If it's really 'Ponytails,' AKA Nassella Tenusissima, it's hardy to zone 7. It can be grown as an annual, but the plants I grew from seed last year only made about a foot tall in their first season.

Zone 5 is very optomistic. However, that said I am growing Stipa tenuissima (Nasella) in my garden & it does over winter. I have a patch of about 9 plants growing in a 'sand bed' mulched with 'pea gravel' ie, good drainage.

It is short-lived for me ~ is this a characteristic of the grass or just a result of growing it in a colder zone?
A.

Feather grass is not reliably hardy where I live, in NC, which is zone 7. However, good drainage seems to be a critical factor in its winter hardiness. If the ground is not well-drained, it doesn't overwinter for me. It also doesn't survive winter if grown in containers, but some of my plants in the ground survived this winter, which was very mild.

I have 27 Stipa Ponytails swaying in the breeze and doing beautifully in my frontyard. I planted them 4 years ago in zone 7, Charlotte, NC and have not had one problem. Not even the cold snap that took all the blooms from my azaleas, didn't phase my Stipa Ponytails. I planted them in the clay soil didn't use any compost but I do use mulch from my recycle center every other year and water when needed. I have found Stipa Ponytails to be the easiest plant to take care of in my yard. I hope this helps you.