Answer: Terra cotta pots have a couple of advantages over plastic pots. Besides being attractive, they're heavy enough to keep a top-heavy plant from toppling over, and they're porous, which is important for plants that need exceptionally well draining soil. Because they're porous, they wick water away from the soil and through the sides of the pots, where it evaporates. Since the sides are damp most of the time, it creates a perfect environment for mold to grow. Some people think the white or green mold adds to the character of the pots. Others think it's unattractive. If the plants you're growing require the quick drainage and extra humidity these pots provide, you can disguise the the unattractiveness by placing the clay pot in a larger decorative pot. It's perfectly harmless, but if you just don't like the looks of the surface mold, you can carefully scrub the outsides of the pots with plain water and a soft brush. Just hold them at an angle under running water and scrub the mold off. The mold will return and you'll have to scrub it off repeatedly, but that's just the nature (and part of the charm) of terra cotta pots. The only way to keep it from forming on the outsides of the pots is to seal them with a clear acrylic coating so water cannot penetrate.
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