Bishop's weed (Aegopodium podagraria) is a ground cover also called goutweed or ground elder. In many states it's considered invasive---a legal name for a bad weed. Winterizing this remarkably persistent member of the carrot family might only be necessary in hot or dry regions where it hasn't invaded and killed off native vegetation. It grows throughout the eastern half of the United States (except for the Gulf coast) as well as the Pacific Northwest and over half of Canada.
Keep bishop's weed neat during late summer and fall. Remove flowers and clear leaves that show signs of sun or heat scorch---new leaves will replace them any time of the season. The plant is theoretically hardy from zones 4 to 8 but it is also a vigorous grower beyond those boundaries.
Control seasonal growth with barriers so that the ground cover grows only in shady, moist areas. Bishop's weed planted in exposed areas where its roots can dry out or bake in the sun will not overwinter successfully.
Mow it down in the fall and rake out the thatch. Although bishop's weed is not as appealing as its cousins parsley and carrots, its leaves provide winter shelter for rodents and other creatures.
Cover the stolons (the fat roots by which goutweed spreads) with a light layer of compost or peat mixed with garden soil. It will cover any roots exposed by raking and provide some nitrogen for early spring.
Apply a one-inch layer of winter mulch to plants in areas where bishop's weed is not native. In hot, dry areas, it will insulate the ground consistently against heating and drying through the winter. In colder zones, it will insulate against freezing, drying winds. Bishop's weed growing in the right location probably does not need any winter protection in zones 4 to 8.