Staff — June 9, 2016

Covering a Slippery Slope
Photos: David Winger for Plant Select unless noted otherwise.

Your client wants to stabilize a slightly sloped area on newly developed property, where the builder has plopped in a couple of shrubs and spread a thin layer of mulch. For now, the incline looks okay; the shrubs are new, the mulch is fresh, and at least it isn’t exposed soil. But you know that the first time it rains, all that mulch will wash downslope and will end up in a damp and dirty pile against the house.

Delosperma floribundum (Starburst ice plant).

The project comes with a pretty tight budget, though, and your client is asking for a solution that stops short of an engineered retaining wall. Can it be done? Provided that the slope isn’t too steep, yes. (Before you agree to the work, be sure to confirm that the slope does not, indeed, require a permanent retaining structure.) With a combination of sturdy groundcover plants and, say, a few strategically placed boulders, the slope can become an asset to the property rather than a nightmare waiting to happen.

Delosperma floribundum (Lavender Ice ice plant).

Pat Hayward, director of Plant Select in Fort Collins, Colorado, recommends a combination of boulders, woody plants and perennials. “Don’t expect the plants to do all the work” on a slope, she explains. “Don’t expect the plants alone to solve your problem.”

Perennial selections

Perennials add seasonal color but may take a few growing seasons to anchor sufficiently. Still, the vibrant hues offered by such sturdy groundcovering selections as South African ice plants (Delosperma) dazzle while they define and help to hold a slope. This succulent, mat-forming, quick-spreading plant, which is a great choice for the West, grows to about 4 to 5 inches tall and spreads to about 1.5 feet. Some cultivars are even shorter and tend to tightly hug the slope, reaching only about 2 inches tall.

Phlox subulata

Photo: iStock

Ice plant generally flowers from May through September in zones 5 to 9, and “when they are in bloom and you have a bunch of different ones, you have to wear sunglasses,” Hayward says. “They glow; they’re iridescent.”

Bloom colors range from purple to orange to pink (and nearly everything in between); foliage is generally a fleshy, medium green and produces transparent flakes, resembling tiny pieces of ice (thus the common name). In warmer locations, foliage is evergreen.

Delosperma prefers full sun and dry to medium, well-drained soil; it’s a natural choice for drought-prone areas.

Zinnia grandiflora ‘Gold on Blue’, a native prairie zinnia, “blooms from the middle of summer to the end of summer, and it takes absolutely the worst sunny, hot, dry conditions,” Hayward offers. “It’s dormant during the winter, but this thing runners; it colonizes like crazy. So it will hold the slope with its roots, but in the winter there’s not much there.”

Zinnia grandiflora (Rocky Mountains zinnia; plains zinnia)

Photo: David Winger for Plant Select

Plant Select recommends this perennial for use on slopes, along driveways and other challenging sites where it can be allowed to spread to form an 8- to 10-inch tall carpet. Large yellow flowers cover the mat of very narrow, light to medium-green foliage. This tough plant is hardy in zones 4 to 8, where it prefers full sun and thrives in dry to xeric moisture conditions.

Although it’s a low-maintenance plant, shearing plants in late fall or spring helps to clear out old growth.

In the Southeast, Wally Pressey, general manager for Classic Groundcovers, Athens, Georgia, recommends orange daylily, Hemerocallis fulva and a double fulva, or ‘Kwanso’. “They’re both spreaders – you’re not going to have any foliage in the winter, of course, they’re dormant – but they spread and they’re tough, too,” he says. “They’re somewhat aggressive, and they’ll grab on and they just keep going. They work in good soils and bad soils, but they need sunlight.”

Hardy in zones 3 to 10, orange daylily is tolerant of a variety of soils. In some areas, it is considered to be aggressive to the point of invasiveness; H. fulva is an infertile triploid plant that does not set seed, but it spreads by rhizomes. It tends to be slow-growing, so if it is carefully and judiciously sited, it can be kept in check. Similar hybrids can be substituted if there’s a concern that the plant may escape its boundaries.

That being said, it can serve as an excellent plant to help secure sloping areas. Pressey says, “The root system is definitely sufficient; thick, meaty roots grab onto everything. Once it’s there, you need a Bobcat to get it out.”

