Concrete planters add an industrial, modern component to your outdoor or indoor living space. You can make concrete planters using a variety of household items, such as wastebaskets, bowls and coffee cans. One way to make concrete planter boxes with a box shape is to use cardboard boxes to mold the planters. Working with concrete is not difficult; you can create inexpensive items for your home and garden with a designer appeal.
Pour builder's sand onto a flat work surface. Mix water in with the sand to create a pile of damp sand.
Fill a medium-sized cardboard box with the damp sand. The medium sized box should be large enough to fit the smaller box in the center with a 5- to 6-inch space between each of the four sides.
Layer 2 inches of damp sand on the bottom of the medium sized box. Center the small box on top of the sand in the medium box. Pack the space between the sides of the two boxes with damp sand. Remove the small cardboard box. The sand should maintain the shape of the small cardboard box inside the large box. Pat the sides firmly as needed to keep the sand compact and together. The sand creates the mold for the planter.
Follow the manufacturer's directions to mix the concrete topping mix. Layer 2 inches of concrete topping mix onto the top of the sand layer at the bottom of the medium cardboard box.
Cut a 1/2-inch dowel into two 3-inch sections. Oil each section of dowel and insert them straight into the concrete mix on top of the sand, spaced at least 2 inches apart. The top of the dowels should be flush with the top of the concrete layer. The two dowels will create drainage holes in the concrete planter. The oil is a release agent to help the dowels slide out of the solid concrete planter.
Cover the outside of the small cardboard box with oil. Position the small box on top of the layer of concrete topping mix. Pour the concrete topping mix between the outside of the small box and the sand layer along each side. Tamp the mix in place as you fill the space. Leave the concrete mix to wet-cure in the sand mold for three to four days.
Remove the small box. Remove the concrete planter from the sand mold.