The ' Marion' cultivar (Rubus L. subgenus Rubus) or Marion blackberry, marketed as marionberry (help·info) , is an indigenous blackberry developed by the USDA ARS breeding program in cooperation with Oregon State University. It is a cross between the ' Chehalem ' and ' Olallie ' blackberries.he marionberry's growing conditions are perfectly adapted to the mild, maritime Oregon climate, with its mild rains and warm summers. The berries ripen throughout late spring and early summer. The harvesting season is typically between July 10 and August 10, with a single acre producing up to six tons in a harvest.