Also known as "swamp sunflower" or "narrow-leaf sunflower".
Helianthus angustifolius is a perennial native to the US-- a wildflower that is well-adapted to use in the home garden, provided that the gardener is aware of a few important facts...
First, they are foliage-only until very late summer/early fall. When it does flower, the blooms are up to a few inches across and numerous.
Second, it can be potentially invasive, so it bears watching. You may need to curb its spread by underground rhizomes (...and self-sown seed).
Third, it wants to grow tall, and it has a habit of flopping over. Providing a support may help, but it is also common practice to cut them back at some point in the summer to promote a bushier growth that is less likely to topple.
I cut my biggest clump back at least twice this summer. It still looked fairly good a month or so ago, but I was concerned that it was growing too tall. Gradually it began to "open up" in the middle, the tall stems leaning out from the center of the clump. Then we got a couple of heavy rains, and that was the signal to flop and "lodge" (which apparently is the technical term for a stem that bends/breaks under its own weight).
The flowers are still glorious and plentiful, but the plants look messy. (Fortunately, the macro lens allows me to focus in on the flowers...)
I still enjoy the flowers, but it's disappointing that the plants are collapsing. I guess I either didn't cut back hard enough or didn't time it right. I'll try again next year!
In the meantime, the stalks are going to die to the ground in winter, anyway, so there's a fresh start just around the corner.
...And there's no denying the beauty of those golden flowers.
It's easily propagated. Just dig up one of the little plantlets that pop up in spring. (Remember, it will want to spread ever outward from wherever you originally plant it, so there will probably be new baby plants every year.) You can also wait for the seeds to mature, harvest them, and plant them in spring (or by winter-sowing). I grew some from seed earlier this year, with little trouble.
Maybe these late-season bloomers have the right idea... Why flower during the hot, humid middle of summer when you can wait for the dry, pleasant days of early autumn? Life is sweeter, this time of the year!