Do not prune this perennial now or in the fall or winter. This is a some what tender plant so the best time to prune them back is in the spring when you see new growth on other perennial plants. Usually this is in April. The foliage will go limp  this winter but it is protecting the roots and base over the winter. You can prune back to within six inches of the  ground. Good drainage is key for most plants with gray foliage so do not mulch with heavy, wet leaves. Bark chips on top of the roots is fine. Keep growing, Marianne Binetti