Koshu koume is the collective name for the small plum varieties long grown in Yamanashi. There are records of it as a product of this province from the late Edo period, which is early for a named regional crop, and as sericulture declined here plum cultivation expanded to take its place.

A single fruit weighs four to six grams. The stone is small, which leaves more flesh in proportion than the size suggests. The Kofu basin gets strong sun in the day and cold at night, and that swing is credited with suiting the fruit.

They are picked while still green, and the packing is meant to be finished by the end of May.

What goes in with them is red shiso, rice vinegar, and salt. Then comes the part that separates this from an ordinary umeboshi: it is never dried in the sun. It stays submerged in its own plum vinegar, a method called dobu-zuke, sunk pickling, and because it is never dried it never softens. The fruit keeps a hard, brittle snap when you bite it. Some makers add eggshell to the barrel to hold that texture. It keeps well enough to be eaten all year, on rice, with tea, in a rice ball, dropped into ochazuke. The defining technique of this pickle is a step that is left out.