The first willow root I ever pulled from a sewer lateral came from a section on River Road in Avonside, autumn of 2014. The house had been red-zoned and the wastewater had backed up into the shower one last time. We opened the 100 mm earthenware pipe at the boundary and drew out a root mat nearly four metres long, the colour of wet tobacco, grown in through a joint the earthquakes had opened by 8 mm. The tree belonged to a crack willow on the riverbank forty metres away. That job taught me most of what I know about why the council and Environment Canterbury keep felling willows along the Avon and the Heathcote, and why the neighbours who protest are not entirely wrong.
What the willows are, and why they were planted
Most of the big riverside willows in Christchurch are crack willow (Salix fragilis), with a scatter of weeping and grey willow in the wetter reaches. They were planted from the 1860s to hold banks stripped of raupō and flax, and for a century they did that job well. A mature crack willow carries a trunk of 800 mm to well over a metre, a crown 15 to 20 metres wide, and a root plate spreading into the bank and down into the saturated silt.
Crack willow branches snap at the collar under their own weight, and every piece that lands in water strikes root. Along the Heathcote through Woolston and Opawa the willows are effectively one clone, all the same age and all approaching the same state of decay.
The three reasons they come out
- Flood conveyance. A fallen willow in the channel is a dam. Root wads catch debris, water backs up, and the opposite bank scours. After the 2011 earthquakes the Avon bed dropped and the channel capacity shrank, leaving no room for obstructions.
- Bank failure. A willow with a 20-metre crown on liquefied silt is a lever. When it goes over in a nor’wester it takes a wedge of bank three or four metres deep with it. The bank did not fail because the tree was removed; it failed because the tree was left.
- Pest status. The regional pest management plan treats willow along waterways as a plant to be controlled rather than protected, and the rules on what may be planted or removed beside a river sit with Environment Canterbury rather than the city council. Crack willow spreads by fragments, so a mature tree upstream is a nursery for every reach below it.
What the roots do that nobody sees
Willow roots are the most aggressive I deal with. They follow moisture, and a cracked or displaced sewer lateral leaks warm, nutrient-rich water into the soil every time a toilet is flushed. The root finds the joint, enters as a hair, and swells. In old earthenware pipe with a joint every 600 mm, there is an entry point every 600 mm. In the eastern suburbs the earthquakes helped: pipes sound for sixty years were left with open joints and sagging bellies. A willow within 30 to 40 metres of a broken lateral will find it within two or three seasons.
A CCTV inspection to confirm root ingress runs about NZ$250 to NZ$450. Cutting roots and hydro-jetting the line buys six to eighteen months for NZ$350 to NZ$700. Replacing a 12-metre lateral in PVC, with trench, connection and council inspection, comes in between NZ$5,000 and NZ$11,000 depending on depth. The root problem and the pipe problem are the same problem; fixing one without the other is throwing money in the trench.
What the felling does to your section
If a council contractor takes out a large willow behind your fence, three things change on your side.
- The water table comes up. A big willow transpires several hundred litres a day in summer; remove it and shallow footings can move as the silt rehydrates.
- The roots die and rot, leaving voids under driveways and paths. Paving lifted by a live root can settle unevenly when the root goes. If a slab has been pushed up, it is worth reading what happens when roots under a Christchurch driveway are dealt with before the surface is repaired.
- The wind changes. A house behind a 15-metre screen for forty years will feel the nor’wester differently, and trees sheltered by the willow will now be tested.
None of these are reasons to keep a decaying crack willow. They are reasons to have the lateral inspected before the tree comes out, because a dying root system stops holding a broken pipe together at about the same time it stops drinking the leaks.
Frequently asked questions
Can I stop the council removing a willow behind my property? Usually not if the tree is on council or Environment Canterbury land and has been assessed as a flood or bank-stability risk. You can ask for the assessment and for a replanting plan; most river corridor removals are followed by native replanting within a season or two.
How far do willow roots travel? Along a leaking pipe or a wet gravel trench, easily 30 to 40 metres, further than the tree is tall. In dry soil without a water source they stay much closer, typically within one to one and a half times the crown radius.
Will removing a nearby willow fix roots in my sewer? Not on its own. Roots inside the pipe die but stay put, and the joint or crack they entered through is still open. Have the line inspected by camera and repair or reline the damaged section, otherwise the next tree along will find the same way in.