Black walnuts are the only all-wild tree nuts in the United States. They vary from the traditional English walnut with which U.S. consumers are most familiar in that black walnuts have a far tougher shell and earthier flavor.

The growth this ingredient is seeing on menus is due to that unique flavor for use in cocktails. The bitter, earthy flavor gives balance to sweeter drinks and provides a welcome dimension to cocktail standards like the Old Fashioned.

Market research firm Datassential reports that it’s currently found on 1.4% of menus, up 22% over the past four years. Currently, 60% of the population knows it while only 35% have tried black walnut. Awareness is highest among Boomers, the firm reports.

Click through the gallery to learn more about this Flavor of the Week and see how one restaurant is using black walnut on its menu.