P. Vivax, have you grown it in zone 5 or 6?
If anyone has, how big of a diameter did your culms get? I was looking online to buy it and saw that one place listed it as being hardy to -5 F and another site listed it as being hardy to zone 5.
I've grown p. nigra henon in zone 6a, and it doesn't spread very fast at all, and if temps drop below zero, the tops all die off and then the new culms come up smaller than before. I've yet to see it get that nice dusky gray color too, though the grove is very narrow and doesn't get much shade.