In the South we put up post every 10-15 feet, with a single wire/cable on it.
The problem I see for really cold zones,3,2,1, is that the new vine come out in Spring,
then over Winter to bloom & fruit next Spring.
If this works great, if not you could mount the wire in a way that you could easily detach it,
to lay the wire & blackberry vine on the ground.
Then cover the wire & vine with a thick layer of mulch, this technique is use with Fig trees in the North.
You could wrap the vines in the Fall, if you do not lay them down.
Then there are fruit tree walls, rock & brick is best, with rocks around the plant to cover the roots.
The day heat from the sun, will warm the plant at night.
Walls cost, so a wind break made fiber, wood will help, rocks or gravel on the bed will still warm the roots.
Please let us know how they fair in the North.
