Trees planted too close to a septic tank or its drainage field can cause serious — and expensive — damage. Tree roots naturally seek out moisture and nutrients, making septic systems an irresistible target. Understanding safe distances, recognising the warning signs early, and knowing when to act can save you thousands in repairs.
Why Tree Roots Are Such a Problem for Septic Systems
Septic tanks, soakage trenches, and drainage fields are buried underground and constantly moist — exactly the conditions tree roots love. Even small hairline cracks in concrete tanks or PVC pipes are enough for fine feeder roots to enter. Over time, roots can:
- Block or crush pipes, causing slow drains or sewage backups inside the house
- Crack or lift concrete tank lids and walls, leading to leaks and ground contamination
- Clog soakage trenches, reducing the system's ability to treat and disperse effluent
- Undermine the structural integrity of the entire system
This isn't just a plumbing headache — a failing septic system can contaminate groundwater and nearby waterways, which is a serious issue under the NZ Building Code and your regional council's resource consent conditions.
What Are the Recommended Safe Distances?
There is no single universal rule in New Zealand, but the following distances are widely used as practical guidelines by arborists, drainlayers, and councils. Always check with your local council and refer to your system's design documentation, as requirements can vary.
| Tree/Plant Type | Minimum Distance from Tank or Drain Field |
|---|---|
| Large trees (e.g. macrocarpa, oak, willow, poplar) | 15–30 m or more |
| Medium trees (e.g. fruit trees, silver birch) | 5–10 m |
| Small ornamental trees (e.g. dwarf varieties) | 3–5 m |
| Shrubs and hedges | 2–3 m |
| Lawn grass and groundcovers | Generally safe |
Willow, poplar, and macrocarpa are particularly aggressive and should be kept well away — their roots can travel extraordinary distances in search of water. If you're unsure about a specific species, a qualified arborist can advise on root behaviour and growth habits.
Warning Signs Your Tree May Already Be Causing Damage
Watch out for these red flags:
- Slow-draining sinks, toilets, or showers throughout the house (not just one fixture)
- Gurgling sounds from drains or toilets
- Sewage odours inside or outside the home
- Wet, soggy, or unusually lush patches of grass above the drain field — this can indicate effluent surfacing
- Visible cracks in the tank lid or inspection covers
- Tree roots visible near inspection points or around the tank area
- Frequent pump-outs needed more often than your system's design requires
If you notice any of these signs, don't delay — contact a licensed drainlayer and an arborist promptly. Drainlaying work on septic systems is restricted work under the Plumbers, Gasfitters and Drainlayers Board (PGDB) and must be carried out by a licensed professional.
How to Find Out Where Your Septic System Is
Before you can assess whether a tree is too close, you need to know exactly where your tank and drainage field are located. Here's how to find out:
- Check your property file at your local council — most councils hold as-built drainage plans for septic systems
- Look for inspection covers (usually round concrete or plastic lids) in the yard
- Review your LIM report (Land Information Memorandum), which may reference the system
- Ask a licensed drainlayer to locate the system using a drain camera or locating equipment if you can't find records
What Can You Do About an Existing Tree?
If a tree is already too close, your options depend on how established it is and whether damage has already occurred:
Root Barrier Installation
A physical root barrier (a deep, impermeable membrane) can be installed between the tree and the septic system to redirect root growth. This is most effective when done early, before roots reach the system.
Tree Removal
In many cases, removing the tree is the safest long-term solution. A qualified arborist can assess the tree, advise on removal, and handle the work safely — particularly important for large trees near underground infrastructure. You can find a local arborist on tradedirectory.co.nz.
Pipe Relining or Replacement
If roots have already entered the pipes, a licensed drainlayer can use a drain camera to assess the damage and recommend relining, jetting, or pipe replacement as needed.
Choosing the Right Trees Near a Septic System
If you want to plant near your septic system, choose species with non-aggressive, shallow root systems. Good options generally include:
- Native grasses and sedges
- Low-growing groundcovers
- Shallow-rooted annuals and perennials
Always check with a local arborist or your council before planting anything near a septic system or drain field.
When to Call a Professional
Call a licensed arborist if you need a tree assessed, pruned, or removed near your septic system — especially for large or established trees where safety and underground infrastructure are concerns.
Call a licensed drainlayer (registered with the PGDB) if you suspect root intrusion, notice drainage problems, or need your system inspected, repaired, or modified. All work on septic tanks and drainage systems is restricted work in New Zealand and cannot legally be done by an unlicensed person.
For healthy homes compliance (relevant to rental properties), refer to Tenancy Services for guidance on relevant standards.
ISO 9001 / 45001 / 14001
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