Gardening on a slope

Want to find out more about gardening on a slope? In this article, our editor Tanya Visser describes how she tamed the steepest of several slopes in her beautiful garden.

Building a house and garden on one of the renowned ‘Thousand Hills’ of KwaZulu-Natal was no walk in the park! I had big dreams and firm plans about a garden ‘on the level’ with an expansive lawn bordered by wide, mixed beds filled with flowering plants despite the natural topography of the land. It turned out this dream was quite a challenge and resulted in a lot of earthmoving.

I suppose I could have just settled for the usual cement retaining blocks often used to retain banks, but my budget was tight, and I wanted a more natural look with bold rocks and dry stone retaining walls. My dad trained as a stone mason and some of the stones used in my wall were the original stones the two of us collected when they were blasting for the new N2 highway to Scottburgh. I moved around with these stones until I had my own property.

A set of curved sandstone steps was also one of my extravagances I was definitely going to have.

Vegetating the slope

The first objective was to plant strong focal plants like bold aloe species and lots of succulents to stand out as contrasting plants, to bind the soil, and also to soften the rock edges with their lush and spreading growth habits. As new aloe hybrids became available and I fell in love with their strong clump-forming and free-flowering habits, I added swathes of them.

No gardener is ever satisfied with the same look for long. So, when the original vegetation stabilising the bank became established, I decided to add some grasses and flowering perennials to supply summer colour when the aloes rested. The slope was also extended with a pool terrace and more stone work.

To stop the soil from washing away I pegged yards of weed matting material between the rocks, and made slits in it through which I planted.

About the dry stone walls

Way back in 2010, my mentor and friend Gerald Schofield, came to my aid by donning his gloves and gathering stones together to start building the dry stone wall to retain the bank at its base. He did an excellent job as a stone mason, teaching me all the tricks and we published a short ‘how to do it’ article in the magazine at the time. Dry stone-walling involves putting together natural rocks using soil, artistic flair and a lot of blood, sweat and even tears.

An attractive aspect of a dry stone walls is all the little plants growing in between the rocks once the wall is completed and the soil has settled – a final touch to an already unique garden feature!

Tips for Dry Stone Walls

• Use rocks of manageable sizes with roughly geometric shapes sourced locally.
• The base foundation for the stone wall to retain a bank, should be a trench about 10 – 15cm deep.
• When the dry stone wall is completed, the entire structure must lean backwards (towards the bank) at an angle of about 10 degrees. Bear this in mind as you go along.
• Dig a little way back into the bank to make sure the wall will fit properly against the bank.
• Stack the excavated soil close by for easy access when backfilling begins.
• Use a four-pound hammer to trim rocks as needed. Protect yourself with gloves and goggles.
• Dry stone walls built this way should not be any taller than 50 – 60cm. Any taller, would require an engineered concrete wall behind it to prevent damage from water. On lower walls the gaps between the rocks act as natural weep holes.

How To Build One

• Start by placing a layer of the largest rocks along the bottom of the trench.
• With this row and those following, the rocks must be selected carefully to ensure that their flattest edge faces the side of the wall that is exposed to view.
• Place a layer of weed matting behind the first stone layer against the bank.
• Backfill behind the first row of rocks with soil and stamp it down firmly.
• Put down a thin layer of soil over the tops of these stones and then put the next layer of rocks in place over them.
• Where possible overlap the rocks in a similar manner to a brick wall. This strengthens the wall.
• Use small pieces of rock to hold the larger rocks in position.
• Continue to backfill with soil stamping it down firmly both behind and in between rocks. Also check that the wall is stable and leaning back at the required 10 degree angle.
• If you need to, you can use a builder’s line as you go along to keep it all level.
• As mentioned in the tips above, make sure your dry stone walls is not taller than 50 – 60cm.

READ MORE: Slopes can be tricky to negotiate! Take a look at these tips

Plants for the Summer Look

Erigeron karvinskianus (fleabane)

If you have spaces to fill but a lack of funds, get this groundcover. It is a fast-growing, evergreen plant that can be plugged into crevices and cracks in stone walls.

Salvia ‘Wendy’s Wish’

This is an endless bloomer with large spikes of magenta-pink flowers. A bonus is the subtle citrus scent.

Miscanthus sinensis ‘Variegatus’

Amongst all the perennial colour, this wonderful grass (my favourite) does its usual job to add dramatic texture with its striped leaves. Perfect to bind soil on a bank.

Alstroemeria ‘Inticancha’ (Inca lilies)

I really went all out for dwarf alstroemerias with their compact and mounding growth habits and abundant flowers. They come back year after year!

Achillea millefolium (yarrow)

A happy mix of these easy to grow perennials enlivens my slope with colourful flower heads good for cutting as well as for butterfly attracting.

Bracteantha bracteata ‘Mohave’

The bright, straw-like flowers of this plant has a high impact in the garden deep into autumn, and you can pick the everlasting papery flowers.

Plants For The Winter Look

Curio mandraliscae (blue chalk sticks)

This plant is a sprawling, mat-forming succulent shrub with upward pointing pencil-like, silver-blue leaves.

Sedum ‘Angelina’ (stonecrop)

A low, evergreen succulent groundcover for sunny areas and poor, dry soil. It forms a trailing mat of golden-yellow leaves with lovely amber tones in winter. Summer brings many clusters of starry yellow flowers.

Helichrysum petiolare (silverbush everlasting)

The best in the land if you love grey foliage! A spreading, all-rounder shrub which produces masses of papery yellow and white flowers in midsummer.

Ruschia lineolata (carpet of stars)

Ruschia is a vygie-type plant that has fleshy, serrated blue-green leaves and produces masses of pretty striped pink to cerise and white flowers throughout the year. It knits together quickly to form a neat carpet.

Aloe arborescens ‘Yellow’

This aloe is a yellow form of the krantz aloe that blooms spectacularly all winter long with curved, blue-grey leaves arranged in rosettes.

Aloe ‘Twice as Nice’

A clump-forming aloe hybrid with striking red and white bi-coloured flowers.

Coprosma ‘Tequila Sunrise’

As tough as old takkies and intensify into rich burgundy hues in winter, this plant makes a statement.

Aloe ferox (bitter aloe)

An iconic South African plant and endemic to KwaZulu-Natal. It is single-stemmed, always forming a ‘petticoat’ of old dry leaves around its stem. It makes a huge candelabra of flowers in winter.

READ MORE: Learn more about the beautiful Aloe ferox here