Well, since the iris experts haven't answered yet, may I suggest an uneducated suggestion. I would have guessed that 2-4 D would be a very bad idea but I actually don't know. However, since the bed has not had regular tending, it seems to me that it would be more effective and productive to simply dig all the iris out. Iris are generally easy to remove since they sit at ground level. Then you could apply 2-4D or whatever and later you could do some gentle plowing etc and add bone meal and then replant good sections. You would wait to do all this at the appropriate time of year for your region.

Chas, I really appreciate your response, I definitely had not thought about that option! Great idea!

Preen is all I use after pulling all the weeds and grass. For the grass the refuses to go away even when pulled many times I use Grass getter mixed with crop oil. 24-D very bad. Renovating the bed is a good start. Preen is a pre emergent so it kills anything that comes from a seed. Grass getter will only kill grasses.