditions it thrived in) ends.]]
* Ward Moore's novel ''Greener Than You Think'' has the surface of the Earth being overwhelmed by a (human-created) variety of super-crabgrass.
* ''Literature/PandorasStar'' has the Primes flood the local lakes and rivers of the planets they invade with a single celled organism that is essential to their reproduction. The pollution of the waters like this kills off most of the other life in them.
* In Creator/HarryTurtledove's ''Literature/{{Worldwar}}'' series, after humanity and the Race achieve an uneasy peace, the Race starts importing flora and fauna from their homeworld, causing ecological damage to the regions of Earth they occupy. In the final book, some white mice get (accidentally) released on the Race's homeworld and start proliferating; the Race throws a fit, [[MoralMyopia completely ignoring the hypocrisy]].
* Creator/ThomasMDisch's ''The Genocides'' has the Earth transformed in a monoculture for alien crops, and the aliens consider humans to be pests and try to exterminate them. [[spoiler:They succeed.]]
* The Red Weed of Gemma Files's ''Literature/TheHexslingerSeries''; unusually for such organisms, it actually ''enriches'' the land it infests -- as long as [[BloodMagic you kill it by feeding it human blood]], which oh-so-conveniently helps funnel power to the deities which seed it and encourages the practice of blood sacrifice in the locals.
* In a short novel by BrianAldiss, a couple of an alien race named as the Charioteers soak a rural English farm with an equally alien enzyme, which massively increases production of everything the farm produces: milk, produce and meat, at the cost of making it completely inedible for humans... but perfect for the Charioteers' needs. To make it worse, humans are not exempt from the transformation. A couple of named characters are transformed into aberrations by exposure and the narrator's LoveInterest barely survives.
* "The Voice In The Night" by Creator/WilliamHopeHodgson features Alien Kudzu lichen that covers an isolated desert island. It's a good thing that it's isolated, because the lichen grows on flesh as easily as rock: possibly the TropeMaker for combining Alien Kudzu with TheVirus.
* In Creator/StephenKing's ''Literature/TheTommyKnockers'', the alien ship releases... [[ImportedAlienPhlebotinum something]] into the atmosphere which is harmful to almost any form of living creature that isn't a Tommyknocker, as part of their terraforming effort. [[TheImmune Having metal in your head combats it]].
* In Creator/StanislawLem's short story ''The White Death'' a civilization of robots lives in [[CityPlanet gigantic city]] beneath the surface of otherwise barren world. When a crashed spaceship, with remains of human crew inside, is found on the surface, the king of the city orders it to be immediately destroyed and broken down into atoms. Single mold spore accidentally avoids destruction and subsequent widespread mildew infestation rises humidity causing the whole civilization to succumb to rusting.

[[folder: Live Action TV ]]
* In the ''Series/DoctorWho'' serial "[[Recap/DoctorWhoS6E5TheSeedsOfDeath The Seeds of Death]]", the Ice Warriors employ pods that release a rapidly-multiplying spore, which sucks the oxygen out of Earth's atmosphere. [[spoiler: The Doctor finds out that ''water'' counteracts the pods.]]
** Also the Triffid-inspired Krynoids, from "[[Recap/DoctorWhoS13E6TheSeedsOfDoom The Seeds of Doom]]", which can possess Earth plants.
** And in the spinoff ''Series/Class2016'', in the two-part episode "Co-Owner of a Lonely Heart"/"Brave-ish Heart", Earth is beset by rapidly reproducing flesh-eating pink flowers resembling cherry blossoms; their sheer volume will suffocate anyone they don't eat. They are never given a name other than "petals."
* In an episode of ''{{Series/Eureka}}'', the ice rapidly spreading over town turns out to be the result of the fungus growing in the ice.
* In ''Series/StargateAtlantis,'' the base gets seeded with the material of the organic ships used by the Wraith, and Wraith ship material begins to overrun the city. Worse, it was actually growing from ''inside'' one of the main cast. (She gets better, and more importantly, [[spoiler: so does Carson, finally.]])
* Double subverted in ''Series/TheXFiles'' episode "[[Recap/TheXFilesS04E11ElMundoGira El Mundo Gira]]". After a bizarre flash of lightning and hot yellow rain, an immigrant farmer is killed almost instantly by what is discovered to be a normal Earth fungus (Aspergillis) instead of an alien pathogen. However,[[spoiler: the fungus multiplies rapidly when exposed to an enzyme of alien origin.]]
