Why are Auckland's autumn leaves so colourful this year? video

In Auckland and Northland we're told not to expect our trees to perform in autumn as it doesn't get cold enough.

Instead Arrowtown and other chilly spots down country have a well-deserved reputation for glorious displays of fiery autumn colours.

But this year is different - Auckland's deciduous trees are in brilliant form.

At Ayrlies Garden and Wetland the swamp cypress and liquidambars are the best autumn performers.

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A hillside of glorious autumn colour at Ayrlies Garden viewed from the wetlands.
SALLY TAGG, COURTESY OF AYRLIES

A hillside of glorious autumn colour at Ayrlies Garden viewed from the wetlands.

 

When she started the garden back in 1964, Bev McConnell was told that trees didn't have autumn colour in Auckland. Despite that advice she started experimenting. 

McConnell wanted the north facing valleys to be "on fire" in autumn. She knew she had to choose trees that suited the conditions. Swamp cypress grow in soils with poor drainage and are the last to lose their rusty orange needles. Liquidambars grow tall and support each and are not as brittle as claret ash. 

Swamp cypress growing in a lake at Ayrlies Garden.
SALLY TAGG, COURTESY OF AYRLIES

Swamp cypress growing in a lake at Ayrlies Garden.

Bev's top tip for getting the best trees is to visit the nursery when the leaves on the saplings are colouring up so you know what you'll be getting when the trees mature in years to come.

Bev's experiments have certainly paid off. All these years later the autumn trees and borders at Ayrlies are spectacular.

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Lynda Hallinan reported that Foggydale Farm in Hunua had its wettest summer in decades with almost a metre more rain than usual.

Flowering cherry, Prunus 'Awanui', at Foggydale Farm.
FOGGYDALE FARM

Flowering cherry, Prunus 'Awanui', at Foggydale Farm.

"But all that flooding had a silver - or gold, orange and red - lining as the autumn foliage has been the best it has ever been, or at least since I moved to Hunua seven years ago," she said.

"The liquidambars and pin oaks are usually fairly reliable for colour but this year even trees that aren't usually attractive in an Auckland autumn, such as the ornamental cherries Prunus 'Awanui' and 'Shimidsu Sakura', were a blaze of copper and gold.

In our wetland, the swamp cypresses and tulepo (Nyssa sylvatica) were sensational - they looked like flaming torches.

Acer rubrum 'October Glory' at Foggydale Farm.
FOGGYDALE FARM

Acer rubrum 'October Glory' at Foggydale Farm.

"The only downside is that we usually get to enjoy Auckland's ho-hum autumn display until early July, whereas this year the leaves have all already hit the ground and are ready to be raked up for the compost heap."

Auckland Botanic Gardens Manager, Jack Hobbs, says that people have come to realise that the trees that deliver so brilliantly in the south don't colour so well in Auckland but there are exceptions like swamp cypress and liquidambar.

"Nyssa sylvatica (or tupelo) is the tree that delivers the most consistently vibrant autumn colour at the gardens. It loves moist soil but it's definitely a tree for larger gardens," Hobbs explained. 

Nyssa sylvatica at Auckland Botanic Gardens.
JACK HOBBS/AUCKLAND BOTANIC GARD

Nyssa sylvatica at Auckland Botanic Gardens.

"For smaller gardens cotinus (smoke bush) doesn't get too big. Some of the lagerstroemias (crepe myrtles) also colour well and are under rated as garden specimens."

As there's a limited range of reliable autumn colour from foliage, Hobbs recommended that Auckland gardeners think about autumn flowers like aloes, camellias and tibouchinas instead.

What's the science behind the wonderful displays of colour in our streets, parks and gardens?

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It seems obvious that deciduous trees react to cold weather by colouring up and dropping their leaves but that is not the full story. 

Deciduous trees produce a flush of new leaves in spring. The leaves look green because of that's the colour of chlorophyll which converts sunlight into energy (carbs and sugars) by the process of photosynthesis. 

The leaves stop growing at the end of summer. They've done their job and are about to drop so the trees absorb the energy back into the branches and roots.

Willows at Western Springs.
SAM VILLAROMAN

Willows at Western Springs.

In autumn when the nights start getting longer and the days get shorter, the trees take this environmental cue to finish sucking energy from the leaves and start plugging up the flow of minerals to the leaves with a layer of cells known as an abscission layer. The leaves can't make any new chlorophyll and so they start to change colour. 

The colours we see are yellow and orange carotenoids and red and purple anthocyanins that are left when the green fades. And when they all break down, the leaves turn brown (thanks to the tannins).

So what causes good colour some years and a poor show the next?

Autumn leaves in Auckland Domain.
ALEX SANTOSA

Autumn leaves in Auckland Domain.

If it's cool but sunny in autumn, the chlorophyll breaks down more quickly allowing the autumn tones to show. The best red hues are destroyed by frosts. Drought causes the trees to drops leaves earlier before they've have the chance to colour up as nicely.

And the best conditions of all? We've just had them - heaps of rain, and a relatively sunny but cool autumn. 

 - Homed

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