Answer: Some of the ornamental grasses do show exciting fall color, but this is not one of them. Cortaderia selloana 'Pumila'usually dies back during the winter in colder zones such as yours, so you would trim it back to remove old foliage and clean out any debris that may have become trapped at the base of the plant. You would routinely remove the flowering stalks as well. If you like the look of the plants and to provide a little extra natural winter protection for the crown, you can wait until late winter or very early spring, but try to have it done by the time the early daffodils bloom in your area.
Based on your zip code, you are gardening in zone 7A or the coldest part of zone 7. Depending on the microclimate of your yard and the weather each year, it might be as cold as zone 6. So I should mention that Cortaderia selloana 'Pumila' is considered hardy only to zone 7, so it may or may not survive the winter for you. Good luck with it!
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