Extreme temperature variations and drying winds can be harmful to many of our commonly planted perennials. Usually the foliage of most perennials starts to die and wither after the first hard frost. The question is when to remove the dead foliage.
One thought is to immediately remove the dead foliage. Many perennials will suffer no harm as a result, as long as you exercise great care when applying mulch. The other approach is to wait until spring to remove dead foliage. Research has found that mums survive the winter better if the old foliage is left standing through the winter. You should always remove diseased foliage to discourage the spread of leaf-spot diseases and fungal problems.
Whether you decide to remove dead foliage in the fall or wait until spring, mulches provide the best protection for your perennials. Established perennials and bulbs benefit from mulches that are applied after the ground freezes, because mulches do not allow soil temperatures to fluctuate as much throughout the winter.
Mulches do a better job of insulating plants when space is allowed for air to circulate. Mulch that packs down to a dense mass during the winter can cause mildews and molds to form. Apply three to four inches of straw, pine needles, or other mulching material in late November or early December when the soil surface freezes. A good organic compost used as mulch is also effective and can be used as a soil amendment in the spring
Mulch should not be removed too early in spring or plants will begin to grow too early. Plants located on the south side of a building or wall will emerge sooner than those in other areas, but may be subject to spring frost damage.
Dehydration is a common problem when snowless winters occur. A layer of mulch several inches thick helps retain soil moisture. This mulch should be coarse and loose to permit air movement to roots. Root tissues continue to metabolize in the winter and requires oxygen for this process to take place. Reduced soil oxygen level increases the aggressiveness of many soil pathogens. Mulches which pack down should be avoided.