Some perennials, however, should be left standing and this begs the question 'to cut or not to cut?' It's easy to make a decision with annuals. After the first frost when they are blackened and looking ugly, pull them out and throw them in the compost bin. Likewise, clean up plant debris from the vegetable garden. When asked what to do with perennials, as with many gardening questions, the answer is 'it depends.' Here are some hints on when to cut and when to wait.
Perennials to leave standing
Perennials that add interest to the winter landscape.
During a snowy winter, some plants provide stunning interest in the form of height and structure. Not only evergreens and the skeletons of shrubs but also grasses and standing perennial seedpods display beautiful winter artistry. Ornamental grasses are most dramatic in the winter landscape with their tall plumes. Consider switch grass (Panicum), zebra grass (Miscanthus sinensis 'Zebrinus'), and feather reed grass (Calamagrostis). Leave them standing until spring then cut them back before the new shoots appear. Siberian iris (Iris sibirica) and blue false indigo (Baptisia australis) have interesting elongated black seedpods that stand out against the snow. Allow the seeds of sedum 'Autumn Joy' (Sedum spectabile 'Autumn Joy') and Joe Pye weed (Eutrochium purpureum) to display their large, round lacy globes all winter long.
Perennials that provide food for birds.
Many birds rely on the seed heads of dried perennials for food. It is comon am to see goldfinches in a stand of purple coneflower (Echinacea purpurea) enjoying the seed treats. The seeds of black-eyed Susan (Rudbeckia) and oxeye sunflower(Heliopsis helianthoides) are other favorites. In addition, overwintering birds find protection in plant stubs and ground covers.
Perennials that help beneficial insects in winter.
Beneficial insects may hide in or near native plants for the winter either as pupae, caterpillars or eggs. The plants provide shelter from their predators such as birds or spiders. Our native butterfly, the viceroy, rolls itself in a leaf and drops to the ground where it stays until spring, providing a good reason not to rake plant debris from around your perennials.
Perennials needing protection.
Don't cut back marginally hardy perennials like garden mums (Chrysanthemum spp.), anise hyssop (Agastache foeniculum), red-hot poker (Kniphofia uvaria), and Montauk daisy (Nipponanthemum nipponicum). They are more likely to survive the cold of winter if you leave their tops to collect leaves and snow for insulation and moisture.
Low-growing evergreen or semi-evergreen perennials.
There is no need to cut back hardy geraniums, heucheras, hellebores, dianthus and moss phlox. Tidy them in the spring as needed.
Perennials to cut back
Cut back plants with disease or insect pest problems to reduce the chance of infection the following season. Bee balm (Monarda) and phlox (Phlox paniculata) with powdery mildew are examples. Even resistant varieties of bee balm and phlox can become infected in bad weather so cut them all back. Remember to destroy, not compost, diseased stems and leaves. It is important to cut back hostas and remove all their leaves from the ground as soon as the frost takes them. Dead hosta leaves harbor slug eggs that will hatch and ruin next year's greenery. I cut back plants with browning or blackened foliage and bare stalks that don't add anything visually to the winter garden: peonies (Paeonia), daylilies (Hemerocallis), brunnera (Brunnera macrophylla), and speedwell (Veronica) for example. Late in the season some plants, including yarrow (Achillea), Shasta daisy (Leucanthemum x superbum), and globe thistle (Echinops), grow new basal leaves. Cut off the stalks without disturbing this new growth.
When cutting down a plant, leave about two inches above the soil to mark its location. This is especially important for plants that emerge late such as butterfly weed (Asclepias tuberosa), rose mallow (Hibiscus moscheutos), and balloon flower (Platycodon grandiflorus). You will be less likely to dig into them accidentally before they appear in spring if you can see a portion of their stalks. Bypass pruners are preferred because they make a clean cut through the stem of the plant. Hedge clippers work fine, but for a large planting, but try a power hedge trimmer for efficiency.
Don't be in a hurry to rush outside and cut plants back. Unless the plant is diseased or infected, wait until several hard frosts have killed back the tops. In the spring, the plant sends up energy from its roots to produce beautiful foliage and blooms. Allow the roots time to reclaim that energy from the dying plant, keeping it strong for re-emergence in the spring. For many perennials, leaving plant tops over winter is fine and may be preferable. In some areas of Pennsylvania, however, where seasons are short, gardeners have more time for cleanup chores in autumn. There's something very satisfying about making an early start on next year's garden.