Tree roots are a leading cause of blocked household sewers. Urban Utilities, the water utility for Brisbane, Ipswich, the Lockyer Valley, Scenic Rim and Somerset, calls tree roots the number one cause of blockages in sewer pipes. Much of the risk comes down to what is planted where. Choosing a plant to suit the distance lowers the risk, though no utility guarantees any plant, and none certifies a plant as having non-invasive roots. If a drain is already blocked, a blocked drain specialist in Melbourne such as Water Serpent Plumbing can clear the roots and tell you which pipes are at risk before you plant.
This guide gathers what Australian water utilities publish on the subject, with the plants they name, and sets it out by distance from the pipe. It is a summary of their advice and not ours, and the sources are linked at the end.
Quick facts
| General rule | Plant at least three metres from the pipe (Urban Utilities). SA Water sets 2 metres or 3.5 metres from a sewer pipe, depending on the plant |
|---|---|
| Directly over a pipe, or up to 1.5 metres away | The six plants Unitywater names, listed below. Not every grass qualifies: Hunter Water rates pampas grass and bamboo as high risk |
| 1.5 to 3 metres | The seven shrubs Unitywater names, listed below |
| More than 3 metres | Blueberry ash, plum myrtle and peanut tree (Unitywater) |
| Before you dig | Find your pipes first, through Before You Dig Australia and your council or utility |
Why roots go for pipes
Roots follow water and nutrients, and a sewer or water pipe offers both. Urban Utilities notes that some roots are as thin as a strand of hair and can work through the tiniest join between two pipes. Once inside, they can trap wet wipes and cooking fats, which can block the pipe. Clearing the roots is often not the end of it, because roots often return to the same pipe they were cleared from. Urban Utilities' view is that prevention is better than cure, since a root blockage can mean an expensive plumbing bill.
Three details are worth knowing. Older pipes are the higher risk: Water Corporation says suburbs more than 20 years old are particularly at risk because they are less likely to have PVC wastewater pipes, and SA Water says houses built since the late 1990s generally have PVC pipes, which are less likely to be invaded. New trenches still matter: Urban Utilities says to avoid planting near freshly laid pipes, because roots are attracted to the softer soil. And the plant that matters is the fully grown one, not the one in the pot, so plan around mature size.
Water mains are at risk as well as sewers. Unitywater's distances apply to water supply pipes too, and SA Water explains that large structural roots can press on water mains and crack them.
What to plant, by distance from the pipe
Unitywater, which serves the Sunshine Coast, Moreton Bay and Noosa, publishes the most practical list we have found, sorted into three distance bands. Urban Utilities explains the thinking: garden plants and grasses have fibrous roots that grow in the upper soil layers and are less likely to cause sewer problems.
Directly above, or up to 1.5 metres away
- Blue flax lily (Dianella caerulea)
- Spiny-headed mat-rush (Lomandra longifolia). Our lomandra care guide covers planting and upkeep.
- Tall sedge (Carex appressa)
- Common tussock grass (Poa labillardieri)
- Kangaroo grass (Themeda triandra)
- Common hovea (Hovea acutifolia), a small shrub
Between 1.5 and 3 metres away
- Dwarf banksia (Banksia oblongifolia)
- Woombye bush (Phebalium woombye)
- Broad-leaved palm lily (Cordyline petiolaris)
- Hairy bush pea (Pultenaea villosa)
- Austral indigo (Indigofera australis), dogwood (Jacksonia scoparia) and sweet wattle (Acacia suaveolens)
More than 3 metres away
- Blueberry ash (Elaeocarpus reticulatus). See our blueberry ash guide.
- Plum myrtle (Pilidiostigma glabrum) and peanut tree (Sterculia quadrifida)
Urban Utilities names three trees to choose: lemon myrtle, tulipwood and native gardenia. It gives no distance for them, so its general rule of at least three metres applies. Its general advice is to favour slow growing trees and shrubs with non-aggressive root systems, and to avoid plants with tap roots, which grow straight down.
The plants linked above are the ones we stock from those lists. If you are choosing anything else, from our native shrubs for example, check its mature size and root habit against the distance bands first.
Other states
The lists above were written for south east Queensland. The principle travels, but the species and the rules differ.
- South Australia. SA Water's tree planting guide sorts plants into schedules with set clearances: 2 metres from any sewer pipe or connection for the plants in its first schedule, and 3.5 metres for those in its second, including in home gardens. It also says no tree or shrub may be planted within 1 metre of a water main or connection without SA Water's written approval.
- Western Australia. Water Corporation publishes a waterwise tree list for councils planting verges and public open space. It rates each tree's root invasiveness as low, moderate or high, and notes that waterwise trees tend to have low-invasive roots. It is not written for home gardens: Water Corporation points residents to its Waterwise Plant Directory and to their local council.
