In a previous article I mentioned that you must make sure that your tools and containers are clean before you put sterile potting soil in them. Tools and plastic containers can be cleaned with soap and water, but when it comes to terra cotta containers; it’s not quite so simple.
Terra cotta containers are inexpensive attractive and durable. The pottery is made of red clay that has been baked until it is hard. But because they are made of red clay, they have a very porous texture. Water a container filled with soil and you will notice that over time, the terra cotta appears to sweat water. If you grow plants in a container such as this long term, you will notice green algae or white residue from fertilizer on the surface of the container.
If terra cotta containers can hold water or fertilizer residue in the permeable clay surface, they can also hold diseases, mold spores or fungus in the microscopic holes as well.
A soft brush may be just fine for plastic containers, but in order to deep clean terra cotta containers, you need a stiff brush. A hard-bristled brush used for cleaning dishes is perfect for terra cotta containers. If you are retiring a dish brush from active use in the kitchen, you can down-grade it to your potting bench for this purpose.
Cleaning a container is a three step process. Fortunately, it is not a difficult one to undertake. First you will need to remove the dirt on the inside of the pot. Next is the step where you remove salt buildup from fertilizers. This is the white crusty stuff that is on the interior, exterior and probably in those microscopic pores of the clay surface. Lastly, you will sterilize your container to kill any diseases. When you have finished these steps, your clay containers will be ready to use again.
If there is any soil in the container, remove that first. The soil may be recycled if you sterilize it. I have covered the process of sterilizing soil in a previous post. Allow any soil that is sticking to the sides of the container to dry before you remove them. Dry soil is easier to remove. Once the soil is dry, scrub the sides of the container with your brush. You may wish to leave the algae green patina alone. Many gardeners prefer this patina, since it makes clay containers look aged.
The crusty salt buildup on your containers is a result of salt left behind by fertilizers. This salt can actually harm plants if it builds up in your soil. To neutralize the salt, make a paste of baking soda and water. Spread the paste over the crusty white salt and allow it to dry. Then scrub the paste away with water and your scrub brush. Rinse the container with water and allow it to dry.
The last step is to remove any bacteria, fungus or mold that can contaminate your plants. To do this, mix a solution containing 1 part bleach and 9 parts water in a bucket, tub or kitchen sink. This basin should be large enough that you can fill it with the solution and then submerge your pot in the bleach water solution. So for example, if you have a tub that will hold 11 gallons of liquid, mix 1 gallon of bleach with 9 gallons of water. Allow plenty of room for water displacement when you put the container into the liquid.
You can also make a weaker solution by mixing 10 parts water. Bleach can wear on a clay container, so less is more. Place the container into the solution. Both the solution and the pot should be lukewarm. Placing a warm pot into cold liquid or vice-versa can cause the container to crack.
Allow the pot to soak for at least 30 minutes. Especially dirty containers may need to soak longer. Use the scrubber on any especially filthy areas on the container. Then lift the clay container and submerge it in a basin of clean water. If you use your sink, transfer the container from one side where the basin is filled with bleach water into another side filled with clean water.
Soak the container for 30 minutes. Then drain the water, fill the basin and allow the container to soak for another 30 minutes. Since bleach will harm plants, you want to soak it completely out of the container.
An Alternative to Bleach:
If you don’t want your container contaminated with bleach, you can heat sterilize it in your oven. But just like heat sterilizing soil, this process stinks. Make sure to open a window before you do this. Or, you can sterilize smaller pots by wrapping then in tin foil and placing them in a crock pot or slow cooker that has been moved outdoors and plugged into an exterior outlet.
After dry scrubbing the container to remove soil, place it on a cookie sheet to catch any remaining soil so that it does not fall into your oven. Place the pot in the oven and bake it for 1 hour at 220 degrees Fahrenheit. Then turn off the oven. Allow the container to cool completely before removing it from the oven.
Soak the container again before planting. A clay container should be saturated with water before planting so that it does not pull moisture from potting soil.