Answer: Part of the beauty of terra-cotta pots is the character they develop as they age. Some of this character is in the white streaks and green mold that they develop. Since terra-cotta is porous, moisture is constantly being drawn from the soil to the outside of the pot where it evaporates. This is good for the roots of your plants but it also sets up just the right environment for salt build up. The salts are relatively harmless, and outside of sealing the pots inside and out, there's no real way to prevent it. You can scrub most of it off with a stiff brush and water, but it will come back again as you use the pot and more water evaporates and concentrates on the rim and the outside of the pot.
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