I'm in zone 5 and I have 5 large clumps of Chinese Silver Grass. I bind them up with a little twine and leave them up all winter. They look nice against the snow. Altho, I have no snow at the moment. It was 60F here yesterday Normal is 30F.

In the Spring I chop them down to the ground with the hedge trimmer and machete. Run my lawnmower over them with the bag on and dump it all in the pile. Once I add a little green grass to the pile it all breaks down fairly fast.

As with most additions to my pile, the smaller the size the faster it composts,

Those stems would be a brown, for composting, after standing all winter. How fast they compost depends on the total C:N ratio as well as the size, since smaller particles tend to be digested fairly quickly.
The groundskeepers at Michigan State University use chain saws to cut the ornamental grasses down in the spring and all of that is composted.

henshaw21, shredding all ornamental grasses is a good start & most of the time the shreds will compost in 2-6 months.
I'm not sure you will not get some seeds in the garden.
If I were you & I got seedlings in the garden, I would pot them up. But that is just me.

Thanks for the replies all. I've got a new pile starting. I'm going to shred up a portion of the ornamental grass and mix it in as a test. If it breaks down as quickly as the shredded leaves I'll use them again in the future.

What do you use to shred your ornamental grasses?