I was led to believe that to overwinter Cosmos Chocamocha you treated them like Dahlia's? I recently potted up last years corm and on inspection today found the corm had rotted. A new corm that I bought this year and potted up at the same time is growing well. Should I have treated it as my Canna Lily and kept it in slightly damp soil in a frost free place over winter? The Canna Lily is now growing strongly.