Any plant that 'engulfs' the bearded iris will not be beneficial. So it depends upon how close and how big the Nepeta are. Make sure a lone rhizome doesn't get overtaken.

Thanks..that is what I thought..I was wondering how close people grow their nepeta to the iris or do they just keep cutting it back.

Your growing season is 12 months?!

Depending upon the Nepeta, 'cause there are dwarf_er cultivars around, I would guess 1.5 feet away for most. That allows a 3-foot width for the Nepeta. Is your bed large enough for that?

Unless the Nepeta is directly south of the iris (providing a 'northern exposure') the shade won't be an issue in sunny Florida :-)