Though Luke Smith initially thought a "plant" simply meant a "vegetable lifeform", Sarah Jane Smith explained that, in the case of Diane Cope to Martin Trueman, "It means he knows her. He's planted her in the audience." In this case, she suspected that Diane was placed in the public to give the illusion that he knows Diane's life from astrology. Her theory was quickly disproven, though, when he began to read Rani Chandra's life and even Sarah Jane's herself. (TV: Secrets of the Stars)
Strood though the Ninth Doctor may have been a plant, "only brought in [to the Big Brother house] to stir things up." (TV: Bad Wolf)