Write down the measurements of the triangle you want to make. Cut three pieces of a 4-inch by 4-inch (4x4) post all the same length to the size of the triangle sides.
Saw off the ends of the 4x4s at a 60-degree angle with the miter saw. Fit these angled pieces together to make the triangle by making the angles face each other. Repeat this step until you achieve the height of the triangle that’s sufficient for you.
Drill holes through the layers and hammer large nails into the holes to secure the triangular box.
Line the inside of the box with landscaping fabric if you are using it outside. The lining protects the wood from moisture or weeds. Fill the triangular planter box with soil and plant your selected greenery inside.