Harry has a "black thumb," not in the traditional sense. Harry's heritage from the House of Black is strong. Any plant that he touches mutates into a poisonous or thorny version. In the most extreme cases, the plant can even attack.

It works on any plant, even vegetables, fruit, and cotton fabric. Ironically, eating the mutated vegetables gives Harry more nutrients. It doesn't work on cooked foods.

Harry is banned from potions and herbology. When Harry touches the potion ingredients, they return to life in monstrous forms. (Snape has a lot of fun experimenting with them.) In herbology, every plant starts to mutate just from being in Harry's presence.

Harry is good friends with Neville, who is fascinated by this ability.

Narcissa was very impressed when she heard about it from Draco.