Try calamagrostis x acutiflora 'Karl Foerster.' It is clump forming and grows to around six feet. I split it last year (into four plants) having grown it for ten years without getting any seedlings.

Stipa Gigantea (Golden Oat Grass). The flower stems will grow to 6-8 ft tall. I have one in our front garden and it's fab.

Miscanthus sinensus ...Zebra Grass...is my suggestion.