Cutting back outdoor plants cleans up the appearance of the garden bed for winter. It also clears out the excess plant debris so that the perennial plants are trimmed and ready when spring comes. Cutting back helps prevent disease in some plants, as disease organisms live in the dead and dying foliage through the cold months then attack the young, new foliage in the spring. Cut back your outdoor plants in fall as they begin to die back or go dormant, often after the first autumn frost.
Cut back perennial plants to a height of 3 inches above the soil level, using sharp shears. Remove all the plant matter and discard it or compost it if it doesn't appear diseased.
Sterilize your shears after cutting back each bed to prevent the spread of disease. Rinse them in a solution of 1 part bleach to 10 parts water then wipe dry.
Place a winter mulch around the plants after cutting them back. Check the depth of any remaining spring mulch in the bed and add enough additional mulch to bring the depth to 3 inches.
Cut back some perennials a second time during mid-summer. Geraniums, for example, benefit from drastic cutting back if they become leggy or stop blooming.