Sorry the pic is sideways It's from my phone.

Get some 18" bamboo barrier. It's extremely heavy duty plastic, and you bury it in a trench all around the stand of Arundo.

You can also contain it by mowing down the new shoots every year.

(Pssst...it's past time to cut or pull out the ugly last-year's canes.)

Thanks,
I will try the bamboo barrier. I wanted to try to contain it because I planned on putting in a new iris bed on the other side a few feet away. I have put in several new beds and I am starting on that area now.

Right now I just mow down new shoots. I usually knock down the old reeds in early spring but never got around to it this year.