If you don't cut down or burn down the old foliage, the plant looks terrible when the new growth starts coming up. It also represents a danger of crown rot.

Big Bluestem will self-sow in your zone.

There are 14 species of Spartina. Some are highly invasive, some mildly and some not so much.

It isn't likely either of these will reach 8' in your zone. 6'...maybe.

I never cut my little bluestem, but they I am lazy and I do not have problem with invasiveness or crown rot. It has to rain for things to rot., but they do seed out. There seed is not hugely fast growing. If you want them large then you need the Big Bluestem. If I was feeling annal, I would cut them back in Late winter.

Wantonamara, how tall does your big bluestem get? I was hoping to be able to plant it next to a fence to add some privacy to yard without taking a lot of space. Thanks