ground cover

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Small plants other than saplings, such as mosses, ferns, grasses, and low shrubs, growing on a forest floor.
• noun A low-growing dense growth of plants, such as pachysandra or crown vetch, planted for ornamental purposes or to prevent soil erosion in areas where turf is difficult to grow, as in deep shade or on a steep slope.
• noun A plant used for such a growth.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Something, particularly plants, which overlays an area of land. May refer to plants specifically grown to prevent erosion of an area.