Herbaceous perennials are defined as plants that die back to the ground each winter and sprout new top growth from underground structures (rootstocks, rhizomes) or crown buds each spring. They persist from year to year and are considered to be more or less permanent. Therefore, it follows that the expired top growth must be removed at some point during the year.
When is the best time to remove the dead tops of perennials? This activity is generally reserved for spring or fall and there are benefits and drawbacks to both. Leaving the tops in the garden until spring helps to increase the survival rate of the overwintering perennials, especially those with marginal hardiness. The tops break the wind and trap snow, providing additional insulation for underground structures and crown buds. Snow cover also moderates soil temperatures so that alternate freezing and thawing temperatures in spring do not cause heaving. Perennial tops protruding from the snow often add an element of winter interest to an otherwise barren landscape. Dried stems and seed heads also provide food and shelter for birds and other wildlife. These structures can also be useful tools for locating plants in the spring. On the downside, leaving dried seed heads throughout the winter can lead to weed problems in subsequent growing seasons. Waiting until spring to cut down perennials increases the risk of damaging new, succulent growth. If spring cutting is preferred, extreme caution must be taken not to damage emerging shoots. Also, working on top of heavy, wet spring soils can increase soil compaction and impede root growth. Perhaps the biggest disadvantage to leaving the tops until spring is that they can harbor overwintering insects and pathogens. From a pest control standpoint, removing the tops in the fall is a necessary practice. A thorough fall clean-up will keep the garden tidy and reduce the potential for future pest invasions. The removed tops make an excellent addition to the compost pile, providing they were not infected with disease during the growing season.
Fall cutting should follow a number of heavy frosts. Cut down the tops but leave approximately 5 cm at the base of the stems. This will help with snow trapping and provide some insulation to the overwintering structures. For perennials that produce buds on or slightly above the soil surface, leaving a few inches at the base of the stems will help to prevent mechanical damage and winter injury. In climates with extreme winter low temperatures and little snowfall, such as on the prairies, a layer of organic mulch around (but not on top of) perennial clumps can stabilize soil temperature, conserve soil moisture, reduce weed competition, and reduce soil compaction. Mulching tender perennials will greatly improve winter survival. Cutting and removal of diseased plants should always be done in the fall. Never advance diseased plant material to the compost pile.
Certain types of perennials are classified as evergreen and, during milder winters with ample snow cover, they will maintain their green foliage into spring. Examples include, moss phlox (Phlox subulata), carpet bugleweed (Ajuga reptans), Bergenia (Bergenia cordifolia), and perennial candytuft (Iberis sempervirens). Evergreen perennials will especially benefit from a layer of additional insulation (mulch or evergreen boughs) to improve winter survival and encourage the evergreen habit. Do not cut evergreen perennials back in the fall. Wait until spring to see if the foliage survived the winter. If at that time leaves look brown and tattered, trim them back accordingly.