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Landscaping on a slope

New Zealand’s hilly landscape can present both challenges and opportunities for landscapers. In this article, expert Jon Muller discusses how to make the most of what mother nature offers

Landscaping in Wellington – as I do – is a challenge, due to its steep topography. However, no matter where you are in New Zealand, a hill is usually close by. While hills can be beautiful, working on them can be challenging. A hazard that has been made painfully clear after a summer beset by extreme weather events is the risk of banks or structures collapsing.

That said, living on a hillside also has its advantages, such as good views and more sun (depending on your aspect), which is why so many of us live on them despite the risks. Good views and good conditions have a big effect on outdoor living, as good light without too much wind is ideal. It also has a big influence on plant selection, as we discussed in Issue 18.

The questions you need to ask

When designing a landscape on a slope, you need to look at the existing features, such as walls, trees and paths. It’s important to check what condition structures are in and consider how well the drainage works before you begin a project. For example, check to see if water runs down a slope into a neighbour’s property or into a stormwater drain. You may need an engineer to check the condition of the structure – keep in mind that if it has been damaged by an earthquake, you may be entitled to help from The Earthquake Commission.

You may need to enlist the help of a drainlayer to check the stormwater. I recently used a drainlayer, who put a camera down a drain to show it was blocked, which was causing water to back up and damage pathways. Given the recent extreme weather, it’s always good to be on the safe side when it comes to water movement.

Tips to avoid slippage

What should you do about existing plant cover? You may have large trees on the slope and dense weed growth. You frequently see hillsides collapsing after heavy rain due to large trees (often pines) putting so much weight on the underlying soil that the hillside slumps. I have been told by a hillside repair specialist that gorse is particularly bad at causing soil fractures, due to its taproots.

So, it’s best to remove top-heavy trees and shrubs from slopes but leave the roots in place, as removing them could cause more destabilisation. If you cut back shrubs like gorse, leave the roots in the soil then either spray the regrowth with a weed killer, or paint weed killer on the stump as soon as you have cut it back.

Another problem arises when the toe or base of the slope is cut – such as with a road – because it removes the footing of the slope and can make it more prone to slippage.

Often bare hillsides are hydroseeded with grass to provide a green cover. To better stabilise it, you can also spray a polymer adhesive additive to the soil to ‘bind’ it. Simply mix the polymer 1:10 with water and it will create a hard surface for up to two years. I used it on my lime sand driveway, which is on a slope, and it goes rock hard – otherwise the lime sand ends up in the drains after rain.

On hillsides that are crumbling or supporting a load, its common to install rock anchors into the bank and then add mesh or sprayed concrete. If the slope is stable, you can use ponga logs placed at the angle of the bank, wired into reinforcing pins banged into the bank. You can also fix the ponga logs over the mesh as a softener and grow climbers up it.

How structures mix with slopes

Slopes have challenges with structures. If you must build walls over 1500mm high, you will need an engineer’s input. It’s better to have a series of walls and terraces than one huge wall if you want to create flat spaces. Keep in mind that ensuring adequate drainage behind any wall on a slope is crucial.

When putting in terraces becomes too difficult, decks provide a great way of creating a flat space over a bank. You can also build a screen on your deck if you want privacy, or clear screens if you still want the view.

Pathways are ideally constructed from exposed concrete for good grip. With that in mind, it’s a good idea to keep the H3 timber framing the paths and steps in situ once the concrete is poured, rather than box up, pour and strip the boxing. Handrails are needed, so choose between timber or metal.

Planting hillsides should allow for maintaining and increasing stability of the slope. On a bare hillside, use plants with fibrous root systems and low “clump” growth, such as:

Groundcovers such as:

Climbers to train up ponga logs include:

On this job, a low retainer wall was built to hold the 'toe' of the slope, and ponga logs fixed to the bank to provide a cover. At the end, the ponga logs were fixed to mesh attached to rock anchors, as that part of the bank was crumbling so needed cutting back and mesh added. Low renga lilies and ferns were planted in the planter and Clematis paniculata used to climb up the ponga logs.

Jon Muller has owned Wellington Gardens Ltd for 24 years. He practices landscape design and construction, with an emphasis on planting design.

He has taught at polytechnics and university, mostly in soft landscape subjects. He has written six gardening books, including Wellington Gardens, and enjoys helping clients find landscape solutions. He also helps students and workers studying for their landscape qualifications.


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