From the Super Mario Wiki
A Hurt Plant is a species of carnivorous plant found on Lavalava Island in Paper Mario. These plants look exactly like Heart Plants, but when touched, they do not drop hearts, and instead attack Mario. Their encounter method effectively prevents a First Strike.
In battle, they attack by diving into the dirt and popping out near Mario to bite him, possibly poisoning him. However, Mario can prevent himself from being poisoned by blocking the Hurt Plant's poisonous attacks.