* The final episode of ''Series/GarthMarenghisDarkplace'', entitled "The Creeping Moss from the Shores of Shuggoth", itself a likely ShoutOut to "The Lonesome Death of Jordy Verril" listed above, is about a virulent spaceborne ancestor of earthbound [[StockYuck broccoli]] that infects and kills Sanchez's love interest.
* In ''Series/KamenRiderGaim'', the otherworldly [[IDontLikeTheSoundOfThatPlace Helheim Forest]] isn't just a side effect of an alien threat, it ''is'' the alien threat. Its spores come through spatial cracks and sprout up wherever they land, also poisoning the soil so nothing else can grow. Worse, anything that eats the plants' [[ArtifactOfAttraction hypnotically delicious]] fruit is ''immediately'' [[FoodChains mutated into feral creatures called Invess that spread its seeds]]. It's bad enough that the forest qualifies as an EldritchLocation that's already overtaken at least one alien world and is poised to consume the Earth in under ten years. In fact, it's heavily implied that the TemptingApple and FoodChains tropes were inspired by past contact between Earth and Helheim.

[[folder: {{Music}}]]
* Music/{{Genesis}} released a song called "The Return of the Giant Hogweed," about the said invasive plant which has had occasional outbreaks all over Great Britain. Of note is that the history of the plant in Britain, as narrated in the song, is entirely true -- except for the part where they turn into Triffids and begin ''plotting'' their next invasion.
* ''Music/JeffWaynesMusicalVersionOfTheWarOfTheWorlds'': "The Red Weed" is the first track on Disc 2, "The Earth Under The Martians." The music ''alone'' successfully conveys a silent, desolate London, wracked with scarlet fever (after a fashion) and choking to death from an alien life-form. Sir Richard Burton's narration is but the icing on the cake.

[[folder: Tabletop Games]]
* ''TabletopGame/DungeonsAndDragons''
** ''TabletopGame/{{Planescape}}'' features razorvine, growing mostly on [[{{Hell}} Lower Planes]], but eagerly acclimatizing anywhere. Its stem is a living razor wire, growing twisted and under tension and thus lashing around if cut. Some try to use it as a security measure, but it's a virulent weed and hard to eradicate. In some places any sod barmy enough to bring in and plant cuttings can be summarily executed. Egarus is a fungus from Abyss which was accidentally introduced on a Prime world, and after discovering that it grows everywhere and that they can't kill it the natives kick it out to the Quasiplane of Vacuum... and it survives ''[[BeyondTheImpossible there]]''. Egarus quietly destroys (as in, 'disintegrate') all sorts of stuff around when it can find anything.
** Infinity Vine in ''TabletopGame/{{Spelljammer}}''. A leafless plant that ''very'' quickly multiplies its mass until it covers everything that got light and air with 10' thick layer (there's not enough light beyond this). Of course, if it's a ship and the extra mass is more than its power source can pull, it's stuck for good. The vine is easily destroyed, but regrows, and is immune to all diseases and poisons. Due to this magical metabolism it vanishes just as fast on planets and instantly dries up in the Flow -- the only reason why the whole Prime Material Plane isn't buried under 10 feet of green ropes.
* ''TabletopGame/Warhammer40000''':
** The Orks are a [[PlantAliens fungoid hybrid lifeform]] that provides a weird variant of this. Each Ork constantly sheds spores (particularly after death) that usually grow into normal mushrooms, but in certain conditions will sprout first [[ArmlessBiped squigs]], then [[OurGoblinsAreDifferent snotlings, gretchin]], and finally more Orks. Thus each Ork is a one-asexual-alien ecosystem, and any planet that has successfully repelled an Ork invasion will probably still be plagued by recurring tribes of Orks for generations to come. The Department Munitorium, ever pragmatic, consider these worlds to be highly valuable as recruiting grounds, as guardsmen recruited from these worlds have a tendency to be disciplined, methodical, and "bloodied" in combat against feral Ork tribes.
** [[HordeOfAlienLocusts Tyranids]] are a more straightforward example. In the initial stages of an assault a hive fleet will choke a world's atmosphere with spores that send the native flora into overdrive, sprouting more spore chimneys on the ground to accelerate the hothouse process. In the planet's last days digestion pools will form, open-air stomachs that Tyranid organisms dive into to deposit consumed bio-matter, which is then piped up via capillary towers to the hive ships in orbit. And at the end all of this is consumed and recycled, [[PlanetEater leaving nothing but a dead world behind]].

