"You've got to treat the cause, not the symptoms," says Bill Hudson, Ph.D., extension educator with the Ohio State University Extension in Marion County. There are several steps you can take now to prevent moss next spring. Make plans to do some winter pruning on trees and shrubs that shade your flowerbed. Then get a soil test to evaluate your soil quality. Adjust your soil's fertility and pH based on the test's recommendations. If your flowerbed contains acid-loving plants such as rhododendrons and azaleas, you may not want to add lime to your soil.
If you have compacted, poorly drained soil, dig in 1 to 2 inches of compost to improve the soil's fertility and ability to regulate moisture. Covering bare soil with 2 to 3 inches of bark chips may also control moss, though Dr. Hudson cautions that slime molds (which look totally gross but cause no harm) often grow on cool, damp mulch. During dry periods in the spring and summer months, water your flowerbeds for 30 minutes once or twice a week instead of for a few minutes each day. This allows the soil to dry between waterings and also encourages your plants to develop more extensive root systems.
But perhaps the best solution is to beat nature at her own game and, after amending the bare spots with compost, fill them in with groundcovers that complement your flowerbed design. Low-growing plants that perform well in shade include lungwort (Pulmonaria spp.), ginger (Asarum spp.), Hakone grass (Hakonechloa macra), and Liriope spp. If all else fails, try to enjoy the moss for what it is—a self-planting, maintenance-free, green groundcover.