Blackberry plant varieties come in two main types: erect and trailing. Winter care for the two types differs slightly. Erect blackberry plants are more cold-hardy than trailing ones, and require less care. Trailing types will need to be pruned and then tucked in for the winter. Erect types will merely need to be pruned. If your winters are mild (do not get below freezing), you will not need to do anything except prune for either type of blackberry.
Identify which type of canes are currently growing on your blackberry: primocanes or floricanes. Think about whether you harvested blackberries this year or not; blackberries only come from floricanes.
Prune the primocanes of erect varieties of blackberry during the later winter months in your area. Choose the three or four strongest canes of each plant, and get rid of the rest using your pruning shears. Prune back any trailing branches on these canes to between 12 and 18 inches in length.
Stake the branches of erect varieties and train them to the stakes. Tie them to the stakes with twine so that the branches point upward.
Prune the primocanes of trailing varieties of blackberry during the early winter months in your area. Use the same rules as for erect varieties.
Apply a thick layer of mulch to the canes of trailing blackberry varieties. Use at least 3 to 4 inches of mulch to provide ample insulation against extreme winter weather.