Concrete is such a durable building material that Roman structures such as paved roads, aqueducts and even the Colosseum--which were made using a form of concrete--survive to this day. You can make sturdy, long-lasting garden urns from concrete by casting them using concrete molds. Concrete urns will insulate a plant's roots against temperature extremes, including freezing temperatures in winter and heat in summer. Additionally, concrete urns will not rust the way that metal urns will.
Put on protective gear including eye wear, breathing protection and rubber gloves. Concrete is caustic and can harm you if you breathe it or get it in your eyes.
Combine cement and water in a mixing bucket. Stir until it's the consistency of a mud pie.
Add 1 cup of fiberglass reinforcing fibers to the mixture.
Coat the inside of each half of a concrete urn mold with vegetable oil. Assemble the mold by aligning each half and snapping the mold in place.
Pour the concrete into the mold. Soak the mold with water from a garden hose. Cover the concrete mold with a plastic tarp. Allow the concrete to set for between two and seven days.
Open the mold to release the concrete urn. Brush the urn with a wire-bristled brush to smooth out the seams. Soak the urn with water and cover it with the tarp for another seven days to allow the urn to cure and harden.