Thorns were added in the chapter: Stretched Thin
Thorns are the second plant-based obstacle to be added into the game, the first one being Algae. Unlike Algae, thorns do not grow when coming in contact with Water.
Thorns being cleared by Poison Water is impossible.
When a Balloon comes in contact with Thorns, the Balloon is popped, and the contents are released like Toxic Ooze but they do not fall.