Ground covers
Though they get scant attention and often go unnoticed altogether, ground covers are really important and a problem for most of my coachees. Adrian Higgins writes that in his garden, ground covers are "the glue that holds the garden together," "providing some visual relief from the more exalted specimens in the garden."
First and foremost, ground must be covered, either by plants of some size or with hardscaping, like pavers for a patio, or by mulch. Large plants cover ground, of course, but on this website, ground covers are short of stature. (Not precise, I know.) Some can survive being walked on occasionally, and some not at all.
Uses
Groundcovers are grown wherever ground needs covering, a purpose that's of increasing interest to readers is in replacing whole lawns. Having ripped out all my lawn in the fall of 2007, I'm keenly interested in this exciting topic and am "trialing" a few alternatives in my garden as I type. Results will be included here.
Filling out nicely or Attila the Hun?
Writer Elizabeth Lawrence wrote, “There are two difficulties with ground covers: first to get them to grow, and then to get them not to… Once established, the ground cover begins to travel, and then it may travel too far. A plant that is hard to keep in bounds may give the gardener more trouble that it saves.”
The problem is, groundcovers have jobs to perform in the landscape: preventing erosion, blocking weeds, and looking good, sometimes even all year long. People with large budgets can simply buy enough of a well-mannered, slow-growing plant to cover large areas instantly, but most of us buy a few of something - or are given a few - and rely on them to fill out and start perform their assigned duties, the sooner the better.
So what's a gardener to do? First, to address the type of invasion that threaten natural areas - don't do that! If your property is near or adjoining one, consult your county or state list of true "invaders" and steer clear of them.
The other question that gardeners need to ask is: How will this plant perform in my garden? Can it be grown among perennials or only under trees? In a mixed border or only by itself as a lawn substitute?
And because the spreading behavior of plants varies by region and is greatly affected by growing conditions, look for feedback from actual gardeners near you, especially feedback about the growing conditions under which a plant causes trouble - or not.
For that, I highly recommend the plant reports at Daves Garden. Just put a plant name in that search 
engine (upper right) and you'll find reports of real gardener experiences with that plant, gardeners all over North America who've grown it in a range of different sites and conditions. It's an amazing resource you can tap into without even joining the Daves Garden community, and it's been a big help in writing the ground cover profiles you'll find here.
The Cost
If the area to be covered is large (as it is in lawn replacement situations) cost is a big factor. I once calculated the cost of covering my small back yard with thyme and it came to over $600, so I quickly understood the widespread of turgrass - it's cheap to grow. In the long run it's not cheap at all, what with the costs of mowing and dumping huge quantities of products on ths stuff.
So might help to consider the long-run cost of your alternatives, and if you DO buy ground covers for a large area, it's cheaper to buy small containers and space farther apart. Add mulch bertween and water, and most will fill out in 3 seasons, according to Higgins. Yep, takes patience.
The Plants
If your site doesn't demand super-short plants that have to endure foot traffic, here's lots more perennials to consider. Or move on up to shrubs and trees. And to read real gardeners' experiences with different ground covers, a terrific source is the plant profiles at Daves Garden. Just put a plant name in the "Search Plants" function you see on the right. More photos of ground covers can be found by using Google Images (a great resource!)
For Sun
- Carex is an evergreen ornamental grass that loves shade
- Ceratostigma/Plumbago
- Chelone glabra/Turtlehead
- Chrysogonum virginia/Green and Gold (only with sufficient water)
- Coreopsis verticillata/Threadleaf coreopsis
- Delosperma cooperi/Ice Plant is great for hot, dry spots with good drainage, even on banks. Cannot take being stepped on, though.
- Hypericum or St. John's wort is a large group of plants, both native and nonnative.
- Liriope is evergreen and tough everywhere. The spreading kind is so vigorous and hard to remove, think of it as very short bamboo.
- Mazus reptans can handle any amount of sun but drought? Not so much.
- Oenothera /Evening Primrose
- Polygonatum/Solomon's Seal
- Pulmonaria/Lungwort
- Sedum acre/creeping sedum for hot, dry spots can take a little foot traffic. I've replaced an entire lawn with this stuff, as shown in my photos.
- Stachys/Lamb's ear
- Thyme, creeping types
- Tradescantia or Spiderwort is native to the Eastern U.S.
For Shade
- Aegopodium podagraria or Bishop's weed - not for every site.
- Ajuga reptans can handle full shade to part sun and is evergreen.
- Allegheny spurge is native to the U.S. Southeast.
- Begonia grandis - hardy begonia has pink flowers in late summer and beautiful foliage all season.
- Carex is an evergreen ornamental grass that loves shade
- Chelone glabra or Turtlehead is native to the East and Midwest in the U.S.
- Chrysogonum virginia/Green and Gold is native to the U.S. Southeast.
- Epimedium/Barrenwort can even handle dry shade.
- Euphorbia amydaloides/ Wood Spurge is my favorite perennial (and groundcover) for shade. It's even evergreen!
- Galium operatum, also known as Sweet woodruff, is lovely and another favorite (MUCH shorter).
- Hakonechloa grass is gorgeous and one of very few ornamental grasses for shade. Grows awfully slowly, though.
- Hellebore is an evergreen that's best in partial shade, also can handle dry shade under trees.
- Heuchera/Coral Bells are awfully popular these days, with good reason.
- Hostas - the old standby for shade.
- Hypericum or St. John's wort tolerates partial shade.
- Liriope, even the creeping type, is better behaved in the shade but still can be a thug.
- Lysimachia nummularia/Creeping Jenny is way too vigorous to share space with other plants but works well in containers and all by itself as a lawn replacement because it CAN take some foot traffic.
- Mazus reptans can handle any amount of sun but drought? Not so much.
- Ophiopogon or Mondo grass is a better-behaved substitute for creeping liriope in warm climates.
- Pachysandra is another standby for the shade, whether we're bored with it or not.
- Solomon's seal is one of my favorites becuase its variegated green and white leaves brighten up the darkest spots.
- Tradescantia (spiderwort) is native to the Eastern U.S.
Very short groundcovers that can take some foot traffic
- Liriope, Mazus, Creeping Sedum, Thyme, Bishop's weed, Ivy (coming soon), Creeping Jenny, Pachysandra, Allegheny Spurge, Mondo grass, Sweet woodruff, Vinca or periwinkle (coming soon).
Evergreens
- In the sun: Carex, Liriope, Mondo Grass, St. John's Wort (many), Thyme and Mazus
- In the shade: Carex, Liriope, Mondo Grass, St. John's Wort, Mazus, Hellebore, Hakonechloa Grass, Euphorbia amydaloides, Allegheny Spurge, Pachysandra.
Good Information in Print
- Covering Ground
by Barbara W. Ellis - Hardy Succulents
by Gwen Moore Kelaidis and Saxon Holt
- The Well Tended Perennial Garden
- Green Roof Plants
- Encyclopedia of Perennials
- Designing with Perennials
- The Complete Flower Gardener
Top photo, Sedum acre with Evening Primrose. Middle photo is Eurphorbia amydaloides. Bottom photo shows Chelone glabra.