There are a few varieties of Pampas Grass (Cortaderia selloana) which are supposedly hardy to zone 7. Both 'Pumila' and 'Pink Feathers' are among them. I've tried both, in my very warm zone 7, and both failed to survive mild winters.

ok. I am waiting for an identification from nc state environmental......after reexamining my plants, there are 7 of them, I am beginning to realize that I have a weed grass growing up from where my pampas was planted. 3 of the 7 plants do actually have some pampas leaves also. Now my question is, how do I get rid of this weed grass that is growing right up along side the pampas grass. can I use a weed killer on the grass if I just pour a little right in the crown of the grass? I can't dig it out as I will surely injure the pampas grass roots. or should I just let them grow happily together and hope the pampas roots/plant will overtake the weed grass. the other 4 plants there is no sign of pampas so I am just going to spray those with the weed killer., when I did them out I will replant with new pampas and heavily mulch this time. any other suggestions would be much appreciated...

There is almost no way to apply herbicide to a weed grass growing in with an ornamental grass. Try pulling out all of the foliage of the weed grass, and keep doing it, in hopes it finally gives up.