White mold on terracotta pot exterior

[estación seca]

The pot had minerals in the clay when you bought it. And it doesn't take a high concentration of minerals in your water to cause the efflorescence. All the minerals in all the water are carried by the evaporating water to the outer surface.

Most of the evaporation from the pot takes place from the clay surface, which is larger in aggregate than the opening at the top. You water the medium so you wash the salts there back into the pot.