A new retaining wall transforms a steeply sloped, difficult-to-use section of garden into usable, level terraced space. But the wall itself is only part of the picture, how you landscape behind and below it determines whether the result is a beautiful, functional garden space or a weed-filled patch that constantly challenges the wall’s drainage system.
In the Blue Mountains, landscaping after a retaining wall has specific considerations: the climate (extreme rainfall, frost at altitude), the soil conditions (often acidic native soil or imported fill), the proximity to natural bush, and the drainage performance requirements that protect the wall’s long-term integrity.
Quick answer (BLUF)
The most important landscaping priorities after retaining wall installation are: confirming drainage outlets are clear and functioning, allowing fill to settle for 4-8 weeks before permanent planting, importing a quality growing medium to replace native sandstone soil (which is low-fertility), selecting plants with appropriate root characteristics, and maintaining the drainage system as part of ongoing garden care. Avoid deep-rooted plants within 1-2 metres of the wall face.
Step 1: Confirm drainage is working before you plant
Before importing topsoil or planting anything, test that the drainage system behind the wall is functioning. This means:
- Running a hose into the drainage zone behind the wall for 10-15 minutes
- Confirming that water appears at the ag pipe outlet within a reasonable time (generally within 15-30 minutes for a well-drained system)
- Checking that the outlet is clear and discharging freely
If the drainage is not performing as expected, address it now, before you’ve spent money on topsoil, plants and mulch. Sorting out a drainage problem after a fully planted terrace requires removing the planting, excavating and reinstating.
Step 2: Allow fill to settle
Newly placed fill material continues to settle under its own weight for 4-8 weeks after placement. Planting into settling fill can result in uneven settlement around plant rootballs, subsidence of mulch layers, and in worst cases, roots in bridging cavities that form as fill settles around them.
For significant fills, more than 300mm deep, allow the full settlement period before planting. During this time, watering the fill (without waterlogging it) accelerates settlement.
Step 3: Soil preparation
Behind a new retaining wall, the growing medium is often one of:
- Excavated spoil placed as fill, typically low fertility, possibly clay-heavy
- Imported topsoil, quality varies widely; get a specification from your supplier
- Native sandstone residual soil, naturally low fertility, acidic (pH 5.0-5.5), excellent drainage
For productive planting, most of these starting conditions need improvement:
- Native sandstone soil: Generally a good base for native plants. Amend with compost (not fertiliser, natives prefer low phosphorus) for ornamentals and vegetables. Check pH, most Mountain natives prefer 5.5-6.5.
- Clay fill: Amend heavily with gypsum and compost to improve structure. Add a quality growing medium (200-300mm) on top for planting.
- Imported topsoil: Use certified weed-free topsoil from a reputable supplier. Cheap topsoil brings weed seeds that will plague the garden for years.
Plant selection for retaining wall terraces in the Blue Mountains
The critical constraint: root behaviour near the wall
Plants with aggressive, spreading or deep root systems can invade the drainage layer behind the wall, block ag pipe, and exert pressure on the wall face. Avoid planting these within 2-3 metres of the wall:
- Trees (any species), roots spread far beyond the canopy
- Bamboo, rhizomes penetrate drainage systems readily
- Ivy, jasmine and other vigorous vines, dense surface roots
- Fruiting trees (figs especially), highly aggressive root systems
Suitable species for terraced Blue Mountains gardens
Ground covers (excellent for slope stabilisation within the terrace):
- Grevillea ‘Groundcover’ varieties, low phosphorus needs, frost-hardy forms available
- Lomandra species, very drought-tolerant once established, excellent drainage
- Myoporum parvifolium, native ground cover, frost-tolerant
- Brachyscome multifida, excellent for lower Mountain elevations
Shrubs (moderate root impact):
- Westringia fruticosa, frost-tolerant, drought-tolerant, no drainage system impact
- Lavender species, perfect drainage partner for well-drained retaining wall terraces
- Hydrangeas, classic Blue Mountains garden plant, manageable roots, thrives in cool climate
Frost-tolerant choices for Blackheath, Katoomba, Mount Victoria:
- Hellebores, exceptional in cold climate gardens
- Deciduous Rhododendrons, the Blue Mountains has outstanding conditions for these
- Prunus species (ornamental cherries), keep at distance from wall
Mulching: do it, but do it right
Mulch is one of the best investments for a new retaining wall terrace, it suppresses weeds, retains moisture, moderates soil temperature and feeds soil biology as it breaks down. Apply 75-100mm of organic mulch (wood chip, lucerne, sugar cane) after planting.
