Landscaping on slopes

Summer 2026 saw several notable landslips following extreme rain events. In this article, horticulture expert Jon Muller discusses how planting can mitigate the effects.

I have just returned from an ocean swim at Mount Maunganui, where I spotted huge slips on the mountain that were especially noticeable from sea. While there, Wellington experienced its own intense flooding and resulting slips.

The Mount is still closed but, hopefully, the parties involved can sort out the issues soon and reopen this epic Maunga. At the time of the slip, there was talk that the cause was too many exotic trees being removed and native trees being planted in their place, which caused instability on the slopes.

While it is always good to plant native plants endemic to the area, it’s important not to remove too much vegetation. Ideally, locally sourced pohutukawa planted on slopes would cling to the soil with their very strong roots.

A ROOT AND BRANCH APPROACH

What should you do about existing plant cover? You may have large trees on the slope and dense weed growth. You often see hillsides collapsing after heavy rain due to large trees (often pines) creating so much weight on the underlying soil that the hillside slumps.

It’s best to remove top-heavy trees and shrubs but leave the roots in the soil, because removing roots can cause further destabilisation. If you cut back shrubs like gorse, leave the roots in the soil and either spray the regrowth with a weedkiller or paint a stump weedkiller on the stump as soon as you have cut it back.

The other issue is when the toe or base of the slope is cut, for example by a road. This removes the footing of the slope and can make it more prone to slippage.

CAREFUL ASSESSMENT IS KEY

The downside of living and working on or near steep hillsides – which includes much of New Zealand – includes both the difficulty of working on them and the downside of banks or structures collapsing. This has been borne out recently with the massive storms we have been having nationwide.

When designing a landscape on a slope, you need to look at the existing features such as walls, trees, or paths. Check what condition the structures are in and:

• Review the drainage.

• Look at whether water runs down the slope onto a neighbour’s property or into a stormwater drain

.• Decide if you need an engineer to check a structure’s condition. If structures have been damaged by earthquakes, you may get help from EQC – there is no harm in trying

.• Figure out if you need a drainlayer to check the stormwater. If I am unsure, I will always get a professional assessment of the drainage systems, as often drains are blocked or can’t take the water loading in heavy rains.

It’s always good to be on the safe side when it comes to water movement given the increasing frequency of extreme weather events.

HYDROSEEDING CAN BE AN OPTION

Often bare hillsides are hydroseeded with grass to provide a green cover. You can also spray a polymer adhesive additive to the soil to bind it. 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, it’s 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.

STRUCTURAL CONSIDERATIONS

If your wall supports a surcharge (which is likely on a slope), or you need to build walls over 1.5m 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. Drainage behind the wall is crucial as always.

Where creating terraces becomes too difficult, decks provide a great way of creating a flat space over a bank. You can also add a screen to your client’s deck if they want privacy, or clear screens if they want the view but not the wind.

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.

Clump plants with fibrous roots you could use:

Arthropodium cirrhatum, renga lilyAustroderia toetoe, toetoe

Groundcovers you could use:

Coprosma kirkii, renga lilyMuehlenbeckia complexa, pohuehue

Climbers to train up ponga logs include:

Clematis paniculata, puawananga

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Jon Muller has owned Wellington Gardens Ltd for 26 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.