Beach plum (prunus Maritima) trees grow wild along the eastern coast of the US. They are prevalent on Nantucket and can be found all over the island. Look for their clusters of white tiny blossoms in May and make note of where trees are located to forage the fruit later in the season. Beach plums are ready to harvest from late August through mid-September. The small round fruit is about the size of a grape with dark purple skin when ripe with thick sweet-tart flesh that surrounds a small stone in the center of the fruit. They are just like conventional plums at the store but miniature and a bit more tart. When harvesting the plums make sure to pick a small portion of unripe or less ripe plums, which are higher in natural pectin and will help the jam set without the need for adding commercial pectin or thickeners.