"In Steven Spielberg's 2005 War of the Worlds film, the presence of the red weed on Earth is clearly intentional. Once they have a strong hold of the planet, the invaders take captured humans and drain their bodily fluids, which act as a nitrogenous fertilizer for the red weed, helping it grow and cover the planet. Though it is never stated that these invaders come from Mars (the "Red Planet"), the script indicates that the weed is abundantly present on their world as well, suggesting the two share a superficial resemblance. However, what exactly makes up their red weed, whether it is a natural vegetation or what gives it its color on their world, is unknown. It is also stated in the script that the weed is fierce enough to flourish in spite of the conditions that forced the aliens to find refuge on a new world. But despite its tenacity to survive such a harsh environment, much like in the novel, the red weed is killed by Earth's bacteria and turned into an ash-like dust and is blown away in the wind before it can be examined."
http://en.wikipedia.org/wiki/Red_weed
