Answer: I think the confusion lies in the fact that Pampas Grass is only marginally hardy in your gardening zone (you're in zone 6 and Cortaderia is hardy only to zone 7). With that said, the best approach is to allow the foliage to remain on the plant rather than cutting it back prior to winter's arrival. If the leaves do freeze and die this winter, they will still provide a protective insulation over the crown of the plant. In the spring you can cut away the winter killed leaves and new leaves will emerge from the crown.
As for harvesting the plumes, you can cut them off now and take them indoors. Their age will determine whether or not they will remain intact or will shatter in the dry indoor air. If you intend to always harvest and use the plumes in the future, be sure to cut them off the plant when they are fresh and new. This will extend their life and prevent shattering when used in dried arrangements.
Best wishes with your garden!
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