I just cut the blooms of my liatris but I'm sure it would be fine to cut down to the basal crown. When they first start to grow from self-seeding, they look like grass coming up. But if you pull them up you'll see little tiny bulbs on the ends. Your seeds from last year are probably still viable. Guess it depends on which seeds you'd like to start to take along to know if they'd be a decent size by Sept. 15th. Never hurts to try.
If you pot up your other plants now they should be good to go by then too.
The coneflowers in my picture are very old NOIDS...no idea where I even got them any more. They're about 30-36" tall.
