Yeah, you should prune back in the fall.
If the grass was not pruned, it might go to seed and then die. So I don't know how long the grass has been like this and whether it gone to seed or not. In any case cut it back and see what is going to happen.
https://en.wikipedia.org/wiki/Cortaderia_selloana
https://baike.baidu.com/item/%E8%92%B2%E8%8B%87/6112856
Yeah, you should prune back in the fall.
If the grass was not pruned, it might go to seed and then die. So I don't know how long the grass has been like this and whether it gone to seed or not. In any case cut it back and see what is going to happen.
https://en.wikipedia.org/wiki/Cortaderia_selloana
https://baike.baidu.com/item/%E8%92%B2%E8%8B%87/6112856