Weeds pushing through retaining wall gaps are a frustrating but very common problem for NZ homeowners, especially in our warm, humid climate where plants seem to grow everywhere. The good news is that with the right approach — removing existing weeds properly and blocking their return — you can keep your retaining wall looking tidy long-term. Here's how to do it.
Why Weeds Love Retaining Wall Gaps
Retaining walls — whether timber, concrete block, natural stone, or timber sleepers — almost always have gaps, joints, or cavities. These spaces trap soil, moisture, and wind-blown seeds, creating a perfect little nursery for weeds. In New Zealand's mild climate, even a tiny crack can host kikuyu grass, oxalis, convolvulus, or other persistent species year-round.
Understanding this helps you target the solution: you need to remove existing weeds at the root, then block the conditions that let new ones establish.
Step 1 – Remove Existing Weeds Properly
Half-hearted removal is the main reason weeds come straight back. Follow these steps:
- Pull by hand or use a weeding tool. A narrow hand fork, old screwdriver, or purpose-made crack weeder lets you get into tight joints. Always wear gloves.
- Get the root out. Species like oxalis and convolvulus regrow from tiny root fragments left behind. Take your time.
- Treat stubborn weeds with a targeted herbicide. A ready-to-use glyphosate-based spray (available at Bunnings or Mitre 10) applied directly to foliage on a dry, still day is effective. Follow the label carefully and keep spray away from desirable plants, waterways, and stormwater drains — this matters under NZ environmental rules.
- Wait for the weed to die fully (usually 1–2 weeks) before removing dead material and moving to the next step.
Safety note: Always read herbicide labels and use appropriate PPE (gloves, eye protection). Check WorkSafe NZ guidance if you're unsure about safe chemical handling.
Step 2 – Fill the Gaps
Once weeds are cleared, filling or sealing gaps removes the foothold new seeds need.
Polymeric Sand
Polymeric jointing sand is one of the best options for block or stone retaining walls. You brush it into the joints, then activate it with water — it sets firm but remains slightly flexible, resisting weed germination and erosion. It's widely available at Bunnings and Mitre 10.
Mortar or Concrete
For a more permanent fix on concrete block or brick walls, you can repoint joints with mortar. This is a straightforward DIY job for small areas: mix a bagged mortar product (available at PlaceMakers), dampen the joint, and press mortar in with a pointing trowel. Allow to cure fully before wetting.
Flexible Sealants
For walls that experience movement (common on NZ hillside sites with expansive soils), a rigid mortar can crack. A polyurethane or flexible masonry sealant from a brand like Sika may be a better choice — check the product data sheet for suitability.
Leave Some Gaps Intentionally
Note: some retaining walls need gaps for drainage (weep holes). Blocking these can cause hydrostatic pressure to build up behind the wall, which is a structural risk. Never fill weep holes — only fill non-drainage joints.
Step 3 – Apply a Weed Barrier Behind the Wall (If Rebuilding or Backfilling)
If your wall is being rebuilt or you're doing significant landscaping behind it, installing a geotextile weed mat between the soil and the wall face dramatically reduces weed pressure from behind. This is standard good practice in NZ retaining wall construction.
Step 4 – Ongoing Prevention
Even a well-sealed wall needs maintenance. These habits keep weeds at bay:
- Inspect twice a year (spring and autumn) and pull any seedlings before they establish.
- Apply a residual pre-emergent herbicide to wall surfaces and surrounding soil in early spring to stop seeds germinating. Ask at your local garden centre for a product suitable for hard surfaces.
- Keep the area around the wall tidy — mulching garden beds near the wall reduces the seed bank in surrounding soil.
- Trim overhanging plants that drop seeds into joints.
Retaining Wall Condition — What to Watch For
While you're dealing with weeds, take the opportunity to check your wall's overall condition. In New Zealand, retaining walls over 1.5 metres high generally require a building consent, and walls on steep or unstable ground may need engineering input regardless of height — always confirm requirements with your local council. If you notice leaning, cracking, bulging, or signs of movement, stop DIY work and get a professional assessment. A failing retaining wall is a serious safety hazard.
For guidance on building consent requirements, visit MBIE Building Performance.
When to Call a Professional
Call a qualified retaining wall builder or landscaper if:
- Your wall is showing structural movement, cracking, or leaning
- Weeds are growing from behind the wall through failed drainage areas
- The wall is over 1.5 m high and you're unsure of its consent status
- You need to rebuild or significantly repair sections
- You're on a steep site with unstable soils
A skilled local tradesperson can assess whether your wall is structurally sound and recommend the best long-term fix. You can find qualified retaining wall builders in your area on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
