Answer: Ornamental grasses need to be trimmed off very short -- say six inches -- in either late fall or very early spring before they start growing for the season. You can use a hand pruner, hedge trimmer or chain saw or whatever tool you prefer to do this. Try to have it done by the time the daffodils bloom in your neighborhood. If you wait longer you risk damaging the new growth for the year.
In late fall you should also mulch around the clump, but not over top of it, using an organic mulch. The mulch will eventually break down and help feed the soil slowly over time, so add mulch as needed to keep it about three inches deep throughout the growing season.
Your zip code places you in zone 6, so you need to look for hardy grasses for zone 6 or colder. Pampas grass is sometimes used to refer to all ornamental grasses, and sometimes it is used to refer to Cortaderia selloana. Cortaderia is not winter hardy in your area so it would not be expected to survive the winter. There are many different ornamental grasses you could grow; your local county extension and professionally trained nursery staff should be able to help you select some that are well suited to the planting location you have in mind as well as your design goals. In the meantime, here is some information to help you get started.
I hope this helps.
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