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PAL Questions: 2 - Garden Tools:
Can I plant groundcovers, shrubs, and trees to stabilize a steep slope?
There are several resources which will help you in selecting plants to prevent erosion and mudslides on your slope.
Please note that these articles are merely suggestions and should not be construed as advice. We are librarians, not engineers!
None of our standard books on trees mentions the soil binding quality of tree roots. However, the Miller Library does have very good technical books and articles on slope stabilization. (For example, Slope Stabilization and Erosion Control: A Bioengineering Approach, edited by R.P.C. Morgan and R.J. Rickson, 1995.)
I do want to note one thing that many articles mention: no amount of established vegetation will hold a steep slope if other forces are present that would contribute to a landslide.
The Department of Ecology website has a list of appropriate plants.
Additionally, there are a number of books with information on the subject. Vegetative Contribution to Slope Stability at Magnolia Park (by Kathy Parker, 1996) recommends Oregon grape (Mahonia), which she suggests for gentle slopes.
Other smaller plants she lists are:
Polystichum munitum (native sword fern)
Vaccinium ovatum (evergreen huckleberry)
Symphoricarpos albus (snowberry)
Larger shrubs in her list:
Alnus rubra (red alder)
Philadelphus lewisii (mock orange)
Sambucus racemosa (red elderberry)
Acer circinatum (vine maple)
Amelanchier alnifolia (serviceberry)
Corylus cornuta (hazelnut)
For steeper slopes, Parker says that they may not be good candidates for vegetative rehabilitation unless you put in some kind of structure. She says that Jute mats can be used in conjunction with native seed, mulch, and shrubs, if carefully anchored. She also mentions a Weyerhaeuser product called Soil Guard.
Steep Slope Stabilization Using Woody Vegetation (by Leslie Hennelly, 1994) has a plant list, as well as a chart which indicates plants used to control erosion, the degree of the slopes, and the rate of success in resisting erosion.
Two titles which focus more on the garden design aspect of planting on a slope are Hillside Gardening : Evaluating the Site, Designing Views, Planting Slopes (by William Lake Douglas, 1987) and Hillside Landscaping (by Susan Lang and the editors of Sunset Books, 2002).
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When a cottonwood tree is cut down, does the stump die, or does it send out shoots that grow into more trees?
And, if a cottonwood tree located on a hillside is cut down, what is the risk of erosion?
As it turns out, some poplars and cottonwoods sucker from the roots and some do not. Determining what kind of cottonwood you have is the key to answering this question.
Identifying tree varieties can be tricky. The best way to get a positive ID is to take a sample to the Hyde Herbarium at the Center for Urban Horticulture (near the University of Washington). It is definitely worth a visit, as it is the only herbarium on the West Coast that serves the public.
Hours, driving directions, how to collect specimens, etc. are at http://depts.washington.edu/hydeherb.
As for your second question, here is what the Washington State Department of Ecology's Vegetation Management: A Guide for Puget Sound Bluff Property Owners has to say (p.25):
Given the importance of tree cover on potentially unstable slopes and the advisability of retaining them for erosion control purposes, a landowner should explore alternative options to tree removal or topping...[if a tree must be cut] stumps and root systems should be left undisturbed...[to reduce the risk of erosion].
The above document is available online at http://www.ecy.wa.gov/programs/sea/pubs/93-31/intro.html.
A companion website from the Washington State Dept. of Ecology contains a great list of groundcovers, shrubs and trees that will help keep your slope intact if you decide to remove the cottonwood. The website includes a Plant Selection guide.
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January 13 2017 10:35:53