Although many short berry species don't require trellises, expert gardeners typically opt to use one for trailing berry species that grow long canes, such as blackberries and raspberries. According to Jan Gertley, co-author of "Classic Garden Structures," using a berry trellis keeps your berries from morphing into troublesome tangles and dense thickets. Training your berries to grow on a berry trellis eases your job of maintaining your plants because it allows you to remove weeds or add mulch and compost without having to fight through the berry canes. Often called vertical trellises, berry trellises run parallel to the ground and resemble a clothesline in shape and structure.
Measure out 10 feet from the base of your berry plant in one direction and mark the location with a stone. Measure out 10 feet from the base of your berry plant in the opposite direction and mark that location with a stone. These points mark the locations of your berry trellis posts; if you have more than one berry plant, measure 10 feet out from the two plants marking the ends of your row to determine the end post locations for the berry trellis.
Dig a 30 to 36 inch deep hole at the first end post location using a clam shell post-hole digger. Insert the end of a 9 foot long, 6-by-6 inch, pressure-treated post into the hole. Check with a level to ensure that the post is straight in the hole. Tamp the base of the post firmly into the ground using a tamping rod. Repeat this entire installation process with the other end post.
Measure 30 inches from the ground on each end post and mark the location with a carpenter's pencil. Turn one of the 2-by-4 inch boards perpendicular to the first post; rotate the board so one of its 4-inch sides touches the post, and position the bottom edge of the board in line with the pencil mark on the post. Secure the board to the post with two 3-inch galvanized screws to create the support board for your berry trellis. Repeat this process on the second end post.
Wrap one end of a piece of galvanized 14-gauge wire around one of the support boards, locating it 6 inches from one end of the board. Secure the wire to the board with a galvanized fencing staple. Run the wire to the other end post; pull the wire taut, wrap it around the second support and secure it with another fencing staple 6 inches from the end of the support. Repeat this process with a second wire 6 inches from the opposite end of the trellis support boards so your berry trellis has two support wires.
Measure 30 inches above the first set of trellis support boards on both end posts and mark the locations with a carpenter's pencil. Install a second 2-by-4 inch support board on each post at the 60-inch location in the same way that you installed the first set. Run two strands of wire from the second support, securing them with galvanized staples.
Wrap the canes of your berry plants around the wires to encourage them to grow along the wires instead of the ground.