Question: Can you slow water runoff at the top of a slope?
Master Gardener Carolyn Langdon advises you to build a small rain garden at the top of your slope. Add a small berm at the top of and/or mid-way down your slope. Put in a berm of logs, branches, soil and/or rocks to slow down the water running off and to allow time for the rain to absorb and for plant roots to establish. The idea is to place any material that will act to obstruct or slow down the path of water. Organic material has the additional benefit of providing texture and nutrients to your soil as it breaks down.
Clearing a shoreline or hillside of herbaceous plants, shrubs and trees can lead to erosion if not re-planted. Longer, steeper slopes, especially those without adequate vegetative cover are more susceptible to very high rates of erosion during heavy rains than shorter, less steep slopes.
Please remember that a rain garden is not the same as a water garden. There are 5 components to a rain garden:
2. Amended filter bed (see wood log trench below)
3. Berm on the low side
4. Rockery to slow water entering the garden if necessary
5. Plants tolerant of water and long periods of dryness (i.e. 2-3 days of standing water)
A rain garden is a low tech solution for a location that periodically gets inundated with water. For example some downspouts can’t handle the quantity of rain and the spill over can cause existing vegetation to die and erosion of soil. Hard surfaces channel water during torrential rain and spring snow melt events that cause erosion particularly on steep slopes. A rain garden and berming might be an affordable solution. A 5X10 foot rain garden 6 inches deep is equivalent to 11 rain barrels.
Dig your rain garden (12-18” deep) and fill with a combination of logs, branches, and wood chips at different stages of decomposition. Add native soil and locally composted organic matter. Plant. Remember good humic soil will store a lot of water. Other practises for extreme sites is to mulch deeply, contour the soil, plant native species adapted to the location and plant densely.
Select the Right Plants
In this situation plants that can withstand short periods of flooding and long periods of dryness are required. Xerioscaping plants i.e. those that tolerate drought won’t do well in a condition that includes extreme wet and extreme dry. Likewise plants that require constant moisture wil not do well.
Do favour native over non-native plants and do not plant fast growing invasive plants however tempting that might be to stabilize your slope.
The following native Ontario plants can tolerate moist and dry soil:
Aster (Aster spp.)
Bergamot, Wild (Monarda fistulosa)
Black-Eyed Susans (Rudbeckia hirta)
Blazing stars, Rough (Liatris aspera)
Evening Primrose (Oenothera biennis)
Bittersweet (Celastrus scandens)
Goldenrod (Solidago spp.)
Verbena (Verbena spp.)
Yarrow (Achillea millefolium)
Sweet gale or bog myrtle (Myrica gale L) Myricaceae (Wax-myrtle or bayberry Family)
Non invasive ornamental grasses, native sedges (they look like grasses and can tolerate some shade), and rushes. Hierochloe odorata or Sweetgrass
Marginal Woodfern (Dryopteris marginalis) can tolerates mid-summer drought if planted in the shade.
Butterfly Weed (Asclepias tuberosa) a Zone 3 Ontario Native that is heat and drought tolerant,
Liatris aspera or Blazing Star, another Ontario native,
Schizachyrium scoparium or Little Bluestem (Ontario native)
Highbush Cranberry (V. trilobum or V. opulus var. americanum). This native shrub likes to grow in open, wooded, somewhat poorly drained locations. In the ideal location, cranberry can become very wide, often three metres or more, and reach about the same height.
Low bush Cranberry or Squashberry (V. edule)
Nannyberry (Viburnum lentago) will grow almost anywhere including shade, but they take on their best form in full sunlight. In full sun their fall leaf colours will be a vibrant purple-red.
Common or Eastern Ninebark up to 3 m, spring flower cluster, berries
Credit Valley Conservation planted native plants in their rain garden. They chose the following plants because they were widely available at local nurseries: Red Osier Dogwood, New England Aster, Tall Meadow Rue, Black Eyed Susans, Canada Anemone. Shade-tolerant native plants included: Common elderberry, Sensitive Fern and Heart leaved Aster.
A dense base of day lilies and irises will give you a fibrous root system. While they aren’t native they are often planted in a naturalized landscape.
To buy plants please try your local garden centre and check other sources listed in our list of suppliers and services here.
For additional plant selection please see Appendix C, List of Plants tolerating both wet and dry conditions. University of Guelph