After harvest is completed, remove dead leaves and other vegetative debris from your garden and add them to your compost pile. Leaving it in the garden can provide a handy over-wintering place for insect pests and diseases.
Hardy perennials can endure subfreezing temperatures well. However, alternate freezing and thawing can cause the ground to heave roots out, often resulting in loss of plants. After foliage has died back, cut off all dead leaves and stems and cover the plants to a depth of 6 inches. Straw, leaves, pine needles or any other coarse, light material is excellent for this. Place dead boughs over the mulch to hold it in place. This mulching should be done AFTER THE GROUND HAS FROZEN in the North, or after the garden is cultivated in the South. A lasting snow cover is nature's mulch.