[[folder: Video Games ]]
* The creep in ''VideoGame/{{Starcraft}}'', which is probably supposed to represent this (although in-game it really only affects zerg base-building). In this case, the creep is not exactly an alien plant, but rather part of the BizarreAlienBiology that makes up the zerg. It's closer to being ''zerg blood vessels'' than an alien plant.
* The sequel, ''VideoGame/StarcraftII'', slightly alters the way in which creep spreads, making it considerably more invasive. Though the idea was scrapped for balance purposes, creep was even originally supposed to damage non-zerg buildings over time. It remains impossible for either of the other races to build on creep, though, making this trope in full effect.
* Undead Blight in ''VideoGame/WarcraftIII'' acts similar to ''Starcraft''[='=]s creep, though there are differences- blight does not recede when its source is destroyed and can be dispelled by building structures of other races on or near blight.
* The Biomass in ''VideoGame/UFOAftermath'', it's actually an attempt to build a giant psionic brain and can be driven back with special base sized psionic devices.
* The [[MinovskyParticle titular substance]] in the ''VideoGame/CommandAndConquerTiberium'' series serves to consolidate minerals for more efficient harvesting, with the handy side effect of being so toxic as to depopulate any planet afflicted by it.
* In ''VideoGame/SpaceStation13'' "Space Kudzu" is a plant that spreads rapidly throughout the station and can only be destroyed by fire or suitable cutting implements. Fully grown Kudzu prevents anything from passing through the block that it is on. Not deadly in itself, but it's great at distracting the crew from looking out for assassinations and bombings.
* Xenofungus in ''VideoGame/SidMeiersAlphaCentauri'' has these properties, but doesn't exactly count since it's native to Planet. On the other hand, one could argue that the human's terraforming efforts (particularly with stands of trees) are an inversion (Earth life infesting an alien ecology).
* Pops up in ''VideoGame/DeadSpace'' -- repurposing dead skin cells found in the dust of the ventilation system, ''something'' starts growing inside the ship, gradually transforming the atmosphere into something toxic for humans... even inside a hermetically-sealed starship, it does a lot of damage and spreads widely. On a planet, it would get out of control in short order and swallow up everything. A scientist's log specifically describes it as a "habitat adjuster" or something like that... a biological terraforming engine, in other words.
* ''VideoGame/StarControl 2'' mentions "Deep Children", modified Mycon used to terraform. They rapidly grow into lower part of crust and provoke tectonic activity until the planet is a FireAndBrimstoneHell comfortable for Mycon, but not for original species of Mycon's preferred targets -- organic-rich water-based worlds.
* The Flood in ''Franchise/{{Halo}}''. It's a fungi-like parasite that [[PuppeteerParasite takes over the bodies of sentient species]] and can convert an entire ecosystem into Flood mass; when it takes over the Covenant capital of High Charity, the entire thing becomes covered in Flood kudzu. It has wiped out entire species, and forced the [[{{Precursors}} Forerunners]] to build the titular [[WeaponOfMassDestruction Halo Rings]], and wipe out all life in the galaxy to start anew.
* Systems can be found in the ''VideoGame/SpaceEmpires'' series that have been infested by some sort of titanic organism.
* ''VideoGame/HalfLife'':
** Toward the end of [[VideoGame/HalfLife1 the first game]] you start to encounter organic bounce pads growing in Black Mesa.
** In ''Half-Life: Opposing Force'' parts of Black Mesa are overgrown with some sort of organic matter native to either Xen or Race X homeworld. This biomass often provides ammo for Spore Launcher, attachment points for Barnacle Grapple and, in one instance, a Healing Pool.
** ''VideoGame/HalfLife2'' has two fauna-based variations: the Antlions and the Leeches have overtaken most of Earth's landmasses and oceans, respectively. Barnacles are even more fitting example, as they seem to grow in every place with a ceiling and enough moisture.
* The body of the many in and around the Rickenbacker levels of ''VideoGame/SystemShock2''.
* The enemies of ''VideoGame/{{Aleste}}'' and ''Aleste 2''.
* In ''VideoGame/{{Evolva}}'', the Parasite proves to be able to fill the whole planet with its mooks. Seriously, just compare the huge number of aliens and the number of indigenes you see in each level.
* In ''VideoGame/WarMetal'', the Blight which is just like the vines in ''Literature/TheWarOfTheWorlds'', and it mutates any life forms it touches into Bloodthirsty.
