Cedar planters add dimension and style to your garden, patio or landscape design. Cedar wood contains an aromatic oil that repels insects and helps retain moisture so the planter will hold up well over time. Make a cedar planter to fit the dimensions of your gardening space; long rectangular planters can also act as borders and stocky square planters can anchor the entrance to a garden path.
Determine the desired size of the cedar planter. The types of plants you plan to grow and the available space are the two main considerations for planter size. Standard sizes for cedar lumber include four-, six- and eight-inch widths. Plan the dimensions of the planter as a multiple of these sizes to avoid unnecessary cutting. For example, a 24- by 28-inch planter requires six four-inch wide cedar boards on two sides and seven four-inch wide cedar boards on the other two sides.
Measure and cut the cedar lumber to fit the dimensions of the four sides of the planter. Using the example in Step 1, the only other dimension you need is the height of the planter. Cut 26 1- by 4-inch cedar boards equal to the desired height, six each for two sides and seven each for two sides.
Place six 1- by 4-inch cedar boards side by side with the ends flush to make one 24-inch wide side of the planter. Cut two 1- by 2-inch treated wood strips equal to the width of the side, less three inches. In this example, the wood strips would be 21 inches long.
Center the wood strips across the cedar boards, one six inches from the top edge and one six inches from the bottom edge. Attach each wood strip to each cedar board with a decking screw.
Repeat Steps 3 and 4 to build the remaining three sides of the planter.
Join the four sides together at the corners with L brackets. Place one L bracket four inches from the top edge and one L bracket four inches from the bottom edge of each corner. Attach the L brackets with decking screws.
Flip the planter upside down and measure the inside dimensions for width and length. Cut a section of treated plywood to fit inside the bottom of the planter. Drill holes into the treated plywood for drainage.
Place the plywood on top of a flat work surface. Fit the cedar planter over the plywood. Attach the sides of the planter to the plywood with decking screws inserted through the bottom perimeter of the cedar boards into the plywood.
Cut a section of thick plastic liner to fit inside the cedar planter. Attach the plastic to the inside top edge of the planter with a staple gun. Puncture the plastic over the drainage holes.