Brrr-ennials

Some perennials such as coneflowers and black-eyed Susans serve as food for birds in the winter. So although some of these plants self-sow excessively in the garden, you may want to leave plenty of the seedheads for the birds. Make sure your perennials are dormant before pruning, then cut down to about two to three inches above the base of the plant. If you cut too close, you may cut overwintering buds. If you garden in colder climes, you may want to prune to about six inches above the base, allowing snow to accumulate and thus help insulate the plant. Always remove any diseased foliage. Most ornamental grasses are grown for their winter interest and so aren't pruned. This Erianthus is an exception; prune to about eight inches tall, or to the top of the brown stems. Ferns don't need to be pruned. Fronds that are left in place help the plant better survive the winter.

Plants that don't require pruning:

• Varieties that overwinter better if not cut back, such as Kniphofia (torch lily) and Alchemilla mollis (lady's mantle), ferns.
• Those that have winter interest such as Lavandula (lavender) and Iberis sempervirens (evergreen candytuft).
• Those with attractive seedheads, such as Dictamnus albus (gas plant) and many ornamental grasses.
• Perennials that attract or offer cover for songbirds: Echinacea purpurea (purple coneflower), Eupatorium maculatum (Joe-Pye weed).
• Short-lived perennials or biennials that should be allowed to seed to ensure permanence in the garden: Lychnis coronaria (rose campion), Linum perenne (perennial flax).

• Perennials that harbor pests or disease over the winter such as botrytis-susceptible Paeonia (peony) and mildew-prone Phlox paniculata (garden phlox).
• Perennials that aren't aesthetically pleasing: Begonia grandis var. evansiana (hardy begonia) and Veronica spicata (spike speedwell).
• Perennials that may reseed and become invasive: Foeniculum vulgare "Purpureum" (fennel) and Centaurea montana (mountain bluet, cornflower).