I do a lot of container gardening, from starting seeds to growing plants that require pampering. I never, ever throw away an intact pot. I’ve even been known to pick them out of a curbside trash pickup pile.
One thing I’ve learned, the hard way, is the importance of cleaning those pots before using them for another seed, seedling or plant. Why? Take a look at one of your used pots — see those white rings around the inside? Those are salt and mineral deposits and they can dehydrate a plant’s roots if they sit up against the rings. Then there are all the gross things you can’t see, such as bacteria and fungal spores. So, even if a container appears clean, don’t take any chances and give it a good cleaning and sanitizing.
I admit, the large pots require a bit of a juggling act to get them sanitized, but when I stop to consider what might still be clinging to them — fungal pathogens, pests, fertilizer salts, etc. — the contortions are worth it.
Small pots can be cleaned in the kitchen sink or outdoors, in a large bucket. I use my bathtub for large pots. The sequence is easy:
Scrub off the gunk
I like steel wool to scrub terra cotta pots, and not the steel wool with soap injected into it. If the gunk is really stuck on the inside of the pot, soak it for awhile and come back and try again.
Plastic pots are generally easier to scrub and steel wool scratches the outside, so I use the scrubbing part of a sponge made for non-stick cookware on those.
Make sure the water is warm and soapy and scrub the pots, inside and out and then rinse in clean water.
Disinfect planting containers
Make a solution of 9 parts of water and 1 part household bleach and soak the pots for 15 minutes in the solution. You might want to wear some latex gloves when you go fishing into the bleach solution to retrieve the pots — bleach irritates the skin.
Now rinse the pot well in plain water and leave it somewhere to dry, preferably in the sun (the sun can help kill off anything that the bleach didn’t).
Lil’ tip: When it’s time to pot up your new plant, soak terra cotta or unglazed clay pots in a bucket of water first. This keeps the clay from soaking up the moisture in the potting medium.