While some perennials are evergreen, most survive the cold months by storing food reserves in their roots, tubers or bulbs. After the first frost, cut back perennial growth to just a few inches above the ground using a sharp hand pruner, garden shears or scissors. Leave the stems above ground to protect the crown — the part of the plant at ground level where the stem meets the roots — throughout the winter.
You may choose to leave certain perennials alone. Since they naturally begin to go dormant in late fall as they prepare for winter, some plants and grasses provide visual interest all year long. Take note of what kind of perennials you have in your garden. Try to understand their natural, annual cycles to decide whether you want cut them back or let them be.
Ornamental grasses usually remain attractive throughout the winter. Late-flowering, daisy-type perennials also remain attractive, such as: Rudbeckia and Purple Coneflower, Achillea, Agastache, Aster, Astilbe, Baptisia, Buddleia, Chelone, Cimicifuga, Eryngium and Eupatorium. It is up to you whether you want to cut them back or not.
It is a good idea to cut back perennials such as Alchemilla, Anemone, Campanula, Centaurea, Coreopsis, Delphinium, Dicentra, Euphorbia, Geranium, Hemerocallis, Hosta, Lychnis, Monarda, Nepeta, Oenothera, Phlox (tall types), Trollius and Veronica.
Evergreen perennials and alpines should not be trimmed in the fall. These include: Ajuga, Alyssum, Arabis, Armeria, Artemisia 'Powis Castle' and 'Huntingdon', Aubrieta, Aurinia, Bergenia, Cerastium, Corydalis, Dianthus, Epimedium (trim in late winter, before new buds appear), evergreen Euphorbia, Helianthemum, Helleborus, Heuchera, Iberis, Kniphofia, Lamium, Lavender, Liriope, Origanum, Phlox (creeping types), Primula, Pulmonaria, Sagina, Saxifraga, Sedum (many creeping types), Sempervivum, Teucrium, Thymus and Viola.
It's best not to cut back these types of perennials as well: Buddleia, Caryopteris, Erysimum 'Bowles Mauve', Fuchsia, Hypericum, Lavatera, Perovskia (Russian Sage), Phygelius and Santolina.