Answer: Perennial grasses may be cut back in the fall or in the early spring, depending on whether or not you enjoy the seed heads as a decorative element in the fall and winter garden. Some gardeners will leave the faded flowers and seed heads all winter on black eyed susans and sedums so that the birds may enjoy the seeds and because they find the seedheads decorative. Other gardeners will dead head or remove the spent flowers as they occur. Most perennials are left until frost kills back the tops, then the browned portions are removed. They may be tidied again in the spring if needed.
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