Susan is also blogging at GardenRant.Liriope
There's no groundcover known to humankind that's as successful as liriope at preventing erosion on even the steepest hillsides, at surviving wet or bone dry conditions, and doing all that in full sun or deep shade. You can yank it roughly out of the ground and cram it back into the ground and it won't complain.
Best of all for budget gardeners who need to cover ground, it'll spread and fill out quite quickly. Just divide the clumps you bought and they'll soon be ready to be divided again and before you know it — no bare ground will be showing, and weeds don't stand a chance in the thick mass of sheer liriope.
The Liriope that Spreads — Only for Certain Spots!
That said, the spreading type of liriope that I've just described will, given some direct sunlight, continue to spread and smother everything in its path that's not substantially larger than it — like a tree. Yes, Liriope spicata spreads SO aggressively by runners, it'll go below cement and come back up on the other side. 
In the photo on the right is an example of where spreading liriope should never be planted — in a mixed border with other small plants — and it'll be quite a chore to remove it without hurting the plants around it. Small gardening error there.
However, there ARE places for spreading liriope and an example is shown in the top photo — all by itself in a contained area, like the right-of-way between the street and the sidewalk. Another might be a steep, shady embankment where there's room to remove each year's unwanted spread without damaging any nearby plants. Or in full shade, where its spread is less aggressive.
The Well-Behaved Liriope
Liriope muscari is the "clumping" kind that does not spread by runners and basically stays where you put it. It blooms purple, not white like spicata, but it shares all the good qualities of the spreading kind — incredible toughness and adaptability — without the invasive behavior. And there's an especially attractive green-and-white variegated variety of this plant, which brightens up any shadey spot.
But budget gardeners, the bad news about clumping liriope is that you'll have to buy many more of them to accomplish the same coverage you'd get quickly and cheaply with the spreaders. And clumpers usually aren't available free from your neighbors, as spreaders are. So it's easy to understand why so many homeowners use the spreading type where they shouldn't, even after being warned.
The Dilemma Posed by Spreading Groundcovers
This one plant epitomizes the dilemma of anyone in need of groundcover for a large area: if a plant fills in quickly, it usually will also spread where you don't want it to go. And certainly the word "invasive" is a flag for anyone. So here are some good follow-up questions to ask:
- In what situation does it spread so aggressively? In the case of spreading liriope, it's only a problem when it gets direct sunlight. I've seen it growing in the shade for years, not spreading at all.
- And how hard is it to remove the unwanted growth? If it's a matter of a few shallow-rooted seedlings, removing them might be a easy enough. But plants like Liriope spicata that spread by long, tough runners are actually much harder to remove than turfgrass, which everyone agrees is hard work. So you're warned.
Details
- Can take sun or shade, soggy or dry conditions.
- L. spicata blooms white in summer; L. muscari blooms purple.
- Hardy to Zones 6-9.
Care
- Sheer back in late winter (although in full shade and there's usually no need because the foliage looks fine even after the winter).
- Easily divided using a cheap steak knife.
- It's not just extremely drought-tolerant but also moisture-tolerant — an amazing feat by a plant.
Good Information in Print
- The Well Tended Perennial Garden
by Tracy DiSabato-Aust is the best-selling book on how to make your perennials healthier and better-looking. - Encyclopedia of Perennials
by Graham Rice is exhaustive and an outstanding general resource on the subject.
Location Matters
My test garden is in the Washington, D.C. area, Zone 7A.
Have comments about this place you'd like to add? Send 'em along.