Prune the blackberries after they bloom in early spring before tying them. Remove all the branches except six of the strongest canes.
Cut the side branches so they each have 12 buds on them -- about 1 foot long.
Install a post in the ground near the blackberries if you are growing them on a hill. This gives more support.
Install a trellis if you are growing them near a structure, such as a building or fence.
Cut pieces of string that are 4 to 6 inches long, and tie each cane to a wire trellis. Leave enough room to allow you to get two fingers between the string and the cane. This ensures that the plant has enough room to grow and that the string won't damage it. Cut a 12- to 18-inch piece of string to wrap around a fence post.