Autumn leaves shower NSW Southern Highlands streets in red, orange and yellow

Australia Broadcast Center

18-05-2025

The days are getting shorter and the temperatures cooler, but as winter draws nearer, the last red, gold and orange autumn leaves bring a colourful warmth to Australia's colder regions.
If you have ever wondered why deciduous tree leaves don't simply transition from vibrant green to dead brown, it all comes down to photosynthesis.
Landscaper Charlotte Webb, a founding member of the Southern Highlands Botanic Gardens, explains the range of autumn colours comes down to the weather.
"This year it's been quite good because we've had some cold nights and the colder the night while the leaves are still on, the better the colour," Ms Webb said.
The 13 hectares of gardens in Bowral were designed to highlight seasons in a cool climate.
Ms Webb said the gardens were at their best in autumn, but heavy rain could dim their display.
"We've had moderate rain, but in a really wet year you'll find the colour won't be quite as good because the sugars don't get trapped into the leaves quickly enough to form that intense colour," she said.
"It's the sugars that do all the turning of the colour and that's what affects the strength of the colour."
Leaves produce sugar through photosynthesis, trapping sunlight and using it to convert carbon dioxide and water into glucose or energy.
Ms Webb said autumn leaves in recent years weren't as vibrant for several reasons.
"In the last couple of years, we haven't had as good a colour because we've had those really wet years and the night-time temperature didn't get low enough, quickly enough, while the leaves are still on," she said.
"When in drought, the colours are magnificent because we get no cloud cover, we get a really cold night and the leaves are still on the trees, so they get trapped in and you get those really vibrant colours."
Horticulturist John Gabrielle said leaves thrived on sunshine and required a combination of factors to produce vibrant autumnal colours.
"Plants are what we refer to as autotrophs, they create their own energy source," Mr Gabrielle said.
"Light plays a significant role in the plants' ability to be able to convert that energy into sugars and carbohydrates, so the light source is also imperative to good autumn foliage colour.
"If you have high light levels combined with low temperatures and good soil moisture levels with good nutrients, you'll get really good autumn foliage."
Across the Southern Highlands many trees are still losing their leaves and filling the streets with decorative bursts of colour.
In the Botanic Gardens, a Chinese pistachio's leaves are yet to turn but when it does, Ms Webb said it would be magnificent.
"It gets into all of those vibrant oranges and reds and is a beautiful tree, one of the best autumn trees you can have," she said.
Ms Webb's husband Chris Webb, who is also a landscaper, said autumn leaves transitioned through a myriad of colours, depending on the tree type, because the sugars in their leaves varied.
Ms Webb said, as buds initially unfurl, tender leaves were yet to form a waxy or hairy layer providing protection from the weather, meaning bad weather, like a late frost, could damage new leaves.
However, the leaf developed protection layers that encouraged it to grow throughout the season.
By midsummer, leaves have stopped growing and remain throughout the season until environmental conditions change.
Evergreen trees, however, drop leaves throughout the year.
"It's continually happening throughout the year because those leaves have to regenerate."
Throughout their life cycle leaves provide energy for the tree, and most importantly seed production.
"The leaf's main part is to keep the plant alive and make the next generation happen," Ms Webb said.