Most perennials are cut back after the first hard frost in the fall. This usually occurs in late September or early October. It is important to clean off all plant debris after the frost to help minimize soil-borne diseases.
Most perennials simply need a good layer of mulch applied late in the fall. The purpose of mulching is to protect the crowns of the plants from the alternate freezing and thawing that occurs very late in fall and in early spring. It is important that the ground is allowed to get cold before mulching, so wait until early to mid-November before covering the plants.
When you add fall mulch, aim for a layer that’s 3 to 5 inches deep (deeper in colder regions). Use a material that won’t compact, like straw, chopped leaves or cornstalks, pine straw or clean hay. It’s especially important to mulch shallow-rooted perennials that are prone to frost heave.
Clear out leaves, before applying mulch. Trimming perennials and cleaning up leaves will avoid future garden problems such as insects, slugs and snails, and powdery mildew on roses and apple trees. Weeding in the fall will help to limit the weeds that will grow back in the spring. You can add all this garden debris to your compost pile for future use; remember to avoid using diseased material.