i have many types of berry and fruit trees here that have survived -40f last winter with no damage! i have 2 variities of blackberry that grows well here but aren't big producers. 1 is a cultivar called babycakes dwarf blackberry the other is a wild blackberry called canadian or smooth blackberry. this is the plant i sent to twisted tree nursery in new york to use to try and breed a commercial grade cultivar to grow in the north. unfortunately the berries are small but if crossed with another cold hardy blackberry with bigger berries it could produce a bigger, better crop. man has pushed plants to thrive in areas they aren't native to, for thousands of years. thats why we have such a diversity now that was unheard of even 100yrs. ago. the canadian blackberry is hardy clear to the arctic circle. shurley a z4 hardy commercial cultivar can be had with the right crosses w/ this. I've submitted 2 more plants given to me from folks in vermont and n.h that are at least z4 hardy but still suffer from small fruit size. darrow is marginally hardy to z4 and is showing potential as a cross w/ my canadian blackberry. has good size also. also HoneyberyUSA has a z3 hardy cultivar called pequot lakes blackberry. they dont say anything about berry size and they are out of stock right now but i will order some to trial here.if theres a will, theres a way!