Hollywood is green with ivy. The vine sprouted in 1992 on Sony Pictures Classics' posters for the hit Howards End. Then ivy was clinging to ads for another period film, Miramax's Enchanted April. Now it's overtaking print ads for Miramax's '30s-era Sirens. But this passion for plants is bringing out the animal among competing studios. ''(Miramax's) campaigns are Howards End wannabes,'' huffs Sony's Tom Bernard, who says the End art was designed to lead a reader's eye to the critics' quotes. ''It's common for a lesser movie to hitchhike on the established film.'' Such poisonous talk doesn't bother Miramax's David Dinerstein, who denies cloning the campaign but sees a link between the posters for the two Miramax films. ''People are calling Sirens a hip Enchanted April,'' he says. ''Ivy is a provocative plant. It dates back to the Garden of Eden. And both movies have plants in them.''