Here in zone 6B New Jersey, Boltonia typically blooms in late August/September and any blooms that time of year cannot be understated. They have a fantastic flower color and can be statuesque if situated in the right spot where they can be supported by other plants.
Unfortunately, I have struggled to get these perennials to NOT topple over once in full bloom: 
Last year, I finally took things into my own hands and cut them back by 1/2 in early June. By the time they started blooming in September, they were about a foot shorter than normal at 2 1/2 feet:
The results were a marked improvement from prior years but I still think I can do better and keep them even more upright.
So this year, I waited two weeks longer before cutting the three Boltonia back by 1/2:
Now we wait and see if this will make a difference at all.
Have a great weekend.
John
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John, you are wise to cut them back in early summer for sturdier branched plants. ‘Pinkbeauty’ tends to have a more
relaxed growth habit.- ‘snowbank’ grows slightly shorter.
Shall be looking forward to see the results of your boltonia,
come this late summer/early fall!
I agree Pink Beauty had the flops no matter how I pruned it. Snowbank takes to pruning much better.
Eileen