The best time to split them is in the spring...
Bur even a very small clump will fall over.
You can prop them, or cut them down to about half height in early to mid june. It will keep them from getting so tall...but then you won't have those wonderful tall flowers peeking over the fence.
Linda C

I had the same problem with mine, which is in Year 2. Also I find deadheading difficult b/c the unbloomed look the same as the already bloomed so I just hang out a while. Do you let yours set seed? I usually don't.

I've divided/transplanted in the summer, after blooming. Never had a problem. Mine are just coming into bloom now, a little late this year.

I have "Klaus Jelitto" and have never noticed a flopping problem, they flower stalks are always upright. Mine are in full sun - could that make a difference? Not sure, just a thought...

Mine are in full sun too. Even with the flopping, the color is fabulous. Mine bloom for about 3 weeks. One poster last year wrote that theirs bloom all summer. Is this true?

Mine flop too. So bad I cut them down in mid bloom. 😩 I hate the way they look split and flopped.