The boxes can be made of cardboard or wood. The smaller box should be at least 2 inches (5. 1 cm) smaller than the big box on each side. The smaller box should be at least 8 inches (20 cm) deep for larger plants and 3–4 inches (7. 6–10. 2 cm) deep for smaller plants. Get sturdy boxes so that the mold doesn’t break when you fill it with concrete.

The smaller container should be at least 2 inches (5. 1 cm) smaller than the larger container around each side. The smaller container should be at least 8 inches (20 cm) deep for larger plants and 3–4 inches (7. 6–10. 2 cm) deep for smaller plants. For instance, you can get 2 pots or 2 plastic bowls. [4] X Research source

The plastic bag will prevent the concrete from sticking to the small container and will make it easier to remove the mold.

If you want the sides of your planter to be thicker, measure and mark the line to reflect the change in dimensions. For instance, if your planter’s walls are 4 in (10 cm)-thick, you’d measure and mark 4 inches (10 cm) from the base of the planter.

You can purchase a can of cooking spray online or at a grocery store. You can coat the inside of the larger box with mineral spirits as an alternative to cooking spray. Pour the spirits into the bottom of the box and spread it around on the inside of the box with a rag.

You can add a variety of powdered concrete pigments during the mixing process to infuse different colors into the planter. [10] X Expert Source Melinda MeservyPlant Specialist Expert Interview. 18 August 2020. Purchase concrete pigments online. Concrete pigments come in a variety of colors like blue and red. [11] X Research source If you run out of concrete while filling your mold, mix more. Try to get out all of the lumps from the concrete before pouring it.

If you have thicker walls, adjust the pour level of the concrete. For instance, if you marked 4 in (10 cm) from the base of the planter, fill the concrete up to the line that you marked.

The concrete should not fill the small container or connect over the top rim of the small container.

Check the instructions on your concrete bag to determine exact curing times.

If you’re having trouble getting the small mold out, you can use a crowbar to pry it out of the larger mold.

If you’re having trouble getting the planter out, don’t be scared to break the larger mold.

Before you paint the planter with masonry paint, put a coat of block primer on the planter so that you have a flat surface to work on. [20] X Research source You can use stencils to paint specific images onto your planters. Tape the stencil to the side of the planter and fill in the gaps with masonry paint. [21] X Research source

You can also grow a new plant by planting a seed in the pot.