Stabilizing Slopes

Cathy Kavassalis – Halton Master Gardener

Homeowners often ask how to stabilize embankments and reduce erosion on slopes. While some situations will require professional assistance, there are a variety of natural techniques that you can use to anchor soils using vegetation. Bioengineering uses live plants combined with dead or inorganic materials to produce living, functioning systems that can prevent erosion while providing habitat. Plants chosen need to match growing conditions. You will need to look at your soil structure, slope, aspect, light, water movement, etc. To support healthy habitats, native plants should be chosen.

Sites that are most susceptible to erosion typically have soils with poor structure. They may have too much sand or too much clay with little or no living or dead organic matter to provide cohesion. Or they may be incipient soils (precursor to soils) made up of coarse material, rocks and boulders. Additions of soil, organic matter and mulch (e.g. wood chips) may be needed in conjunction with bioengineering techniques, before you can add plants.

There are a variety of really cool techniques that can work, like “soft armouring” using logs, root wads or vegetative mats. Here is a very brief summary of techniques. You will need to read through the links below to learn which are best for your particular site conditions:

  • Contour Wattling — bundles of branches called wattles or fascines are placed in shallow trenches along a slope.
  • Brush Layering — alternate layers of live branches and soil are used to create a series of reinforced benches along a slope. Dormant willow branches are often used for this. Dogwoods can work as well.
  • Trench Packing — live plants (e.g. whips of dogwood) are placed in a trench, perpendicular to the flow of water. This works for gullies that often run down slopes.
  • Brush Matting — a mat composed of interwoven, branches is secured to the soil by live stakes, wire, twine, or live branches (usually willow) to slow water movement.
  • Cribwall — a frame is built with untreated logs and filled with soil and live cuttings.
  • Live Cuttings — Live cuttings can be used to secure materials in place and to increase plantings on a slope. Live cuttings can be from 45-120cm (18 -48in) in length. Longer cuttings are used for live staking of wattles, while shorter cuttings are used for plantings.
  • Coir fascines or biodegradable geotextiles constructed of straw, coconut coir, wood fibres or blends (similar to brush mats) can be used for slope stabilization and protection of stream banks while plants are establishing. (Do not use synthetic geotextiles e.g. polypropylene, which contaminate the environment with plastic debris).

In Ontario, native willows (Salix spp.), dogwoods (e.g. Cornus stolonifera, C. obliqua), cedars (Thuja occidentalis), aspen (Populus tremuloides), Cottonwood (Populus deltoides) and alders (Alnus incana) are often used for wet sites. For drier areas with poor soils, snowberry (Symphoricarpos albus), white pine (Pines strobus), red elderberry (Sambucus racemosa), dogwood (e.g. Cornus alternifolia, C. rugosa), fragrant sumac (Rhus aromatica), brambles (Rubus spp.), viburnum (Viburnum lentago), etc. Species should be chosen that are regionally appropriate.

See the Forest Gene Conservation Network for woody plants appropriate for your ecodistrict . They also have a species Information table to help you match your soil and shade conditions to the right plants 

The key to success is to plan for plant succession. Introduce a diversity of species so that shade intolerant species (like aspen on the banks of a stream) will eventually be shaded out by a taller shade tolerant trees (like cottonwood) and a diversity of understory plants can continue to self sow or spread via rhizomes (like brambles). Please avoid the use of invasive species. Plants such as creeping periwinkle is quite shallow rooted and does not hold soil well. A deep rooted native like Bearberry/ kinnikinnick (Arctostaphylos uva ursi) would be a far better choice, (roots can extend to depths of 90cm on moist sites and up to 180cm in drier areas). Creeping periwinkle is an invasive species that creates dense shade and prevents other native plants from germinating; it releases chemicals that inhibit the growth of native woody seedlings, and generally creates an environment hostile to native biodiversity.

Toronto has recently adopted a new Green Standard (see link below), which comes into force in May 2022 and sets strict rules for sustainability on sites near ravines. Developers on this type of site must plant the landscaped area with 100% native plants, including all trees, shrubs and herbaceous plants. While this does not apply to existing homeowners, it reflects the city’s recognition that native plants are the appropriate choice for embankment stabilization and securing a healthy environment.

Plants for Erosion Control:

  • Amelanchier alnifolia – Saskatoonberry/serviceberry
  • Aquilegia canadensis – wild columbine
  • Cornus amomum, C. racemosa, C. sericea – dogwood
  • Crataegus spp.,
  • Cephalanthus occidentalis – buttonbush
  • Diervilla lonicera – northern bush honeysuckle
  • Elymus hystrix – bottlebrush grass
  • Eurybia divaricata – white wood aster
  • Lindera benzoin – spicebush
  • Lonicera involucrata – nlack twinberry
  • Physocarpus opulifolius – ninebark
  • Platinus occidentalis – American sycamore
  • Rhus aromatica – fragrant sumac
  • Ribes americanum – black current, R. cynosbati – eastern prickly gooseberry
  • Rubus canadensis – Canadian blackberry, R. idaeus, R. occidentalis, R. odoratus,
  • Salix bebbiana, S. discolor, S. exigua, S. lucida, S. nigra, S. purpurea, S. petiolaris, Sambucus canadensis, S. racemosa,
  • Solidago caesia – bluestemmed goldenrod
  • Spiraea alba – steeplebush
  • Symphoricarpos albus – snowberry
  • Viburnum lentago – nannyberry

Further Resources:

Cover Image: Gro-Lo Sumac (Sumac aromatica ‘Gro-Lo) – Laurie’s Blogspot

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