Amsonia is a favorite genus of mine, a small genus in the Apocynaceae, with 16 species in the USA, a few more in Mexico. There are a few so-called Amsonia outside of North America, but these are now accepted as the genus Rhazya.
Of the more popular ones is Amsonia hubrichtii, the threadleaf bluestar, from Oklahoma and Arkansas. It needs room to fully appreciate the billowing mound of thread-fine foliage, famous to turning yellow or gold in the autumn. Another species less frequently encountered is A. rigida, from southestern USA. It is one of the smaller species, an excellent well-behaved perennial with small broad leaves, among my top 20 favorite perennials. It grows approximately 2' tall. I find that Amsonia hybridize with wild abandon when brought together.
In theis single photo, I put numbers on the plants, they are as follows:
1. Amsonia hubrichtii
2. Amsonia rigida
3. A willowy hybrid that is perfectly intermediate between the two, A. rigida x hubrichtii (or the other way round).
4. A swarm of hybrids down-slope of the parent plants, every leaf size and shape from thread-leaved hubrichtii ones to the broad-leaved rigida types.
I hope to follow up with photos of these plants in flower.