What is crossed with a blackberry and loganberry to produce a boysenberry?

The boysenberry is a cross among the European Raspberry, a European blackberry, an American dewberry and the Loganberry.

The exact origins of the boysenberry are unclear, but the most definite records trace the plant as it is known today back to grower Rudolph Boysen, who may have obtained the dewberry/loganberry parent from the farm of a man by the name of John Lubben (who called it a "lubbenberry"), who in turn may have obtained it from Luther Burbank.

In the late 1920s, George M. Darrow of the USDA began tracking down reports of a large, reddish-purple berry that had been grown on Boysen's Northern California farm. Darrow enlisted the help of Walter Knott, a Southern California farmer who was known as a berry expert. Knott had never heard of the new berry, but he agreed to help Darrow in his search.

Darrow and Knott learned that Boysen had abandoned his growing experiments several years earlier and sold his farm. Undaunted by this news, Darrow and Knott headed out to Boysen's old farm, on which they found several frail vines surviving in a field choked with weeds. They transplanted the vines to Knott's farm in Buena Park, California, where he nurtured them back to fruit-bearing health. He began selling the berries at his farm stand in 1932. When asked what they were called, Knott said, "Boysenberries," after their originator. His family's small restaurant and pie business eventually grew into Knott's Berry Farm.