On the TV Tropes Swamp Thing page, it's stated that a bonkers crossover was in the works at Vertigo between Poison Ivy and Swamp Thing:
"A Swamp Thing/Poison Ivy story was written in the mid-90s for publication was shelved due to to conflict between Vertigo and Batman editors. The story would have revealed that Alec Holland and Poison Ivy had an affair and that Ivy spent years bleeding Alec Holland dry financially and forcing him to steal chemicals for Ivy to use in her criminal capers, by threatening to out the affair to his wife Linda. Years later, Ivy and Swamp Thing would meet for the first time as Ivy attempts to gain plant-based elemental power and Swamp Thing would unleash unspeakable vengeance against Ivy for the years of torture Alec endured; when she uses her power to turn herself into an elemental, Swamp Thing would use his own powers to override Ivy's usage of her own new power, turning her into a tree THEN stripping her of the ability to turn herself back via destroying the artifact she used to gain her powers, trapping her in an And I Must Scream hell. The story never got published because the Batman editors got tipped off that the Vertigo editors not only would not let them undo the story so Ivy could appear in future Batman stories, but they would attempt to STRIP Batman editors and pretty much ANY OTHER DC WRITER of any chance to use Ivy again, by claiming that since she appeared in a Vertigo book, she was now irreversibly Vertigo property."
However, after some relatively thorough research, I couldn't find anything on this proposal that had been shot down? Does anyone have any proof that this story was in the works at DC?