Empty any soil or loose material from the pot before cleaning.
Place the flower pot into a bucket. Fill the bucket with 1 part bleach and 9 parts warm water. Make enough bleach solution to fill and cover the pot. Allow it to sit overnight, or for 24 hours.
Empty the bleach solution, and scrub the pot with a cleaning brush to remove any stubborn residue.
Rinse the pot thoroughly with warm water.
Rinse the bucket thoroughly, and fill it with warm water. Place the pot into the bucket to soak for three or four hours.
Remove the pot from the bucket, and allow it to air dry.