- New South Wales. Hunter Water publishes the list of plants to avoid that follows.
What to keep well away from pipes
Hunter Water in New South Wales rates plants by the damage they do to sewers. Its ratings are the clearest we have seen.
- Extreme: camphor laurel, fig trees and rubber plants, poplars, willows.
- Very high: coral trees, large gum trees, jacaranda, liquidambar, Norfolk Island and Bunya pines, Illawarra flame tree, casuarinas, white cedar, pine trees, plane trees, pepper tree, elms.
- High: bougainvillea, pampas grass, silky oak, hollies, Norfolk Island hibiscus, privets, magnolias, oleander, Canary Island date palm, bamboos (Hunter Water gives the botanical name as Phyllostachys species), rhus trees, brush box, wisteria.
Unitywater adds golden cane palm, large lilly pilly species, umbrella tree, poinciana, paperbarks, mango, black bean and West African tulip tree, and notes that some of these are also weeds. Urban Utilities adds bottlebrushes.
The utilities do not all agree, which shows that root risk is a matter of degree and of place. Hunter Water rates jacaranda very high, while SA Water allows it at 3.5 metres from a sewer pipe.
We sell several of these, including lilly pillies, magnolias, bottlebrushes and jacarandas, and we say so gladly. They are good plants in the right place, and the right place is well away from the pipes. Urban Utilities' general rule is at least three metres. SA Water sets 3.5 metres from any sewer pipe for jacaranda and for the lilly pilly Acmena smithii, and 2 metres for the bottlebrushes on its list. Unitywater and Hunter Water simply list them as plants to avoid near pipes. Where the pipe runs close, choose from the lists above.
Before you plant
- Find the pipes. Lodge an enquiry with Before You Dig Australia. In south east Queensland, ask your council for a drainage plan of your block; in South Australia those plans are held by the Office of the Technical Regulator. Some utilities publish maps: SA Water has its Healthy Pipes tool and Unitywater an interactive map. These show the utility's own mains, not the private pipes on your block, and SA Water says its locations are indicative only.
- Measure to the mature plant. Work from the width and height the plant will reach, not the size it arrives at.
- Avoid planting near freshly laid pipes (Urban Utilities).
- Consider a root barrier. Urban Utilities recommends installing one: heavy cloth or plastic set in the ground to contain a plant's roots, sold at hardware stores and nurseries. SA Water says a barrier can allow a tree closer to its pipes than the usual clearance, but only where SA Water has assessed and approved it first.
- Ask before you commit. Urban Utilities, Unitywater and Hunter Water all say to ask your local nursery. Hunter Water adds an arborist, and SA Water takes enquiries about tree species.
The same thinking applies above ground. A tree whose canopy will reach the house drops leaves into the gutters, holds moisture against the tiles and rubs the roof in wind; Roof Restorers Perth explains how overhanging trees damage a roof, and it is worth reading before you choose the spot as well as the species.
If a pipe does block
The pipes on your own land are generally yours to look after. SA Water says a blockage in the pipes on a property, before they join its main, is the owner's responsibility to clear and repair, using a registered plumber. Hunter Water says the owner engages a licensed plumber to find out whether the blockage is in the private drain or in Hunter Water's network, and that it arranges the work where the blockage is in its network. Check with your own utility, since the arrangements differ.
If tree roots have already entered the drainage system and caused a blockage, have it assessed professionally rather than guessing at the cause. Sydney homeowners can call a blocked drains plumber in Sydney to find where the roots are getting in and recommend the repair.
A word of caution
Unitywater says of its own list that it is a guide only, is not all inclusive, and carries no guarantee. The same caution belongs on every list here, this page included. Roots respond to soil, water and the condition of the pipe. An old or cracked pipe is far more likely to be invaded, and SA Water notes that large roots can crack even a previously sound pipe. Use these lists to narrow the choice, then check locally.
Sources
- Urban Utilities: Plant smart to protect your pipes
- Unitywater: Think before you plant
- Hunter Water: Planting near sewer pipes
- SA Water: Planting the right tree in the right place, with its Tree Planting Guide
- Water Corporation: Selecting the right tree
- Shrek Septic Services (Toowoomba and the Darling Downs): Expert Guide To Planting Around Your Septic Tank, a septic contractor's view of the same question
If you are ordering these online, read what to do when your plants arrive first. The first two days after delivery matter more than the first two months.
On a septic block the planting rules change: our guide to what to plant over a septic drain field gathers the state guidelines on cover plants, tree distances and the signs of a failing trench.
