Part 1 Planting slopes at Phipps Conservatory

Pittsburgh, Pennsylvania surprised me. The city has a whooping 42 percent tree canopy! In comparison, Washington DC has 36% and Portland, Oregon in the wet Pacific Northwest has mere 30% tree cover. Even Canadian cities known for their leafy nature have less tree cover– Toronto 33%, Vancouver, Ottawa, and Montreal all at 20% tree cover. But enough about trees!

Another visual surprise is Pittsburgh’s topography. Hilly. Steep slopes. It’s a great place to see how gardeners tame their slopes. Since there’s too much information to cover in one blog, I’ve divided it this entertaining topic into three blogs.

planted slope at Phipps Conservatory, Pittsburgh, PA

Part 1. Planting a slope: using plants to stabilize a slope

Part 2. Taming a slope: Terraces & decks: structures that allow a gardener to use their garden

Part 3. Terrace or plant slopes?: What a gardener needs to know before they decide between planting, terraces or decks.

Planting up a slope is popular low-cost way to stabilize the slope so it doesn’t erode or — at the most catastrophic point – cause a landslide. Slopes need tough plants. Plants that can take a beating, but still keep on growing. Most plants – trees, shrubs, perennials and grasses – used to stabilize slopes have a fibrous root system.

The downside of slopes

Thin layer of topsoil
Poor soil: nutrients leached away
Drier soil.

 

Phipps Conservatory and Botanical Gardens built its Center for Sustainable Landscape on a hillside. Since Phipps is a strong advocate for advanced green-building practices and sustainable gardening, it’s the perfect place to see best practices and plants to stabilize slopes. Margie Radebaugh, director of horticulture and education confirmed, “all of the plants used in the landscape are natives or cultivars of natives.” and that the slopes were planted two years ago.

“Jute mesh erosion control was placed on the hill to help hold the soil in place,’” says Radebaugh. “It will decompose with time, and, in fact, has already started to do so.” By the time the jute netting decomposes, the slope’s soil will be firmly held in place by plant roots!

Phipps Conservatory generously shares its plant list used around the Center for Sustainable Landscapes. Since the list covers all the plants used (including non-sloping areas), I asked Radebaugh for specifics.

“At this point, one of the most prevalent plants is partridge pea (Chamaecrista fasciculate)”, says Radebaugh. “Partridge pea is an annual, but it will reseed and probably stay with us for a few years…it was planted to help hold the soil while the other plants became established.”

As for perennials, the main ones growing on the slopes at the moment include whorled milkweed (Asclepias verticillata), ox-eye sunflower (Heliopsis helianthoides) and spiked grayfeather (Liatris spicata).

 

Trees planted on the slopes

Botanical name Common name
Betula populifolia ‘Whitespire’ Whitespire gray birch
Nyssa sylvatica Blackgum
Populus tremuloides Quaking aspen
Quercus alba White oak
Quercus imbricaria Shingle oak
Sassafras albidum Sassafras
Amelanchier canadensis Shadblow serviceberry

Quaking aspen, a fast growing, short-lived tree, will eventually be removed as the oaks gain in size

Shrubs used on the slopes

Botanical name Common name
Arctostaphylos uva-ursi Bearberry
Ceanothus virginiana New Jersey tea ceanothus
Ilex glabra ‘Densa’ Dwarf inkberry
Juniperis communis ‘Effusa’ Effusa juniper
Potentilla fruticosa ‘Primrose Beauty’ Primrose beauty cinquefoil
Rhus aromatica‘Gro-Low’ Gro-low sumac
Vaccinium corymbosum ‘Blueray’ & ‘Bluecrop’ Highbush blueberry

 

The hillsides were also planted with the Alkaline Ornamental Meadow, Low Ornamental Meadow and Warm Season Grass seed mixes.

 

Alkaline Ornamental Meadow seed mix

Botanical name Common name
15% Aster oblongifolius ‘October Skies’ October Skies Aster (perennial)
30% Bouteloua curtipendula Side oats grama (grass)
15% Coreopsis verticillata Thread-leaved tickseed (perennial)
15% Rudbeckia fulgida var. fulgida Black-eyed Susan (perennial)
25% Schizachyrium scoparium ‘Prairie Blues’ Little bluestem ‘Prairie Blues’ (grass)

 

Warm Season Grass seed mix

Botanical name Common name
15% Bouteloua curtipendula Side oats grama
30% Sporobolus heterolepis Prairie dropseed
30% Schizachyrium scoparium ‘Prairie Blues’ Little bluestem Prairie Blues
25% Asclepias verticillata Whorled milkweed

 

The plant list for the Center of Sustainable Landscapes does not have the percentage composition for the Low Ornamental Meadow seed mix. And neither did Radebaugh. However, we can guess what might be included in the seed mix by looking at 20 plants listed under Ornamental Meadow Seeding.

“The landscape will evolve and change over time,” says Radebaugh, “with some plants increasing in number, some decreasing or disappearing over time, and others may still appear, from seed, at a later date.”

“As the trees grow, they will provide shade, which will further change the composition of the landscape, says Radebaugh. “The roots of the wide variety of plants growing together will hold the soil in place over time.”

 

And that folks, is all she wrote on planting slopes. My next blog: Part 2:  Taming a slope: Terraces & decks!

 

References and more information

Billie Jo Jannen. SF Gate Home Guides. The Best Way to Keep Mulch From Migrating Down the Hill It’s On

Ian Robertson. Fine Gardening magazine. Shrubs for slopes

Good Oak Ecological Services. Slope Erosion Control
Good article explaining why perennials & grasses are more useful to slope stability than trees and shrubs. And it also covers different kind of netting & mulch used on slopes.

Phipps Conservatory and Botanical Gardens Plant List for the Center of Sustainable Landscapes

State of Washington. Department of Ecology. Plant Selection Guide, best plants for erosion control or slope stabilization project.

Today’s Homeowner: Using pantyhose to anchor plants on a steep slope

University of Minnesota Extension. [Plants for] Steep Slopes

University of Minnesota, Extension. Ground covers for rough sites

 


Written by Cristina da Silva
Wednesday, September 10, 2014 in Plants & Soils

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