I have two large unsealed and unpainted terra cotta planters that have spent some time outside and now appear to be stained in some way. The stains look like wet dirt (see photo). I have tried scrubbing the outside (as gently as I dare); the stains fade when the pot surface is wet, but return once it's dry again.
What is this? Is water seeping from the inside of the pot? I do have large plants in there, but I have a wetness probe and the pots aren't wet on the inside -- water can also drain out the bottom, so it's not pooling in there as far as I know. Is it mold or something? I've used Formula 409 and a scrub sponge on it (again, trying to be gentle). I'm worried that the terra cotta is porous and could somehow absorb chemicals so I'm trying to be cautious, but I'd love to get the stains off!
Any ideas on how to remove these stains?
