Black raspberries provide juicy fruit during the second year after being planted, and they can easily grow in a small area in a backyard. The bushes provide an aesthetic value to the landscape when trellised, and they can be used as a privacy wall once they mature and climb up the trellis. When building the trellis, doublecheck the area's dimensions where you want to secure the trellis, and measure the boards accordingly. The size depends on your preference, whether you want it in a 2-foot by 3-foot space or to cover an entire wall. An ideal size is 4 feet by 4 1/2 feet.
Create the skeleton of the blackberry trellis. Set down the five 4 1/2-foot boards on an even, flat surface vertically. Make the edges line up evenly so the layout creates a box shape, then space the boards 4 inches apart.
Take one board from the set of longer boards and place it across the top of the first set, horizontally. Measure 4 inches down from the top edge of the first set and slide the horizontal board to this point. Keep the horizontal board centered and perpendicular to the vertical set.
Measure down another 4 inches from the first horizontal board and draw a horizontal line across all the boards, even and straight. Continue to repeat this process of measuring down 4 inches and drawing a line until you reach the bottom of the vertical set. Set the remaining horizontal boards across the marks, keeping parallel with the first one. Keep all edges even and centered.
Use a hammer to nail at each intersection so the horizontal and vertical boards are fastened together.
Pick up the blackberry trellis carefully to make sure it is secured and won't fall apart. Flip the trellis to the other side, if necessary, to secure each board's intersection with a second set of nails.
Stain or paint if desired to match your house or paint it green to blend in with the blackberry bush once it grows up the trellis.