* This makes up the entire ecology of Pragia in ''VideoGame/MassEffect2''. It was a planet that was originally used for testing genetically engineered plants until [[GoneHorriblyRight the plants eventually got loose and infested the entire planet]]. By the time you visit it during Jack's loyalty mission the planet has been abandoned and is used exclusively as a base of operations by numerous SpacePirates, and [[spoiler:Cerberus.]]
* One of the disasters in ''[=SimPark=]'' is literally kudzu. Another one is Alien Invasion, which acts as an alien kudzu.
* The acid Cephad Zoa in ''VideoGame/WakingMars'' eventually takes over any environment it's planted in, converting the fertile soil to an acid type so nothing else can grow.
* ''VideoGame/MarioAndLuigiPartnersInTime'', in keeping with the War of the Worlds parody, has the purple alien mushrooms that the Shroobs spread over the Mushroom Kingdom. With their weapons and technology, the Shroobs can actually [[AndIMustScream turn Toads and other Mushroom Kingdom inhabitants into these mushrooms]].
* In ''VideoGame/ZombiesAteMyNeighbors'', the Red Weed found in ''War of the Worlds'' is an occasional enemy, though since it's sessile, it functions more as an expanding obstacle. The Red Weed is the focus of the stage "Where the Red Fern Growls," where it's the ''only'' enemy type and the goal is to rescue the people surrounded by the stuff. [[LogicalWeakness Weed whackers]] utterly destroy the stuff.
* Presumably ''[[Videogame/MetroidPrime Metroid Prime's]]'' Phazon would behave in this manor, but none of the 5 planets seen hit by Leviathan Seeds are allowed to suffer its effects long enough to see. However, given the "terraforming" nature of Phazon and the glimpses of the interior of the Seeds and the planet Phaaze itself, it's not a far stretch of imagination.
* One could only image what it would look like if The Beast from ''Videogame/HomeworldCataclysm'' were to make contact with a planet.

[[folder:Visual Novels]]
* In ''VisualNovel/SayaNoUta'', [[spoiler:one of the endings have this happen to the entire planet when [[EldritchAbomination Saya]] 'blooms'. Not just the planet but also humans are affected, in ways resembling Fuminori's MeatMoss perspective. This is appropriate, given that earlier in the story Saya compared herself to a dandelion spreading its seeds in a desert.]]

[[folder: Web Comics ]]
* [[http://www.pbfcomics.com/?cid=PBF248-Transmission.jpg This]] ''ComicStrip/ThePerryBibleFellowship'' strip shows one more surprisingly clingy and adaptable lifeform in this role.

[[folder: Web Original ]]
* One of the way of terraforming planets in ''WebOriginal/OrionsArm'' is to release plants like this on the surface where they will progressively alter the environment to suit human life.
* The Wiki/SCPFoundation has [[http://www.scp-wiki.net/scp-506 SCP-506]], an inedible genetically engineered variant of zucchini that grows rapidly in increasingly harsh conditions. As long as there is organic matter it'll germinate.
* Earth life is so deadly compared to everything else in Literature/TheJenkinsverse that an alien ecosystem is slowly losing to ''human fecal bacteria''.

[[folder: Western Animation ]]
* The ''[[WesternAnimation/AdventuresOfTheGalaxyRangers Galaxy Rangers]]'' episode "Marshmallow Trees" had shades of this, but by accident. The Kiwi ambassador was trying to share his planet's FantasticFruitsAndVegetables with a human colony in the form of the titular trees. Unfortunately, while the trees were tested and found to be perfectly safe on Kirwin, the conditions of the colony world caused them to grow out of control.
* ''WesternAnimation/JayceAndTheWheeledWarriors'': The Monster Minds have vines which not only grow all over, but ''between planets in interstellar space''.
* ''WesternAnimation/TheTransformers'' had Morphobots, alien robotic [[ManeatingPlant robot-eating plants]] that spread quickly enough to fill an entire valley in a matter of hours, in the episode ''Quest For Survival''. The Autobots ship them all off to a planet infested with robotic insects, after the Morphobots eat all the Insecticons' clones.
-->'''Bombshell''': You didn't tell us they ''bite back''!
-->'''Megatron''': But those three ingrates got away!
* In the two part Season 4 premiere of ''WesternAnimation/MyLittlePonyFriendshipIsMagic'', Equestria is being taken over by Plunder Vines, which grew from seeds planted by Discord shortly before the Royal Sisters sealed him away, but were kept in check by the Tree of Harmony's reserve power, which is running out.

