In Season: Available year round because of their dehydrated nature, but autumn is the time to catch prunes at their freshest By definition, a prune is a dried plum. But while all prunes are plums, not all plums are prunes. Certain varieties of plums have such firm flesh and high sugar content that they can be dried with little loss of their original plumpness and flavor. Traditionally, plums were harvested and sun dried to create tasty prunes. Today, most prunes on the market are dried through dehydrators. High production orchards in Northern California generate approximately 99 percent of prunes in the United States, using French Agen plums almost exclusively.