My son rents a 1960s weatherboard place on a hillside in Karori, the kind of Wellington section where the back garden is a 35-degree bank and the front gate is 40 steps below the front door. Along the top of the bank stand five trees, three macrocarpa and two pōhutukawa, all leaning downhill towards the house by between 10 and 20 degrees. In July 2024 a northerly gusting to 130 km/h at Kelburn had him ringing me at ten at night to ask whether they were about to come through the roof. I own a 25-metre macrocarpa in Christchurch, apparently the family expert, so I went up in August and had a proper look.
Four were fine and one was not, and the difference was not in how far they leaned. It was in how they had got there.
Lean that grew in
A tree that has grown on a windy hillside all its life leans because it has adapted. Wellington’s northerlies and southerlies push young trees over, and the tree responds by laying down thicker wood on the compression side and growing its crown away from the wind. The result is a trunk that curves rather than kinks, with a crown lopsided towards the sheltered side but balanced over the root plate. Arborists call this wind sculpting, and on the Karori hills it is the normal shape of anything over 10 metres.
The signs of a grown-in lean are about time. The bark on the compression side is thick and undamaged. The soil at the base is level and vegetated on the uphill side, not cracked or heaped. The trunk is oval rather than round, wider along the axis of the lean, because the tree has been reinforcing itself for decades. The pōhutukawa in particular, with their multiple stems and dense low root mass, are close to impossible to windthrow once established.
Lean that arrived recently
The fifth tree, the middle macrocarpa, had a different story. Its trunk was straight, not curved; it leaned as a whole, like a post pushed over. On the uphill side there was a crescent of cracked and lifted soil about 2 metres long and up to 80 mm high, with fine roots showing in the gap. On the downhill side the soil was compressed and slightly mounded. That is a root plate starting to rotate; once it lifts, the roots on the tension side are torn and the tree loses most of what held it up.
The tree was about 18 metres tall and 15 metres from the house, on a bank above it. If it went, it would not fall short. Recent movement, evidence of root failure, a target within the fall line, and the next big blow a matter of weeks: that is the combination that turns a leaning tree into a hazard.
What to look for after a storm
If you have leaning trees above a Wellington house, the check after any wind event takes ten minutes. Walk to the uphill side and look for fresh cracks, heaved turf or exposed roots within a metre or two of the trunk. Look at the base for new splits or bark tearing on the tension side. Compare the lean to a photograph from before; a change of 3 to 5 degrees in a mature tree is a serious warning. Look up for hangers. And listen: a tree with a moving root plate creaks at the base in wind, not in the branches.
On a bank like Karori’s the soil itself may be moving. Saturated winter ground on a steep greywacke slope can let a whole root plate slide even when roots are healthy. Trees on the brow of a cut slope are the ones to watch most closely.
What it cost to sort out
The middle macrocarpa came down in September 2024, dismantled from the top in sections because there was nowhere to fell it. Steep access with no vehicle route meant everything was lowered by rope and carried out by hand; two climbers and two ground workers for NZ$3,800 excluding GST. The stump was left in the bank to hold the soil, cut to 300 mm, which on a slope is often the better choice than grinding. A hazard assessment of the other four trees was included and came back as monitor and re-inspect in three years.
The lesson was to stop asking how far a tree leans and start asking whether the lean has changed. A tree that has leaned 15 degrees for 40 years has proven itself in several hundred storms; one that has leaned 8 degrees since last month has proven nothing. That is exactly the question a Wellington arborist answers from the ground in half an hour, and that is cheaper than the alternative.
Frequently asked questions
Is a leaning tree dangerous? Not necessarily. A tree leaning for years with a curved trunk and undisturbed soil at the base has adapted to its site. A tree with a straight trunk at a new angle, cracked or heaved soil on the uphill side, or exposed roots is failing and needs urgent assessment.
How much lean is too much? There is no fixed number. Recent change matters far more than the angle; a shift of 3 to 5 degrees in a mature tree after a storm is a serious warning even if the total lean is modest.
What does it cost to remove a tree on a steep Wellington section? Sectional dismantling with rope access and hand carry-out typically runs NZ$2,500 to NZ$6,000 excluding GST for a 15 to 20 metre tree, depending on access and crew days.