Celery
Celery was originally a biennial that grew in marshland areas and had a pungent, medicinal flavor. It was well known in the Middle Ages as a medicinal plant, but it wasn't used as food until much later. Due to its unpleasant taste, it was not used as a vegetable until the Italians and French developed a sweeter tasting, thicker stalked variety in the 1500s.
Gardeners in Italy, France, and England were heavily involved in improving the flavor and growth of celery during the late 17th and early 18th centuries, and European colonists brought it with them to America in the early 1800s.