There is no need to trim pampas grass (Cortaderia selloana) back for the winter. In fact, some people grow it just to enjoy the plumes in their winter gardens. Come spring, cut out the winter-killed growth at ground level before new growth begins. This is also the proper time to propagate it by digging and dividing the existing clump. While there is no requirement to cut your pampas grass back, there is nothing to prevent you from doing so. If you need to cut it back because of its size, cut the entire clump back to ground level. This needs to be done with care because the edges of the pampas grass leaf are razor sharp and can inflict severe damage to fingers and arms. Pampas grass is hardy in Zones 7b through 10. In zones colder than 7b, treat Pampas grass as an annual.