[[folder: Truth In Television ]]
* The TropeNamer is [[http://en.wikipedia.org/wiki/Kudzu kudzu]], aka the Vine That Ate the South. Brought over from Japan to the [[DeepSouth Southeastern United States]] to prevent soil erosion along railroad trackbeds, it was quickly realized that it grows over large patches of ground, choking out native plant life by denying it sunlight and nutrients. It also loves growing over trees, choking them to death in shade, pulling down branches, or just outright ''uprooting the tree''. It happily does the same to utility poles and power cables, which causes all SORTS of problems. Its growth is roughly a foot per day; take a trip for a week, come back to find it's covered an entire wall of your house. And the seeds can take years to germinate; if you can kill the notoriously hardy thing, you'll often find out a year later that its children have returned for revenge. Oh, [[GoneHorriblyRight and it prevents soil erosion along railroad trackbeds]].
** Then we have ''[[http://en.wikipedia.org/wiki/Imperata_cylindrica Imperata cylindrica]]'', also known as Cogon Grass. This literal red weed chokes more acreage in the southeast than even the more infamous Kudzu and, while not quite impaction on the environment as Kudzu, it is unusually flammable.
** Going back farther, when European settlers first came to the Americas, they brought several plant species with them. Normally not thought of as being aggressive, when introduced to the environment, they went absolutely berserk. Before kudzu, the Deep South was overrun with peach trees (see the correlation between peaches and the state of Georgia), and Argentina was smothered under feral spinach. That said, being edible, these weren't as bad as kudzu. (Technically, [[http://en.wikipedia.org/wiki/Kudzu#Food_and_beverage kudzu is also edible]], but that requires a lot of processing and is a pain.)
** In the Pacific Northwest, there's been a lot of work done for decades to prevent a pair of European plants, Scotch Broom and Gorse, from becoming this. Success has been poor: Scotch Broom is toxic to local herbivores while Gorse happens to be MadeOfExplodium -- it catches fire ''very'' easily and burns extremely hot.
* You've got kudzu? The Soviet Union will one up you with giant hogweed, also known as [[NamesToRunAwayFromReallyFast Stalin's Revenge!]] This thing is just as easy to grow, and its sap is a potent, insidious, slow-acting poison causing burns like mustard gas! It can kill kids who don't know better and tamper with it. It's also fifteen feet tall and grows in massive growths of hazardous shrubbery, like some toxic alien jungle. You have to wear a hazmat suit if you want to clear a patch of this plant monster.
** And its smaller but deadlier cousin, poison hemlock, otherwise known as the plant used to kill Socrates.
* [[ArsonMurderAndJaywalking Zucchini]]. They will overrun your garden if you aren't careful. So will their cousins, pumpkins, and certain wild-type cultivars of strawberries -- which spread by sending out runners that root themselves and grow into a new plant, so that each plant can generate two or three new ones every year -- can be rather surprisingly challenging to keep contained.
* Anyone who has a [[http://en.wikipedia.org/wiki/Populus_sect._Aigeiros cottonwood tree]] in the vicinity of their yard knows how quickly your yard can be inundated by stray saplings. Not exactly alien in areas where it's native, but it's still a hassle to pull the excess of saplings out of your lawn.
* ''Caulerpa taxifolia'', a type of seaweed running rampant in the Mediterranean Sea.
* Water weed (Elodea), after its introduction in Europe and Russia was nicknamed "water plague" for its habit of growing into a sargasso in still or slow-moving waters.
* The prickly pear cactus was this in Australia for a time. Originally introduced as an effective and self-repairing barrier for large cattle stations it eventually grew out of control and chocked off the fields it was meant to guard. It was finally brought under control by the introduction of the Cactoblastus moth which eats it (and local insect eaters will eat them just as readily as native insects). A rare case of introducing a species to Australia that actually ''worked''.
* [[http://en.wikipedia.org/wiki/Japanese_knotweed Japanese knotweed]], as well as its close relative, Giant knotweed. Both are popular garden plants that can quickly fill up the whole garden if allowed to grow unchecked. In the UK, a single plant growing in your garden can ruin your house's mortgage value and it has to be disposed of like a biohazard rather than disposed of in landfill. It can also reappear up to 10 years after being dug up if any root fragments are missed. "Popular" isn't the right term...
* ''[[http://en.wikipedia.org/wiki/Mentha_spicata Mentha spicata]]'', or spearmint, is grown in pots, otherwise it does this -- but underground.
