Mushrooms are fungi that typically grow in grassy or wooded areas where they feed on the living or decaying food matter produced by green plants.s the buttons grow, the shape of the mushroom becomes more recognizable. The stalk of the mushroom grows very quickly, typically as a result of the water it absorbs. Most mushrooms will reach their maximum height within 48 hours. The mushroom cap expands like an opening umbrella as the stalk shoots up.