Lamiastrum (yellow archangel)

Photo: iStock

If you’re working with a shady or semi-shady slope, Pressey recommends Lamiastrum (yellow archangel or false lamium). Maturing at about 12 inches tall and spreading to 18 inches, this slow-growing groundcover forms either a mat or a radiating clump, depending on the cultivar. It requires moist, well-drained, rich soil for optimal performance, but will occasionally tolerate dry to average soils – as long as it’s not in full sun. Medium to dark-green leaves often sport a silver variegation, offering flashes of light in the shade garden. Distinct yellow flowers with a hooded upper petal and slightly protruding lower petal appear in April and May, and can reappear sporadically throughout the season. Yellow archangel is hardy in zones 5 to 8.

Evergreen Phlox subulata (creeping phlox or moss phlox) “is going to spread, and it will work on banks and hillsides,” Pressey states. “It gives you a good flower show early in the spring, and forms a carpet. Some of the varieties mound a bit; some of them hug the ground tightly.” It’s hardy in zones 3 to 9.

Hemerocallis fulva (common daylily)

Photo: iStock

Plants will grow to about 6 inches tall with a 2-foot spread in full sun, although those sited in very hot locations may appreciate a bit of dappled sun. This classic groundcover plant blooms in a variety of colors, ranging from white to pink to blue-purple, with flowers covering the carpet for up to 4 weeks in spring.
“Here, in Georgia, it’s a plant you see on roadside banks, in ditches and it’s kind of an old-timey thing,” Pressey says. “People actually steal it out of one another’s yards here; there’s always a news story about having the phlox stolen. But obviously, if it’s something that’s worth stealing, people like it.”

Woody selections

As in any setting, a bit of variation is a good thing. Plant a monoculture, and you run the risk of a wipeout. On a sloped site, the combination of woody plants and herbaceous perennials also helps to assure that the soil holds fast.

Prunus besseyi (native sand cherry)

Photo: Plant Select

“The woodies cover more ground faster, you can get a 6-foot spread on that slope a lot faster than you would [using] an herbaceous plant,” Hayward advises. Reliable root development is key: “Herbaceous plants could take a while, so you could interplant some ground covering shrubs with some ground covering herbaceous plants.”

A few words about invasives

Yes, many groundcover plants that are described as “vigorous,” “enthusiastic” and “aggressive” may also be considered invasive. After all, one of the defining characteristics of a groundcover is its ability to spread. However, invasiveness depends on region, microclimate, physical confines of the planting site and many, many other environmental conditions, as well as the plant’s tendency to produce prodigious seed, to self-sow or to crowd out nearby plants. What’s invasive in one area may simply be vigorous in another, and no one is suggesting that known thugs be grown, specified or planted. So if you see a plant recommended here that has invasive tendencies in your area, please understand that it may not be invasive elsewhere; it is not our intention to promote problem plants. If you have questions about how aggressive a plant is in your area, check with local Extension personnel or the USDA’s National Invasive Species Information Center at

Hayward suggests Rhus trilobata ‘Autumn Amber’, a Plant Select recommendation that’s tough as nails and suitable for sloping sites. “If you put it at the top of a boulder, it’ll cascade straight down the boulder. It’s completely flat,” she describes. “It’s a three-leaf sumac that is a native to the Southwest; it’s xeric. It has sort of an orangey-yellow fall color, but it’s got glossy green leaves” that provide a delicate, graceful look, despite the plant’s rugged performance. “It’s great for pollinators, bees and butterflies,” Hayward adds.

Growing only to about 14 inches tall and spreading to 8 feet, ‘Autumn Amber’ prefers full sun and thrives in moderate to xeric moisture conditions in loam or sandy soil in zones 4 to 8. Maintenance is minimal, although an annual trimming may help to shape the shrub if you’re filling a small space.

Rhus trilobata (Autumn Amber sumac)

Photo: Pat Hayward

Another slope-stabilizing shrub is Prunus besseyi Pawnee Buttes®, (native sand cherry), hardy in zones 3 to 8. Not only will it help to stabilize slopes, it attracts pollinators with its prolific, white, early spring flowers; wildlife enjoy the resulting heavy crops of black cherries in summer.

Growing 15 to 18 inches tall and spreading to about 6 feet, the shrub is covered with lustrous green foliage that turns a bright copper in fall. It performs well in sun to partial sun and prefers moderate to xeric moisture conditions in clay, loam or sandy soil.

Rhus trilobata (Autumn Amber sumac)

Photo: Ross Shrigley

Stabilizing a sloping property can be a tricky undertaking, but with a combination of plants and sturdier structures like rocks and boulders, that sad little layer of mulch can be avoided. Remember: Don’t rely on the plants alone. They work in concert – woodies and perennials – and they need a bit of time for the roots to establish and hold the soil. If you have any doubts about the amount of grade, consult with a soil specialist or an engineer.

Read more: Edgeworthia chrysantha

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