Concrete planters are sturdy, strong and often look like decorative stone. If you've used a concrete planter for a plant that has died (or that has been potted elsewhere), and now wish to re-use the planter, clean it first. Cleaning and sanitizing planters prevents diseases from spreading plant to plant. A concrete planter's rough surface doesn't always lend itself to easy cleaning, but with the proper equipment you should be able to prepare the planter well enough to safely house a new plant.
Place a plastic bag near the planter. This is to hold the dirt you remove from the planter. If the previous plant suffered from a disease, or the dirt appears moldy, you should dispose of this with your yard waste--otherwise, you can just put it in your compost pile or another area of your garden.
Put on your garden gloves. Use a spade or small shovel to remove dirt from the pot. If the pot is small and light enough, you can turn it on its side, on the plastic bag, to let gravity help you with this part. Otherwise, use the spade or shovel to remove all you can, first.
Turn on your garden hose and fill a clean bucket with water. Add 2 to 3 tbsp. of a mild, biodegradable dish detergent.
Wet a scrubbing brush in the detergent solution and scrub the inside of the pot, where it is likely the most dirty. Rinse the concrete planter out with your garden hose. Scrub the inside again if any dirt remains (and rinse once more).
Scrub the exterior of the planter with the scrubbing brush dipped in the detergent solution. Rinse as you did the interior of the planter.
Mix a solution of household bleach and water in a clean spray bottle. The North Dakota State University plant disease management Web site recommends using a ratio of 3 cups of bleach per 1 gallon of water to sanitize garden instruments. After you've mixed the solution in your spray bottle, spray the entire interior area of the concrete planter. This should kill any harmful microbes remaining. Let the planter dry completely before using it for planting new plants.