Herbs sort of like a dryish soil, so you may be keeping the soil a little moist. Terra cotta is permeable, and by its nature will allow water seepage through the walls. As water evaporates, the dissolved solids (salts) are left behind. Moss, mold, lichens, even algae can find the moist/semi-moist environment hospitable & "decide" to grow there.

It will be difficult to discourage things that want to grow there, unless you resort to frequent mechanical removal (like steel wool). Many, I among them, enjoy these unique patinas on terra cotta. I do know that your level of frustration with the fact that it's there would be greatly diminished if you decide to embrace it rather than be offended by it.

Perhaps someone can/will offer a remedy that isn't toxic to what's IN the pot. I can't think of anything I'd use - just off the top ....

Take care OF. It's nice to see you here. ;o)

Al

I, too, prefer to allow moss and other things to grow on my outdoor terra cotta pots... I think it adds a certain aged, shabby/chic look! I do like to thoroughly wash and scrub used pots with a diluted bleach solution before bringing them indoors for use, however. I don't want anything funky to have the chance to harm my indoor plant collection.

Moss and lichen on outdoor pots is one thing... mold on indoor pots is not acceptable to me. Cutting back on the watering usually helps... molds prefer damp.

Thanks you guys for your thoughts. I guess I should have said that these are indoor pots, at least for now. They might be going outside after all because I don't want mold spores floating around in my house. And I agree about the look it gives to outdoor pots and other things. We'll just see.

Try white vinegar.
Bill

Hydrogen peroxide diluted with water works really well. 50/50 in a spray bottle is safe for the plants. Just spray on and scrub off then rinse. For whiteish areas apply some CLR but take care not to get on plants. If using vinegar I would only use on empty pots. Vinegar can kill plants.

Jill K