How you amend the soil depends on what you're starting with. Perlite and vermiculite will break down and compress over time. Gravel or expanded shale will not. I sometimes use the expanded shale. Adding sand can make clay worse if you don't add enough of it. Mostly I add my homemade compost. More recent information suggests that mixing in compost to the planting hole is counterproductive (bathtub effect), but that hasn't been my experience in gardening here for 30 years. IMO the more compost the better.
"Don't let it flower after September"... just want to clarify. I think you folks are talking about new plants and first flowers, right? I let my established coneflowers bloom and set seed thru fall so they begin dormancy.
Old fashioned purple coneflowers are definitely the toughest I have.
Karen
