Low, dense growths called ground covers can take many forms -- woody shrubs, vines, proliferating perennials and bulbs. Those that are in locales with extreme lows sometimes benefit from a winter mulch. Top-quality mulches include fir bark, sawdust, bark, tree leaves, gravel and rocks. When to apply it depends on how well-established the ground cover is. If it's new, apply mulch when fall plants are bedded. This delays the ground from freezing and ultimately gives the ground cover more time to get established. However, if your ground cover is already acclimated, mulch after the ground is frozen to keep it that way. Straw, shredded leaves or other loose mulch also prevents ground cover from drying from wind gusts. Also, to ensure ground cover survives winter well, plant it at a time of year when it will have the most time to get established before unfavorable weather hits. If you're in a region with cold winters, plant in the early spring. If you're in a mild area, plant in the fall or winter.