Blackberry trellises are simple to build and require only a few dollars' worth of materials. To build a simple trellis, sink two 8' pressure- treated 4x4 posts 3' deep in the ground (Image 1). If you're working in sandy soil, you can add a quick-setting mortar mix to anchor the posts. In clay soil, simply tamp down the soil to hold the posts in place.
The posts can be set anywhere from 10' to 20' apart. Make two marks on each post, one mark at 2-1/2' off the ground and the other mark at 4-1/2' off the ground. Then, at those marks, attach 9-gauge coated wire to the posts with staples (Image 2).
During the first growing season, blackberries don't need to be trained to a trellis. Starting the second year, the canes should be tied to the trellis. One method involves loosely tying primocanes to the wire as they develop. The best time to start is before the buds swell in early spring. When the canes are tall enough to reach the top wire, tie them horizontally along the wire. Another method some gardeners use it to train only the floricanes to the trellis and let the primocanes sprawl on the ground.