They are toxic to each other when they have different stems. Think of it as chess, pieces of the same color (stems in the case of symbiosis) cannot kill each other. I didn't have time to implement a feature that shows affected tiles by a plant when you mouse over it, the best way to check that would be by replanting the seed or memorizing the patterns, which are pretty simple. There's only 3 different flowers in the game so the player would only need to learn 3 patterns (the eye flower affects all tiles in the same row and column, the diamond flower affects tiles diagonally and the carnivore flower forms a ring).