A weed creature was an intelligent, semi-mobile form of seaweed that once inhabited the Atlantic Ocean near the coasts of England. They were sighted by mariners as early as the 18th century, but the Seventh Doctor once described it as "primeval," existing since the days of the dinosaurs in Loch Ness. (PROSE: Christmas on a Rational Planet)
Weed creatures fed on gas and had stingers. A weed creature could also release poisonous gas or a foam that allowed it to mentally control those it touched, who in turn gained the ability to exhale gas. It was vulnerable to oxygen and high-pitched sound.
In the late 20th century, a weed creature was sucked up by an off-shore Euro Sea Gas rig, and, in the space of a few days, took over the crews of the off-shore rigs. It later took over Oak and Quill, two engineers at the refinery commanded by John Robson, and used them to commit various acts of sabotage around the refinery. It also took over Maggie Harris, Robson and Van Lutyens.