In February 2019 I was called to a stormwater line that had stopped flowing behind a 1.6-metre timber retaining wall in Rototuna, on the northern edge of Hamilton where the housing runs into the old peat paddocks. The wall held up the neighbour’s lawn. Behind it, the drainage coil that was supposed to keep water off the timber had not passed water for a season. When we dug down at the outlet end we found the coil packed solid with fine, rust-brown roots, and the roots led back under the wall footing, across the boundary and into a row of Lombardy poplars that had been the farm shelterbelt before the farm became sections.
Why peat and trees make an awkward pair
Waikato peat is not soil in the sense a Canterbury drainlayer means it. It is compressed plant matter, often several metres deep, holding many times its own weight in water. Drain it and it shrinks; the surface of drained Waikato peatland has dropped by a metre or more over the last century. Dry it in summer and it cracks, then swells again in winter.
A line of large trees on peat is a pump. A mature Lombardy poplar can move 300 to 500 litres of water a day in a Waikato summer, and the peat within 10 or 15 metres of the trunks shrinks more than the peat further away. Any structure sitting across that gradient, such as a wall running along the tree line, ends up with its ends settling at different rates.
Poplar, willow and alder send long lateral roots along the top half-metre, and those roots go looking for exactly the conditions a retaining wall creates behind itself.
What a wall offers a root
From a root’s point of view, the backfill behind a retaining wall is the best ground on the section. It is loose, free-draining scoria or crushed rock with air in it, and at the bottom sits a perforated pipe carrying a steady trickle. Compared with dense, airless peat, it is a buffet.
So the roots go there. They grow along the back of the wall in the drainage layer, thickening year on year. They enter the coil through the slots, and once inside they branch and fill it. On the Rototuna job the coil was 65 mm and the root mass inside had the full diameter of the pipe for eleven metres. Water that could not get into the coil sat against the timber, the timber stayed wet, and the wall began to lean.
Roots also travel under the footing, following the underside of the compacted layer because it is another drainage plane, which is how a tree on one section ends up lifting paving on another.
Whose side of the boundary the problem is on
The wall in Rototuna belonged to the uphill owner. The poplars belonged to the section over the back fence. The roots were on everybody’s side.
- The wall owner can cut roots on their own land, including roots that have crossed the boundary, provided the cutting does not destabilise or kill the tree. Cutting every root on one side of a 20-metre poplar is a different matter from trimming feeder roots out of a drainage layer; a qualified arborist should say which is which before anyone starts.
- The tree owner is not generally obliged to remove a healthy tree because its roots are inconvenient, but a tree whose roots are demonstrably causing damage is a different conversation, and a poplar shelterbelt on a residential boundary is rarely something anyone fights to keep.
- Either party can install a root barrier on their side. A vertical barrier 1 to 1.2 metres deep, installed after the offending roots are cut, is often enough on peat because the roots are shallow. Expect NZ$150 to NZ$300 per lineal metre installed.
Fixing the interface, not only the pipe
On the Rototuna wall we replaced the drainage coil, and I told the owner it would block again inside five years unless the tree side of the problem was dealt with. The neighbour took out the four nearest poplars at about NZ$1,800 to NZ$2,400 each including chipping, and the root barrier went in along the fence line. Then the wall itself had to be looked at, because a season of standing water had rotted the bottom rail. Whether the timber is rebuilt on a footing suited to peat is best answered by a Hamilton retaining wall contractor after the roots have been dealt with, not before. A new wall behind a live poplar row on peat is a new wall with a five-year fuse.
Frequently asked questions
How close can a tree be to a retaining wall on peat? For poplar, willow or alder, keep the trunk at least 1.5 times the mature crown radius from the wall, typically 8 to 12 metres, or install a root barrier when the wall is built. On peat every large tree changes the moisture under the wall.
Can roots push a retaining wall over? Directly, rarely; the usual failure is indirect. Roots block the drainage behind the wall, water builds up, and the extra pressure plus wet timber does the rest. A large trunk growing hard against a wall face can lift or bend it over decades.
Who pays when a neighbour’s tree roots wreck the drainage behind my wall? Usually you pay for your own wall and drainage, and the tree owner is only liable where the damage is clearly caused by the tree and they were warned. A dated letter with photographs from a camera inspection, sent before the repair, settles most of these disputes.