Planting a ground cover, a plant that will grow thickly enough to keep out weeds yet low enough to act as a background to other plants, can be very tempting, holding out the prospect of a minimum maintenance garden. Bear in mind, however, that you must mulch, fertilize, water and weed it until it fills in strongly and that you must choose one that grows tall and thick enough to stop weed seeds from sprouting.
Clumping Ground Covers
A number of perennials make nice ground covers if grown close together, and, like most perennials, the clumps thicken as they age so you'll be able to divide them to get more plants to add to your bed. Though they spread slowly, they are usually tall enough to discourage weeds and you could combine them with another type of ground cover to fill in the gaps.
Plants in this category include daylilies, hostas, hellebores, oregano and catmint. Some will spread well by self-seeding, including the perennial geraniums and lady's mantle.
Vine-like Ground Covers
Some vines sprawl horizontally and branch so frequently that they make a thick layer of leaves and stems that shade the ground completely. These are best used in larger beds since individual branches may be 5 to 10 feet long. It is possible to prune them back, and will help thicken the ground cover, but keeping them small would be too much extra work.
Vines for covering ground include English ivy, winter jasmine, climbing hydrangea, trumpet vine, Carolina jessamine and, for a quick annual cover, winter squash.
Ground Covers that Spread by Runners
Runners (also called "stolons") are stems that run across or just under the surface of the ground, rooting at the nodes, the places on the stem where leaves and branches sprout. Plants that spread like this are often quick, inexpensive ways to fill in a bed because you get new plants from every node and each may be separated and transplanted when it is well-rooted.
Common ground covers in this category include ajuga, pachysandra, lily of the valley, Carpathian bellflower, sweet woodruff, camomile, creeping lily turf, strawberries, sedums, creeping thyme, waldestinia and periwinkle.
Low Shrubs As Ground Covers
A number of shrubs spread out horizontally, much like vines, and should be treated in a similar fashion, used only in large areas and pruned back occasionally to help them branch. These include memorial rose (R. wichuraiana,) bearberry, some ornamental varieties of rubus (blackberries,) low growing junipers and low cotoneasters.
Other shrubs are clumping but can be thickly planted as a ground cover. These include heathers, dwarf cherry laurel, Sarcococca and germander.