West Hills Innovative Stormwater Demonstration

Final Report, Chapter 5: Implementing Recommended Practices
Revegetation to Stabilize Steep Slopes

Steep slopes generate faster and more erosive flows, so the grassy areas between and around existing trees should be planted with native trees & shrubs to intercept rainfall during our small, frequent rain events and to slow runoff by breaking up the flows during more intense storms. In addition, the complex and deep roots will hold the soil and pull water out of the ground, even when it’s not raining. Projects proposed for this practice are mostly in more natural, unusable, steeply sloping, open space areas.

This practice is a recommended practice for stabilizing slopes to prevent landslides. (See "Homeowner's Guide to Landslides" by Dr. Scott Burns of Portland State University, page 10.)

Figure 5-1 Before planting, steep grassy slopes generate sediment laden runoff and are at a higher risk of sliding at project 5 and many other project sites.
Figure 5-2 After planting steep slopes are stabilized and runoff is significantly reduced
Planting Strategy

Planting strategy: A formal planting plan for some areas may be desired but for these areas is not strictly needed; however, a strategy should be developed by a landscape professional experienced in habitat restoration. This professional may be a landscape contractor, designer, or architect.

This strategy should include a palette of appropriate native trees and shrubs and how they will be irrigated for two summers. If the right plant is put in the right place, sometimes irrigation will not even be needed, so this too should be considered. (See more below on plant establishment and irrigation.)

Installing the plants: Work can be done by volunteers, but these areas are quite steep. Hiring a professional landscape contractor who is experienced in planting restoration sites is recommended and this is the cost incorporated into the estimate.

Regardless of who does the actual planting, bare root or small potted trees and shrubs are recommended, which have some significant advantages over planting large (1 gallon and up) trees and shrubs.

Small pots or bare root plants:

  • Adapt faster to their new conditions
  • Require less water to adapt
  • Have a higher rate of survival
  • Cost less and are easier to plant

Stakeholder Considerations

Protect views: The planting strategy should ensure that no views will be impacted by the plantings. Trees topped by homeowners can be seen throughout the natural areas, and this is a very damaging practice for a variety of reasons. Plant shorter shrubs where views may be impacted (in most cases, this is the top of the hill) and taller trees where their mature height will not impact views (in most cases, this is the bottom of the hill).

Aesthetics: Shrubs and trees that flower at different times of the year might be carefully combined to be seen from the road.

Specifications & Materials Sources

Free plants from Clean Water Services: Clean Water Services is willing to provide us with thousands of small, bare root native trees and shrubs that would be ideal for these slopes, for free. Rich Hunter, the Water Resources Program Manager, at (503) 681-3600 or (503) 681-3638 and hunterr@cleanwaterservices.org has offered to assist the HOA with the first step in the process of getting plants.

Planting density: In restoration projects in our region, Clean Water Services recommends 1800 shrubs and 600 trees per acre as appropriate.

Restoring the soil: Soil restoration is essential to getting the plants established with minimal or no additional water. Many of the steep slopes have been compacted by equipment or covered in fabric and are likely to have a lower than optimal soil biology count or diversity.

While compost amendment (as described in the fact sheet developed for the FHHOA) is feasible and helpful for stabilizing slopes up to 2H:1V (Click here and scroll down to the second photo on this website from WA Department of Transportation), working on steep slopes with existing intermittent trees and shrubs will make this practice unreasonably expensive. Instead, as a minimum, mycorrhizae is recommended to be added to these very disturbed and highly compacted slopes. Avoid products that are manure based or commercial weed and feed products, since these will run off the steep slopes during the establishment period and degrade water quality with nutrients, excessive amounts of which can cause algae blooms and unsafe health conditions.

One such product that meets the specification and can be hydroseeded onto these steep slopes is Permamatrix, a locally sourced product from Sunmark Environmental in Troutdale, OR.

Time of year: Shrubs and trees should be planted in the fall when they will have the longest oppportunity to establish during the rainy season.

Irrigation for establishment period/1st year: As evidenced by the landslide on SW Terrwilliger in 2008 that destroyed a number of homes, irrigation on steep slopes must be done with great care. For this reason, conventional piped irrigation systems are NOT recommended. Instead, as is commonly done, hire a landscape company with a low pressure water truck to deeply water plants in the root zone only (generally the area between the trunk and the dripline/canopy) once in July and again in Aug after initial planting.

Although irrigation for more formal landscapes is usually recommended in our region through the first two summers after planting, the planting density accounts for some percentage of plants not expected to survive and planting in the fall will greatly reduce those losses.

This follows the guidance provided on page 10 of "Homeowner's Guide to Landslides".

Invasive species: Garlic mustard is an invasive species present in the FHHOA at natural sites such as these slopes, and staff at Clean Water Services is concerned about its spread because it dominates forest floors and appears to impact habitat value. Disturbance activities could easily spread this aggressive and damaging weed. More information on how to identify and eradicate this weed if on the Clean Water Services website and who to contact can be found on their website - click here.

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