I have 4 containers of Rubrum on my deck which were gorgeous last year. I cut them back - maybe too early. They are still brown altho it is almost March. How long should I wait to see if they will come back before buying new ones?
I forgot to mention that we had an unusually cold wither this year - sometimes in the low to mid twenties at night. From what I'm reading her, maybe the cold killed it. But if I want to wait and see, when would new shoots normally begin to appear?
Pennisteum setaceum 'Rubrum' - purple fountain grass - is not hardy in zones below 9 or 10. It will experience dieback in temperatures much below 40F. It is considered an annual in all but the warmest areas of the country. You will need to replace them.
I'm in zone 8b and have not cut my Pennisetum Rubrum back yet. I'm close to the coast, so temps are a bit better for me than even 20 minutes inland. I find that their survival is hit and miss here, but if I cut back early I almost guarantee them not surviving. If I leave purple fountain grasses up until new growth is flushing out at the base and then cut back, they often survive. Looks like they may be ok this year. Larger clumps down the street at our nearby Starbucks already have new growth coming up. They planted theirs 3 years ago and have not replaced them yet. We had temps here dipping down to 30 (even slightly below) this winter. Low 20's likely did the grasses in where you are. I've seen some neighbors bag theirs for protection, like they do with some of the lesser hardy palms in our area. That may help as well in future winters, but for now I bet you need to replant.
I also find that Pennisetum Rubrum that has survived a cold winter with borderline freezes will have a hardened off base and not as much nice fleshy growth that year. They'll push more aerials, get floppier, and just not look quite as nice. I guess it comes down to $ when deciding to replace them or not at that point.
Here is a link that might be useful: Purple Fountain Grass