This game element consists of two parts: the Venus Flytrap and the Soil. As long as soil patches remain on the board, the plants will continue to spread - one new Venus Flytrap grows on an empty soil tile after every move.
To clear the board, you must act in two stages: first, match pieces or activate bonuses next to the Venus Flytraps to remove them, then do the same to clear the Soil itself. Only once all the soil is removed will the plants stop spreading.