Old World climbing fern (Lygodium microphyllum) is the most invasive plant in Florida, surpassing even melaleuca (Melaleuca quinquenervia), according to this article from the Orlando Sentinel. Native to Australia, the fern is taking over portions of the Everglades, creeping along the ground, finding its way up trees, and smothering them with dense foliage that blocks out sunlight. It's difficult to fight such a species with herbicides, since it tends to be intricately intertwined with its victims. Researchers are currently studying two insects as potential biological controls, but for now the invasion continues. You can listen to an audio program on the topic by clicking here.