* If kudzu is the vine that ate the South, then Himalayan blackberry is the vine that ate the Pacific Northwest. Fortunately, it's not quite as fast growing and isn't toxic or prone to exploding in flames, but the thorns that grow on its vines and leaves make it unpopular with most browsing animals. Birds and small mammals love its delicious berries, but this just means that its seeds are deposited in a nice pile of fertilizer. It does have one major enemy that's proven quite effective at controlling it, though: goats. Goats will happily eat its leaves and even vines, quickly destroying the plant. So by putting up some temporary fences around an overgrown area and leaving goats to take care of the vines, it can be quickly eliminated with relatively low impact to the environment (compared to other methods of control such as burning it or using chemical herbicides).
* While neither fungi nor plants, the [[http://en.wikipedia.org/wiki/Quagga_mussel Quagga mussel]] and [[http://en.wikipedia.org/wiki/Zebra_mussel Zebra Mussel]] fit this trope to a T. They are invasive species that came to the Americas as larvae in ship ballast tanks, they [[ExplosiveBreeder expand]] [[MeatMoss onto any surface they can find]], hard rocks and metal and muddy bottoms alike (which has caused problems when they clog intake pipes), and because of their sheer numbers they consume huge amounts of food, which destroys the phytoplankton population of any body of water they settle in. This in turn can cause the collapse of the native food webs of the areas they infest, as the removal of the phytoplankton essentially starves the entire food web from the bottom up.
* Many in the U.S. fight the annual battle with crabgrass and dandelions in their lawns.
* Outside of its native habitat, garlic mustard (''Alliaria petiolata'') produces hundreds of seeds per plant, kills soil organisms needed by other plants while "terraforming" the area to its own liking, and is unpalatable to many herbivorous animals.
* Water hyacinths (''Eichhornia'' spp.) quickly crowd waterways, choke out other plant life, and may even deoxygenate the water enough to suffocate fish and other aquatic animals.
* Technically, none of us would be here if not for the original Alien Kudzu, cyanobacteria (a.k.a. "blue-green algae"). Before they evolved, Earth's atmosphere was virtually free of oxygen and anaerobic Archaea dominated its ecosystems. Once they started spewing out volatile oxygen as a byproduct of photosynthesis, they took over the world and drove the oxygen-intolerant Archaea into niche habitats like hot springs, hypersaline waters, or deep under the ocean floor. The chloroplasts of modern plants are their distant descendants, so one could argue that ''all'' vegetation is Alien Kudzu from the Archaea's POV.
** It's important to note that cyanobacteria ''continues'' to be Alien Kudzu, even today. Under certain conditions, cyanobacteria can reproduce explosively, creating [[https://en.wikipedia.org/wiki/Algal_bloom harmful algal blooms]] that can have dramatic ecological effects, like killing off every fish in a lake. Some species of cyanobacteria also produce [[https://en.wikipedia.org/wiki/Cyanotoxin cyanotoxins]] that can cause health problems for humans, which is especially concerning when cyanobacteria grow in water reservoirs used for human consumption, as the water undergoes treatment prior to consumption that removes harmful microbes like cyanobacteria, but ''doesn't'' remove any cyanotoxins that may have been released into the water.
* Some plants could be considered that if they weren't so damned useful. Potatoes for instance originally come from high up in the Andes (and most types of them do not grow anywhere else), but almost everybody who lives in a temperate climate can tell tales of thrown away potatoes suddenly germinating on their compost heap. Those tubers are hard to kill, easy to grow, and so nutritious that Ireland basically ran on them for a century, until a disease killed most of them and caused one of the worst famines in European history.
** The famine that hit Ireland also hit other parts of Europe, particularly France, Germany, and the Austrian Empire. However, both of those countries still grew significant amounts of grains, while in Ireland the landowners had converted most of the grain-growing land to cattle-raising land to satisfy English demand for beef, cheese, and butter. On the other hand, though, France, Germany, the Austrian Empire, and non-Austrian Italy also suffered bad grain harvests in the mid-1840s. This is why the decade is often called the Hungry Forties, and also a big reason why a wave of revolutions hit Europe in 1848.
* In Finland the local plant invader is Lupine. It was imported in 1700s as a garden plant. Since then it has spread everywhere and there are very few stretches of roadside that don't have at least some Lupines. Lupines have a nasty tendency to pump so much nitrogen into the soil that it chokes just about any other kind of native plant, except the ones which were already nuisances, like nettles and willowherbs.