Ive never heard of suffolk marsh reeds, but if you are talking about Norfolk or Common Reed (Phragmites) then the part that remains above the ground in winter is simply the dead part of the old stem. All the energy is back in the roots or rhizomes. The reed will grow again from this point. In East Anglia where the reed is cut for thatching, this stimulates the growth of the plant so reed can be cut back in this way in the garden and it will sprout agin in the Spring.