She met Swamp Thing in an arc of Justice League International, I think. Whichever one got canceled after DCYou and had Stargirl.
Her reaction was like "Wait, if you're here, what do you need me for? I just talk to plants and shit, and this guy IS the Green!"
And after the battle, she ran off with his head, which was what was left of him, to nurse him back to health.
And she might have met him whenever he invaded Gotham, but I haven't gotten around to old Swamp Thing yet.