doesn't leave much to the imagination, does it...:
"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. "