Soil washing away on a steep bank is one of the most common landscaping headaches for NZ homeowners, especially after heavy rain. The good news is that with the right combination of plants, ground cover, and simple structures, you can stabilise even a challenging slope — often without needing a building consent.
Why Steep Banks Erode So Quickly
When rain hits bare or sparsely covered soil on a slope, there's nothing to slow the water down. It picks up soil particles and carries them downhill — a process called surface erosion. NZ's clay-heavy soils and intense rainfall events (particularly in the North Island and West Coast) make this worse. Compounding the problem, steep banks are often left bare after construction or landscaping work, giving erosion a head start.
Step 1 — Slow the Water Down with Mulch
The fastest, cheapest first step is to apply a thick layer of organic mulch (bark, wood chip, or straw) across the bank. Aim for at least 75–100 mm depth. Mulch:
- Absorbs the impact of raindrops before they dislodge soil
- Slows surface water runoff
- Retains moisture to help new plants establish
- Breaks down over time to improve soil structure
For very steep banks, biodegradable erosion control matting (also called geotextile or jute mesh) is worth the extra cost. You peg it into the soil and plant through it — it holds everything in place while plants establish, then breaks down naturally. These products are available at Mitre 10 and Bunnings.
Step 2 — Plant for Root Strength
Plants are the long-term solution. Their roots bind the soil together and their foliage intercepts rainfall. Choose plants suited to your region and the bank's aspect (sun, shade, wind exposure).
Good NZ options for bank stabilisation
| Plant | Type | Notes |
|---|---|---|
| Kikuyu or Buffalo grass | Grass | Fast-spreading, good for sunny banks |
| Flax (Phormium tenax) | Native | Deep roots, very tough, suits most regions |
| Coprosma species | Native shrub | Dense, low-maintenance, good coverage |
| Liriope | Ground cover | Handles dry shade well |
| Creeping juniper | Ground cover | Excellent on dry, sunny banks |
| Mondo grass | Ground cover | Tidy, low-growing, suits shaded banks |
| Manuka (Leptospermum) | Native shrub | Pioneer plant, great for erosion control |
Plant densely — closer spacing means faster coverage and less bare soil exposed to rain. Water plants in well and keep them moist until established.
Step 3 — Install Timber or Rock Check Dams
For moderate slopes, horizontal timber pegs or logs placed across the bank (called check dams or contour swales) slow water running down the face and give it time to soak in. Space them every metre or two vertically. Untreated hardwood or treated pine sleepers both work — just check that any treated timber is rated for ground contact.
Rock or river stone placed at the base of the bank or in drainage channels helps absorb the energy of water before it can carry soil away.
Step 4 — Consider a Retaining Wall
If the bank is very steep (steeper than roughly 1:1, or 45 degrees) or large, a retaining wall may be the most practical solution. Common options include:
- Timber sleeper walls — affordable and DIY-friendly for lower walls
- Concrete block or Allan Block systems — modular and relatively straightforward
- Gabion baskets — wire cages filled with rock, great for a natural look
- Concrete or masonry walls — more permanent, usually require a professional
Building consent and the NZ Building Code
This is important: under the NZ Building Code and the Building Act 2004, retaining walls over 1.5 metres in height generally require a building consent. Walls under 1.5 m may be exempt under Schedule 1, but this depends on factors like surcharge loading (e.g., if a driveway or structure sits above the wall) and proximity to boundaries. Always confirm with your local council before you build — rules vary. Check MBIE Building Performance for guidance on consent exemptions.
For walls that do require consent, the design may need to meet NZS 3604 or be engineer-designed. Your council can advise.
Drainage — Don't Overlook It
Poor drainage is often the hidden cause of bank failure. If water is pooling at the top of the bank or saturating the soil, no amount of planting will fully fix the problem. A subsoil drain or ag pipe installed at the base or top of the bank can redirect water before it causes damage. This is worth discussing with a landscaper or drainage specialist.
When to Call a Professional
Some situations are beyond a straightforward DIY fix:
- Large or high banks (over 1.5 m) where a retaining wall is needed — a qualified landscaper or civil contractor should assess the site
- Signs of deeper instability — cracking soil, slumping, or movement suggest a geotechnical issue that needs an engineer
- Drainage problems that aren't obvious — a landscaper or drainlayer can identify the source
- Consent-required retaining walls — an LBP (Licensed Building Practitioner) may be required to carry out or supervise the work
A qualified local landscaper can assess your bank, recommend the right combination of solutions, and carry out the work safely. You can find experienced landscapers in your area on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
