When files are placed in the Trash, the information previously stored in the "Trashed" files are still physically located on your disk. To shred the information which was stored in "Trashed" files, choose Shred Trash that can be found under the Shred menu or by Control-clicking on the SafeShred icon in the dock.

Note: Like all files and folders that are shredded, there will be no way to undelete or recover items which have been shredded, even if they were first placed in the Trash.