Wellington sections rarely come flat. From Brooklyn to Khandallah and out to the Eastbourne bays, houses perch above banks of weathered greywacke and clay that were cut when the roads went in. Those banks need something to hold them, and the usual answer is a mix of hard structure and planting. Getting that mix right is what separates a slope that slowly greens over from one that slumps onto the driveway after a wet July.
Plants and walls do different jobs
It helps to be clear about what vegetation can and cannot do. Roots bind the top half-metre or so of soil, slow surface runoff and reduce the scouring that starts small slips. Foliage intercepts rain before it hits bare ground. What plants cannot do is hold back a deep mass of saturated soil on a steep cut. That is the job of a properly designed wall with drainage behind it.
A practical way to think about it: walls handle the steepest and most loaded parts of a slope, such as the cut beside a driveway or below a house, while planting handles the gentler banks above and below, and softens the face of terraced walls. On many sites the best outcome is a series of lower walls with planted terraces between them, rather than one tall wall with a bare bank above.
Choosing species for wind and thin soil
Wellington’s slopes face two big stresses: relentless wind and shallow, quick-draining soil on the ridges contrasted with soggy clay in gullies. Plants that cope well include:
- Harakeke and wharariki for their dense, fibrous roots and tolerance of wind and poor soil.
- Coprosma repens and other coprosmas, which form tight, salt-tolerant cover on exposed faces.
- Karo and kōhūhū for taller shelter, kept well back from wall crests.
- Muehlenbeckia and native grasses such as Poa cita, which knit the surface quickly on steep banks.
- Hebes and kawakawa in more sheltered pockets with a little shade.
Avoid large, heavy-crowned trees directly above a wall. In a southerly gale, a tall tree acts as a lever on the soil it is growing in, and that force ends up on the structure below.
Planting technique on steep ground
Planting on a slope is harder than it looks. A few habits make a real difference:
- Plant small. Grade-sized natives in small pots establish faster on harsh banks than large specimens, and cost far less.
- Cut a small level shelf for each plant, with a slight lip on the downhill side to catch water.
- Use jute or coir matting on bare faces to hold soil until roots take over, cutting crosses to plant through it.
- Plant in autumn or early winter, so roots develop before the drying winds of late spring.
- Space plants closely, around 600 to 900 millimetres for groundcovers, so the canopy closes within two seasons and weeds struggle.
Budget roughly five to fifteen NZD per plant for natives from a local nursery, plus matting and mulch. A bank of 50 square metres might take 80 to 120 plants.
When the wall needs attention first
No amount of planting will fix a wall that is leaning, a bank that is actively slipping, or water that pours out of the slope after rain. Those are signs the structure or drainage needs work before any garden goes in. For substantial cuts or failing older walls, getting advice from experienced builders of retaining walls in Wellington early means the planting plan can be designed around the wall, with space left for drainage outlets and maintenance access. Walls above certain heights or supporting a surcharge usually need building consent, so check with Wellington City Council or the relevant district council.
Frequently asked questions
Can I plant on the face of a timber retaining wall? Trailing groundcovers planted along the top can soften the face, but avoid anything with woody roots growing behind the boards, as it can push them outward over time.
How long before planting stabilises a bank? Groundcovers and flaxes usually knit together within two to three years. Until then, matting and mulch do much of the work of holding surface soil.
Should I water new plants on a slope? Yes, through their first summer at least. Water slowly or use a dripper, because fast watering on a slope simply runs off and can wash out soil around the roots.