You can make a concrete garden planter to suit the shape and size you need for your garden. Using molds, you can create planters that are square, rectangular or round. Once dried, the planter can be painted to coordinate with your garden design. You also can leave the concrete unpainted and allow the elements to age it, giving your planter a rustic, antique appearance. Concrete planters add texture and visual interest to your container garden. You can use them for flowers, herbs and succulents.
Choose two molds of the same shape with the inner mold being 2 inches smaller in width and length than the outer mold and at least 1 inch shorter in height.
For example, if you have a box that measure 12 inches in width and 12 inches in length, and measures 12 inches in height, your inner mold should be 10 inches by 10 inches with a height of 11 inches.
Purchase ready-mix concrete. The aggregate rock in the mix should be between 1/4 inch and 1/2 inch. Check the packaging for coverage to determine how much concrete you need to purchase.
Set up a workspace. Lay down a large tarp on a stable surface. Have two or three bricks, a screwdriver or small stick and cooking oil, along with your molds, in your workspace before starting.
Mix up the concrete. You want the concrete to be somewhat dry, so use approximately 15 percent less water than recommended for making a concrete slab.
Spray the inside of the outer, or larger, mold with cooking spray. Pour concrete into the mold to a depth of 1 inch.
Spray the outside of the inner, or smaller, mold with cooking spray. Place the inner mold into the larger mold so it rests on the freshly poured concrete. There should be a one-inch gap on all sides between the outer mold and the inner mold.
Place one or two bricks into the inner mold to hold it in place. Slowly pour the concrete into the gaps between the two molds. You can scoop the concrete into a small container and pour it into the gaps.
Continue to pour the concrete until the concrete is level with the tops of the two molds. Use a screwdriver or thin stick to remove the air pockets from the concrete. Do this by gently "stabbing" the screwdriver into the concrete and then lifting it out. Add more concrete to level off if necessary. Smooth the top of the concrete with a trowel.
Allow the planter to dry for 24 hours. Gently turn the planter over and remove the outer form. You might need to tap it lightly with a small rubber mallet or hammer to loosen it.
Turn the planter back over and remove the inner mold. Allow the planter to air dry for three days before painting or planting.