The ditchlily, Hemerocallis fulva is the original daylily, without it we wouldn't have the beautiful daylilies we have today. They were brought here with the colonists because they provided food (all parts of the plant are edible) and medicine and because they also provided beauty. As the colonists settled or moved on, the ditchlily went with them, so they spread from east to the south and west as it was settled.
Just as Rita says, they are very good at providing erosion control, too. And I've read that in some areas they are used to provide a natural firebreak because they hold so much water.
