It's ok to divide Japanese iris in summer after they bloom. Read this about the dubia.

http://www.srgc.net/forum/index.php?topic=8910.0

generally speaking... it is the time of year.. where plants grow new roots.. to store energy.. for next year ... so its a good time to do some dividing ...

the downside is that doing it this time of year... might result in really ugly stock.. for the rest of the season ... but your goal is more plants... rather than pretty plants ...

but part of that is how far you are going to divide .... just cutting the plant in half is one thing.. separating them down to 20 pieces is another ...

short of a hundred degrees.. heat is probably the least important variable ... proper watering would be my prime concern ...

these general ideas are not plant specific ... dont always be looking for such ... research is good.. but dont fixate on such .... just do what you gotta do.. when you gotta do it ...

ken

Actually I am not sure about dividing my irises yet. I digged up a a clump of that iris this early spring and barerooted it before potting it up. So not sure that the roots are even established in the pot enough to divide it yet.

I don't mind the ugly stock. As long as it will come back next year. I ended up digging up a nice clump of houttuynia cordata 'chameleon' from the ground and split it into like 7 different pieces. I washed off all the native soil on it and pretty much it bareroot. Then I potted it in a 5-1-1 mix and placed the pot on a saucer of water(so they don't dry out and these plants can live in water).

Houttuynia is one of the most aggressive plants known. DON'T EVER LET IT ESCAPE FROM A POT!