I'll be interested in seeing more experienced responses to this thread. I'm going into my second season with several clumps of seed-grown Cortaderia.

Technically, the grass is evergreen, but, according to Greenlee, in colder climates, will go partially or totally deciduous, and requires cutting back. My almost-year-old clumps appear to have gone completely deciduous. The blades are tan over their full length. This has been one of our warmest winters on record, with a low of 15F so far, so I hope they will come back.

Cortaderia IS evergreen and as such does not go completely dormant. What appears to be a deciduous habit in colder climates is merely dieback. Obviously this would prompt cutting back in spring in those areas where the grass does turn a tan or straw color. Where it remains evergreen, annual cutting back is not required, just a simple trimming or tidying is sufficient. There is a commercial planting of Cortaderia not far from my work that is routinely trimmed to keep it tidy - the grass is very carefully shaped from the outside with the outer edges kept short and allowing the inner growth to achieve full height with flowering stems. Not a common approach, but it does keep the grass very compact and upright in a windy situation and does speak to its versatility in trimming or grooming.