A Biting Plant Flower can be found very early in the game by feeding fruit to a Biting Plant in the Forest of All Beginnings. Since this method is easily avoided by taking the fruit yourself, a second and more commonly acquired Biting Plant Flower is located in the Underground Maze. The flower is used by being added to a cauldron. If both items are obtained and one is put into the cauldron, the other is useless and cannot be removed from the inventory. That said, you can put the second flower into the cauldron along with the first as long as the flower isn't the last item added to the cauldron.
Contributed by Shellcreeper
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