Keep mulch clear of the wall face: Mulch piled against the wall traps moisture against the wall materials and accelerates degradation. Keep at least 100mm of clear space between the mulch layer and the wall face.
Do not block the drainage outlet with mulch: Ensure the ag pipe outlet is accessible and unobstructed. Mark its location so you can find it easily during ongoing garden maintenance.
Keep stair drainage clear too: Where the terrace includes stairs through or alongside a wall, mulch and plant growth can block the cross-fall drainage on each tread just as easily as it can block an ag pipe outlet. See our guide on stairs through retaining walls for how stair drainage is designed to work alongside the wall’s main drainage system.
Keep mulch off seating surfaces too: Where a low terrace wall doubles as bench seating around a fire pit or entertaining area, keep mulch and low planting clear of the coping itself, not just the drainage outlet. See our guide to seating walls and fire-pit terraces for coping and clearance detail specific to that application.
Keep planting clear of lighting fixtures and conduit runs too: If your wall has step lights, up-lights or coping fixtures wired in during construction, dense planting or mulch can bury them within a season or two of growth. See our guide to lighting retaining walls and wiring during construction for where conduit and fixtures typically sit relative to the drainage layer, so you know what to keep clear as the garden fills in.
Ongoing drainage maintenance
The drainage system behind your retaining wall is part of your garden infrastructure and requires annual attention:
- Check the ag pipe outlet after each major rainfall event (especially the first heavy rain of the season)
- Clear leaf litter from around the outlet area
- Remove plant roots that have colonised the outlet before they block it
- Avoid compacting the soil within 300mm of the back of the wall (compacted soil reduces drainage layer performance)
If you notice the outlet running less vigorously than it did when first installed, investigate, early intervention is far cheaper than remediation after wall damage has occurred.
FAQs
Can I plant a vegetable garden directly in the terrace soil behind a new retaining wall?
Yes, but add compost and ensure drainage is excellent. Retaining wall terraces in the Blue Mountains can be wonderfully productive vegetable gardens, the level, well-drained terrace surface, natural frost pockets (useful for some crops), and reliable cool-season rainfall make for good growing conditions. For a closer look at sizing walls and beds specifically for food growing, see our Terraced Vegetable Gardens on Mountain Slopes guide.
My new terrace has mostly fill. What’s the quickest way to establish it?
Import 200-300mm of quality growing medium, spread over compacted fill, and plant densely with a mix of ground covers and shrubs. Dense planting suppresses weeds, stabilises the surface and establishes the root network that ultimately holds the terrace soil together. Temporary erosion control matting is also useful during the establishment period.
Will native plants in the terrace damage my retaining wall?
Most natives with modest root systems (lomandra, grevillea groundcovers, westringia) are low-risk within 1.5-2.0 metres of a wall. Larger native species, banksias, tea trees, wattles, should be kept at greater distance. The risk increases substantially if the drainage layer dries out and roots seek moisture within it.
Does the same planting approach work around a driveway entry wall?
Largely yes, though keep in mind an entry wall’s garden bed often carries stormwater from the driveway surface as well as the retained slope, so drainage capacity and outlet placement need extra attention. See our guide to driveway gateposts and wing walls at entry for how entry-specific drainage is designed before the wing walls and planting go in.
Does planting design change for a curved wall?
Yes, slightly. Dense planting along the outer face of a convex curve tends to read naturally, while the inside of a concave curve often looks best kept relatively open so the curve itself stays visible as a design feature. See our guide on curved retaining walls versus straight runs for the cost and engineering differences that go with choosing a curved layout in the first place.
How soon after construction can I start planting?
For fill areas: wait 4-8 weeks for settlement. For retained natural ground areas: you can plant almost immediately, but keep root balls out of the drainage zone immediately behind the wall until the drainage system is established.