Last year I planted some P. Viridis Robert Young. The plants were only 1 yr but had great root mass. This past summer they flourished. After this past winter all the leaves have turned brown. Could this be topkill? Or do they loose their leaves this time of year naturally? I'm in zone 7. I have several other Phyllostachys varieties in similar locations that are fine. I purchased this off ebay so maybe it's not really Robert Young???