The call came at 6.40 on a Sunday morning in late September: a Cashmere homeowner on Hackthorne Road with a gum tree across the driveway and a radiata pine leaning on the neighbour’s carport. Overnight a southerly had dropped about 15 centimetres of wet, heavy snow on the Port Hills, the kind that sticks to everything. I clear power lines and cut up storm damage for a living. Spring snow is a different animal, and every few years it reminds Christchurch why.
Why wet snow is so much heavier than it looks
Dry powder snow weighs around 100 kilograms per cubic metre. The wet snow Canterbury gets when a southerly arrives at 0 to 2 degrees weighs 300 to 400, and it sticks. A branch holding 10 centimetres of that over a surface a metre wide and four metres long is carrying an extra 120 to 160 kilograms, out near the tips where the lever arm is longest. Wind turns that static load into a dynamic one, and something gives.
The dangerous window is spring. In midwinter deciduous trees are bare and evergreens have hardened off. In September and October evergreens carry soft new growth and sap flow is up, so the timber is at its most brittle relative to the load.
Evergreens catch the load; deciduous trees shed it
A bare oak or plane tree presents thin branches with nowhere for snow to sit. A macrocarpa, radiata pine, Lawson cypress or eucalyptus presents a dense sail of foliage that catches every flake. That is why the calls after a spring snowfall are nearly all conifers and gums.
The worst offenders I see around Christchurch, in roughly this order:
- Eucalyptus — long horizontal limbs with weak attachments. Gums drop limbs under snow faster than anything else.
- Radiata pine — older trees with long bare stems and all the weight up top. Whole tops snap out.
- Macrocarpa — the big shelterbelt trees hold enormous loads and fail at old codominant unions.
- Cedar and cypress — flat plates of foliage that stack snow like a shelf.
- Leyland cypress hedges — rarely break, but splay open and never quite close again.
Where the branch actually breaks
Look at any snow-broken branch and the failure is usually at one of three places: an included bark union, where two stems have grown together with bark trapped in the join and no real wood connecting them; a point of old decay, often a lopping cut that never sealed; or a long horizontal limb that has lengthened for years while its attachment stayed the same size. None of these are visible from the ground in a snowstorm. All of them are visible on a dry day in February to someone who knows what to look for.
What to do during and after
During the snowfall, stay away from the drip line of any large evergreen, and do not knock snow off a big tree with a pole. I have seen a broken collarbone from a limb springing back when its snow was shaken off. Small hedges and ornamental conifers can be brushed gently from the side. Anything with limbs above head height, leave.
Afterwards, before anyone starts cutting, look for hangers: branches that have snapped but are still caught in the canopy. They drop when the sun softens the remaining snow, usually mid-morning, and they are what hurts people. Near power lines, treat every branch as live and ring the lines company first.
Clean-up costs in Christchurch after a snow event run roughly NZ$400 to NZ$900 for a single fallen limb cut up and left on site, NZ$1,200 to NZ$3,000 for a broken top removed by a climber, and NZ$3,500 upwards where a tree has to be dismantled off a roof or carport. Prices rise for a week after any regional event because every climber in Canterbury is booked.
Reducing the load before next spring
You cannot make a gum snow-proof, but you can take the worst of the risk out. Shortening the longest horizontal limbs by a metre or two, thinning the dense outer foliage on cypress and cedar, and removing anything already dead or cracked all cut the load and shorten the lever arm. Where a tree has a strong lean, check whether it is old and stable or new and moving; there is a useful rundown in the piece on lean and windthrow on Wellington hillsides, and the same logic applies on the Port Hills. Watch the seasonal outlook on NIWA from August: the southerly outbreaks that bring wet snow to Canterbury usually show up a few days ahead.
Frequently asked questions
Why do evergreen trees break in snow when deciduous trees do not? Evergreens hold their foliage through the snow season, so snow has a surface to sit on; deciduous trees are bare and it falls straight through. Spring snow is worst because evergreens carry soft new growth.
Should I knock snow off my trees? Only on small hedges and shrubs you can reach from the ground, and only by brushing upward from the side. Never shake or pole a large tree; released limbs spring back and hangers drop without warning.
How much does it cost to clear a snow-broken tree in Christchurch? Roughly NZ$400 to NZ$900 for a single limb cut up on the ground, NZ$1,200 to NZ$3,000 for a broken top removed by a climber, and NZ$3,500 or more if the tree has to come off a building.