Perennial Grasses for Erosion Control on Slopes and Banks

Why grasses work for erosion control

Many perennial grasses and sedges have dense, fibrous root systems that knit the soil together near the surface, while their leaves slow rainfall and runoff before it hits the ground. The Lady Bird Johnson Wildflower Center recommends grasses for erosion control for exactly this reason. Unlike a mown lawn, most ornamental and native bunchgrasses are not cut short, so they build up more root and leaf mass.

A few caveats are worth knowing from the start:

• Grasses do not work instantly. Until they establish, the soil is vulnerable, so temporary protection matters.
• No plant fixes a drainage problem. If water is pouring down a slope in volume, you need to manage the water as well as the soil.
• The best species depend on moisture, sun and soil, which can vary from the top of a slope to the bottom.

Good grasses for slopes and banks

Prefer plants native to your region where you can (in the central US, see Midwest and native prairie grasses). They tend to cope with local weather and are less likely to spread where they are not wanted.

Dry, sunny slopes

Upper parts of a slope usually drain fast and dry out:

• Little bluestem (Schizachyrium scoparium): a deep-rooted native prairie bunchgrass with good fall colour.
• Big bluestem (Andropogon gerardii): tall and deep-rooted; best on larger areas.
• Sideoats grama (Bouteloua curtipendula): drought tolerant and medium in height.
• Indiangrass (Sorghastrum nutans): tall, upright, handles drier soils once established.
• Prairie dropseed (Sporobolus heterolepis): a fine-textured clumper that tolerates dry conditions.

For more on grasses that handle both dry spells and strong sun, see what type of grass tolerates both drought and full sun. For sandy banks and dunes near the sea, see ornamental beach grasses for coastal gardens.

Shady slopes

The Wildflower Center’s list for a steep, shady Iowa slope included eastern woodland sedge (Carex blanda), inland sea oats (Chasmanthium latifolium), Canada wildrye (Elymus canadensis), bluejoint (Calamagrostis canadensis), as well as big and little bluestem and Indiangrass for the brighter patches. Many sedges and some woodland grasses cope with shade; most prairie bunchgrasses want good light to do well.

Wet banks and ditches

At the bottom of a slope, or along a ditch or stream bank, soil is often moist or periodically flooded:

• Switchgrass (Panicum virgatum): adaptable to both moist and drier conditions.
• Fox sedge (Carex vulpinoidea): spreads readily by rhizomes, which Hoffman Nursery notes is helpful for holding soil and slowing runoff, as long as spreading is acceptable.
• Tussock sedge (Carex stricta) and soft rush (Juncus effusus): good for the wettest spots.
• Bluejoint (Calamagrostis canadensis): a native of wet meadows.

See ornamental grasses for wet areas for a wider list and notes on matching plants to how wet the ground is.

Groundcover grass-likes

Spreading, grass-like groundcovers can cover a bank quickly. A Florida nursery recommends lilyturf (Liriope spicata; see liriope vs mondo grass) planted about a foot apart in strips running across the slope, noting that it spreads by underground roots and needs edging if you want to confine it. That vigour is useful on a bank but it can escape into lawn and beds, and in some regions it is considered a problem plant, so check local guidance before using it.

How to stop erosion on a sloped yard

Plants are one part of a combination of fixes:

1. Find where the water comes from. Downspouts, paved areas and a neighbour’s yard often send water to the same gully. Redirecting or spreading that flow (for example, extending a downspout to a safe discharge point or creating a rain garden at the base) cuts the problem at the source. See grasses for rain gardens for plants suited to catching runoff.
2. Protect bare soil. Erosion control blankets made of biodegradable material, straw mulch or a thin layer of compost can hold soil and seed in place while roots develop.
3. Plant across the slope. Rows and bands running across the flow slow runoff better than rows running straight down.
4. Use more than one layer. Combining grasses and sedges with deep-rooted perennials and, on larger slopes, shrubs gives stronger root structure than grass alone.
5. Consider terracing or retaining walls for steeper slopes, as plants alone may not be enough.

Planting on a slope so it establishes

The Wildflower Center points out an important problem with seeding slopes: rain can wash seed down the bank before it germinates. Their suggested solutions are erosion control blankets (seed is sown beneath the fabric and grows up through it, or plants are inserted through cuts in the blanket) or a compost-and-seed application using specialised equipment.

For most homeowners, potted plants or plugs set through a blanket or mulch are the easiest route:

• Plant in spring or early fall when rainfall is more reliable, and match timing to your climate.
• Space plants fairly closely so they knit together sooner. Spacing recommendations vary by species, so follow the plant label or our spacing guide.
• Water through the first growing season, and weed carefully so competitors do not overtake young plants.
• Keep foot traffic off the slope until plants are established.
• Cut grasses back once a year in late winter or spring.

Ornamental grasses to avoid for erosion control

• Invasive spreaders such as ribbon grass, giant reed (Arundo donax) and common reed (Phragmites), which can take over banks and waterways.
• Plants that self-seed into natural areas in your region. Pampas grass, some Miscanthus and some fountain grasses fall into this category in certain areas. Check your local invasive species list, and see non-invasive ornamental grasses for safer clumpers.
• Tender or short-lived grasses that die back or fail to form dense root systems.
• Lawn grass on a steep slope that needs frequent mowing. Mowing a steep bank can be unsafe, and a mown turf often holds less deep root mass than a native planting.

When to call a professional

Most gardeners can handle a gentle slope or a small gully. Seek professional advice if the slope is steep, if erosion threatens a building, driveway, retaining wall or septic system, if there are active slumps or cracks in the ground, or if the bank is along a stream where regulations may apply. A local landscape contractor, extension office or conservation district can assess the site.