Answer: Cut it back quite short, to just a few inches tall. Do it before it begins to grow in the spring. I should mention that this plant, Cortaderia selloana 'Pumila' is considered winter hardy into zone 7. Based on your zip code, you are gardening in zone 6B or the warmer part of zone 6. Depending on your microclimate, it might be as warm as zone 7. So this plant may or may not be reliably winter hardy for you. To try to give it a little extra winter protection, wait to cut it back until spring and be sure to give it a generous layer of mulch over the root area in the late fall.
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