In the words of the old joke: “The world can be divided into two groups. Those who believe the world can be divided into two groups, and those who don’t”.
But, joking aside, among the most intense divisions in the gardening world is over the utility, or otherwise, of deciduous trees. Especially in autumn.
Some people just love them – the colourful display they provide more than compensates for the relatively small period of clutter they cause when they fall. However, others hate them with gusto, and spend inordinate amounts of time with leaf blowers and rakes, cursing all the time.
I confess I belong to the former group. We don’t have many deciduous trees in the garden – a few Japanese maples which colour up beautifully with orange tones, and a couple of magnolias whose leaves generally present a drab sight in autumn – so we don’t usually have a big problem with leaf litter.
The neighbours’ oak tree, perched on our boundary, does shed a lot of leaves at this time of the year, but it is colourful before is shucks off its foliage, and the wind usually takes care of the few leaves my half-hearted attempts at cleaning up account for.
We are lucky that our neighbourhood has some of the prettiest of trees. Just across the road there is a medium-sized Ginkgo. These ancient, pre-historic indeed, conifers have one of the loveliest autumn displays. The maidenhair fern-shaped leaves turn butter yellow at this time of the year, staying on the tree for a long time before falling.
There is another tree just down the road, then another I pass each day on Russell Street, and I glory in them all. There are also a couple of spectacular specimens in Queen Elizabeth Park which are both at their peak about now. One grows alongside a maple on one side that turns blood red, and on the other side, a light-coloured speckled elm that turns silver before the leaves fall. It’s an interesting combination! In another section of he park, another Ginkgo grows alongside a cherry that also colours for autumn.
The odd thing about these autumn colourations is that they are actually present all through the growing season but masked by the activity of chlorophyll. As autumn approaches, the trees start to receive less sunlight, triggering their chlorophyll to break down and allowing the other pigments in the leaf to become dominant.
Although the process is started by less sunlight, other conditions can affect how good the colouring season is. Yellow and orange colourings are formed by different pigments, but red leaves are a result of anthocyanins, and they are, in turn, affected by environmental factors.
In summer, such as the one we have experienced this year, with more sunlight and dry weather, sugars are concentrated in the sap, inducing the tree to release anthocyanins to help capture the energy trapped in the leaves.
Once trees can no longer get any energy out of the leaves they shed them. This allows them to survive the cooler conditions of winter and prepares them for the spring ahead. The leaf fall is nature’s form of recycling – the nutrients on the leaves help feed both the microbial and life in the soil – the bacteria and fungi – and also the small critters , who all get to work breaking down the leaves and helping replenish the soil.
In some parts of the world the autumn colour season is vital for tourism. The north-east of the United States is famed for the colouration that overwhelms the native hardwood forests in the fall, drawing thousands of visitors.
It seems climate change will influence these displays, with the potentially increased cloud cover reducing the amount of sunlight the trees receive, shortening their period of colour. It will be interesting to see how that plays out.
Closer to home I have been watching the effect on some liquidambar trees that line our street. Those on the southern side, which are closed to streetlights, seem to lose their leaves later, which makes sense. The trees, which I discovered recently were planted in the late 1960s, are nowhere near as large as those in Pownall Street, have a nice range of colours, with some trees taking much darker shades.
I know that there are named liquidambars that take on almost purple shades in autumn, while others are yellow, orange red, or a mixture of all of these.
Last year I picked up some of the liquidambar seed capsules from our footpath – spikey walnut-sized balls covered with spines – and managed to raise a couple of seedlings. I was interested to see what colour they would turn this season, but the leaves just faded and fell off the little trunks. They have been growing in a glasshouse, so I hope that next year, when they are potted on and growing outside, they will take on some more interesting tones in the autumn.
