Experts and BHG readers answer.
Perennial Border Plant
Well there are really no perennials that bloom for that long period of time. With perennials, in order to get this long period of bloom, you'll need to plant several species, some which bloom in summer and some in fall. Only annual flowers bloom all summer into the fall. You can look at all your options by doing an Advanced Search on the Plant Encyclopedia on this website. Plug in your zone (or zip code to find your zone) and then use the radial buttons to indicated your needs. It should give you some options, but understand that no perennial will do exactly what you want in terms of long bloom period. Most will bloom for a month at best.