While the most critical contributor to the success of any segmental retaining wall (SRW) will always remain detailed planning and competent workmanship, planting will transform a functional structure into an eco-friendly environment that can create a dazzling display of colour, scent and texture. Add to that the possibility of drought and water restrictions, it is obvious that planting a retaining wall should not be taken lightly.
When the City of Cape Town planned a new head office for the Western Cape Electricity Department, Belville, Cape Town – a Green Star development designed by Bruce Eitzen, New World Associates Landscape Architects – the plans included an underground and large above-ground parking lot, necessitating cut and fill slopes that needed stabilisation.
To conserve space, and to create a green and aesthetically pleasing space around the building, Terraforce blocks were specified by NWE Consulting Engineers, because they are specifically designed to conserve existing natural areas on the project site by maximizing the amount of open space relative to the development footprint. They also maximise site usage and thereby reduce the total percentage of land area needed for development. Being hollow core, the blocks additionally support vegetative growth – are easy to fill with good quality soil – which provides improved storm water quality and better erosion control.
The gardens and plants in the Terraforce blocks, planted by Urban Landscape Solutions, were constructed in winter 2013 and had established over three summer seasons, before the water was turned off in November 2016 with the onset of Stage 3B water restrictions – meaning no irrigation. Over 90% of the planting has survived. The secret to the success? Good soil preparation, careful plant choice, and the advantage of three years “pre-drought” plant establishment.
Says Eitzen: “Before planting, a soil wetter, TerraCotten, was used to boost soil retention, composting and mulching. Next came good plant research and selection, which had the result of an installation that survived the drought, saved on replanting costs and reduced potable water consumption. Plants that thrived were notably the widespread coastal plants, Metalasia muricate, three Rhuses, Searsia crenata, S. laevigata and S. glauca, all Cape Flats sand fynbos species.”
Here is our list of water-wise and indigenous plants most suited for retaining walls, as recommended by Liesl Wasmuth, Managing Director and Landscape Designer, Graund:
Drought resistant plants
- Aptenia cordifolia (indigenous) is a tough, trailing succulent ground cover that will grow in any soil and is perfect for coastal conditions. It has a bright cerise red flower in spring and summer.
- Disphyma is also a succulent creeper, covers quickly and efficiently with little water.
- Othonna capensis, an indigenous succulent plant with small grey tubular leaves and small yellow flowers.
- Carpobrotus edulis (sour fig) extremely hardy succulent
Remember that all plants, even these, need watering when planted and for some time after to establish properly. Once they have established, they can survive on their own.
Plants for those who like it local
The plants in the water-wise section are all indigenous, but a few more include:
- Arctotis (African daisy) forms a dense grey matt with colourful daisy-like flowers. Also quite hardy, but needs to be cut back from time to time.
- Carisssa macrocarpa, great for covering low stone walls. Good in coastal conditions.
- Osteospermum ecklonis (creeping marguerite) for flower power like the Arctotis
- Plectranthus neochillus is a groundcover that is very easy to grow and useful for those difficult parts of the garden that are both dry and shady where root competition is a problem. It has fleshy green-grey foliage and lovely blue flowers for almost all year round.
Please also have a look at the plant guide put together in collaboration with Kirstenbosch Botanical Gardens, Cape Town: Terraflora
Client: City of Cape Town
Terraforce Installer: Greymo Construction
Consulting Engineers: NWE Consulting Engineers
Civil Contractors: Haw & Inglis Civil Engineering