Terra-cotta pots need to be properly cared for so they'll do their job well the next year. Before storing, disinfect pots to rid them of soil residue that may carry disease harmful to new plantings, especially to vulnerable seedlings. Scrub the insides and outsides of your pots with plain room temperature water and a stiff brush (we used a bottle-cleaning brush) to get rid of soil that could harbor insects or disease. To prevent buildup of dangerous and unwanted mold, let pots dry completely before stacking and storing.
Cleaning Terra-Cotta Pots
Martha Stewart Living, October 1998