It very much depends upon the grass, and your climate.

Warm season grasses pretty much stop growing when they flower, and will have a difficult time becoming established before winter. They like to be planted/transplanted while they are in their growth cycle, before they flower.

Cold season grasses can be planted/transplanted up until mid-Fall, because this is one of their growth periods, and they will establish themselves before winter.

All of this is modified somewhat by the nature of your climate. In my zone, I can get away with more fall activity, because my growing seasons are longer than your zone 5.

Pick up a copy of Rick Darke's Color Encyclopedia of Ornamental Grasses. He explains the issue in much more detail.

Donn, thanks for your explanation. It's good to know the reasons behind the rules! It looks like Carex and Hakone are cool season and Pennisetum is warm season.

I've requested the book you recommend from the library. It's checked out to someone else, so I'll have to wait.

Thanks for your help.

I'm glad you asked this question. I was wondering the same thing. I am interested in planting a row of Calimagrostis acutiflora 'Karl Forester' this Fall. Will they be okay?

The book says you can divide 'Karl' in fall or spring. It's a cool season grass, hardy to colder areas than your zone, so it should work ok. I'd do it soon, though. You want it to have a chance to get well established before your horrible winter begins.

Donn,
Thanks for the advice. I know, I know, we have horrible winters. Believe it or not, the last couple of years they have been considered "mild" winters!