Red weed is a plant-like organism from Mars.It is uncertain whether it was introduced intentionally, or by accident.
The plant, brought by the aliens, rapidly spread across the countryside, colouring the affected areas with a red tint. This was thought to give Mars it's red colour.
Red weed in the novel is composed of stubby fingers, resembling small cacti, and, when underwater, wide fronds, resembling lilly pads. Red weed can grow to a height of at least six feet on land, and can rapidly clog waterways, hiding bridges, and causing widespread flooding. The red weed is apparently edible, although it has a sickly metallic taste.
After about fourteen days, the red weed introduced by the Martians begins to develop white patches. These eventually spread, and the plants become fragile, eventually getting crushed into powder. It is assumed that this is caused by a bacterial infection, similar to that which brought down the Martians. The narrator informs us that seven years after the war of the worlds, the red weed is no longer present anywhere.
Red creeper, another name for a martian plant, may actually be a variant form of the red weed itself.
In his visit with the man on Mayburry hill, the narrator notes that the red weed gives off a purplish glow at night. There is also some indication that the plants may be capable of some movement.
The blood from humans could have been one possibility for the downfall to the Martians, as the blood would also contain forms of bacteria and virus to which the invaders had no immunity.
In the 1978 musical of War of the Worlds, by Jeff Wayne, the red weed is portrayed as having claw like leaves, and the ability to creep along the ground. It even threatens to consume one of the characters. Fast moving, or growing red weed with a voracious appetite for human flesh appear occasionally in comic books, and other incarnations of the movie, but since the narrator walked through the red weed unharmed several times in H.G. Well's original novel, this does not seem to have been his intent.
In the League of Extraordinary Gentlemen, Volume two, Red Weed is being grown on earth again after the invasion, supposedly in order to lubricate the martian technology that has now become common. Large fields of it are seen in the north of Great Britain. In the 2005 adaptation of War of the Worlds by Steven Spielberg, the red weed is portrayed as a form of fast-growing vine and is clearly intentional. The plant is brought by the alien invaders to be used to areoform or mark their territory on Earth, and possibly be used as a food source. The plant is fertilised by using processed human, or, in some possible cases, animal blood, which could be manufactured by a standard Tripod. The end substance is then sprayed across the areas intended for red weed growth, and the plants quickly grow on it. Near the end (like in the book) the red weed turns white and can be crushed to dust before it can be closely examined.Speculation from an SFX magazine suggests that the red weed was originally intended to be used as a breeding medium for the aliens, before the design team changed it to vine-like design. This speculation is also backed up by some concept art for the movie.
In the War of the Worlds graphic Novel, Red Weed is dropped as bio-weapon bombs from Flying Machines. they land on, and grow voraciously, on one unfortunate character.