Transplanting blackberry bushes is best done in fall, after fruiting has passed. Moving a bush to follow changes in sunlight or expand the size of your berry-patch is easy, so long as you protect yourself from thorns. As you transplant, you can also help your existing plant start new ones for even more blooms and berries next summer.
Cut back old canes (those which bore fruit this year) to a height of 2 inches. If transplanting space is an issue, trim back new growth to a height of 6 inches. Fruiting may be less the season after transplanting, but establishing roots is critical to long-term success. Leave one or more new-growth canes long if you want to expand your stock of bushes. Remove any dead or diseased branches down to the crown of the plant.
Dig carefully, at least 8 inches to 1 foot away from the plant, loosening soil gently. You can expect to go down at least 1 foot to remove established roots. Loosen soil gently, and ease, don't pull, roots out of the ground.
Wrap bare roots in plastic if transplanting immediately. If transplanting a bush from a nursery pot or if soil seems dry, immerse roots in a bucket of water for an hour before proceeding. Do not leave plants sitting in water overnight. Roots can be damaged.
Dig a hole twice the size of the root cluster and line it with rotted leaf mold or other compost. Transfer plant and fill with soil and water. Tamp down soil gently. Long roots can be curved gently into the hole but should not be wadded or crushed.
Water regularly until the ground freezes.
Expand your plant stock by "tipping" any new-growth canes you chose to keep long. This means gently bending the cane to soil level and covering the end with an inch or two of soil. Some gardeners prefer to leave the end-tip exposed to mark the location of the new plant. The tip will grow its own roots and start a new blackberry plant.