The plant was one of several carried over from Mars but it was the only one that was able to spread across the countryside, coloring the affected areas with a red tint. The dominance of red over green in Martian flora is thought to be what gives Mars it's red color.
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, though it has a sickly metallic taste.
After about fourteen days, the red weed began to develop white patches which eventually spread, and made the plants increasingly brittle until they crumbled into powder. It is assumed that this was caused by a bacterial infection, similar to that which brought down the Martians. The narrator notes that six years after the invasion, the red weed is no longer present anywhere.
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, at one point threatening to consume one of the characters.
Fast moving, or growing red weeds 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 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 aeriform or mark their territory on Earth, and possibly be used as a food source. The plant is fertilized 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.
In the 1998 video game, martians seed conquered territories with Red Weed. It serves no gameplay purpose other than to designate martian controlled land.