Arundo donax 'Variegata' is a really attractive grass. I'm surprised though that the variegated reed got as tall as 16'. Usually the variegated is much shorter than the species, more like 8 to 10'.

Being in zone 6, this grass does not pose a threat as an invasive, like it would in southern CA or similar. It won't even set seed where you are. The concern you'd have would just be where to place it in the bed and will it crowd out other plants. I'd actually say you have more concern with the Phalaris than with the Arundo. I've found that the Arundo, even though it grows from Rhizomes, maintains a tight enough clump to control. It also is very easy to dig out strays, although you'll want to cut back the rhizome all the way to the source or it'll just root again.

The phalaris will eventually grow into the carex and crowd them out. Since it's a very surface spreader, you can contain it with edging and barriers. They won't self-sow, so it's all a matter of maintaining it within the bed. If you're persistant, you can keep it only where you want it, but creating barriers will be so much easier. Or place the phalaris in a container amongst the arundo. Same effect, just easier to control.

You know, I thought it was getting really, really tall last year, it just never stopped growing. On the back of the tag it does say 10' tall, so I looked it up on the internet and I think the tallest is supposed to be 12'. However, everything on that side of my yard is huge. I don't know what it is, my daylilies come up sooner, they are greener and thicker and just all around healthier than the rest of the yard. Maybe someone had a mad poopfest there before we moved in!!! Luckily we planted this in front of a tv tower so it can be tied up. I will post some pictures when it gets big this year. It is one grass I am very, very proud of.

Thank you for your information.
Joyce

I have been growing Arundo Donax for several years in Kentucky. Mine have reached 18 feet in height!!!! I give them lots of water, they cannot get enough water. I will feed them miracle grow the first couple of waterings.
If you water in a certain direction, they will spread in that direction at a faster rate.
I cut them down in the beginning of the season, and I cannot believe how big they have gotten.

The rhizomes are huge!!! The biggest I have seen in a plant!!
I can dig a tiny part off the plant, and it will take off no matter where I plant it. It is very tamed and graceful in the Kentucky area. I would hate to see it in its full glory, thank goodness for the cold weather.