Trees for Shade

Trees have many features but above all they provide vital shade on a nasty hot day. As our summer temperatures rise the inadequate choices made by our Australia's urban planners about 50 years ago have become revealed.

Our infatuation with our native flora began at about that time so natives took over as street trees of choice throughout Australia. Whilst it is vital that we love our native plants it is just as vital that we understand their strengths and defects. Do they actually shade our paths and roads?

Eucalypts dominate Australia like no other tree anywhere in the world. They evolved in association with our song birds who carried out the pollination of flowers in exchange for nectar that they depended on.

Hence the planting of eucalypts, particularly scarlet flowering gums, was seen as both patriotic as well as ecologically desirable in that they attracted parrots into our suburbs. It was only later that we realised that one of their defects was that almost all gums provide inadequate shade canopies (just 30% cover) when today's cities are desperately in need of first class shade trees that provide a canopy of 100% shade.

Our botanic gardens have the best mature tree selections to help us choose the best shade trees. Riding the boom of interest in our native flora, the Royal Botanic Gardens, Cranbourne, 50km east of Melbourne was planted about 40 years ago to focus on our arid centre natives, with very poor shade.

When our first settlers arrived, Melbournians where so unused to the oppressive heat waves they were desperate to create a public garden that they could retreat to on searingly hot days. But the garden created by its first Director, Baron von Mueller (1857) was scientific and systematic and his focus on pines reinforced the dry arid look, rather than a pleasure garden and this approach so infuriated the public who wanted their garden to be cool and refreshing to overcome the heat.

So our first Director of the Botanic gardens was removed and transferred to the Herbarium to be replaced by William Guilfoyle.

Guilfoyle was chosen to create the lush green landscape garden that our English colonists craved; but it took nearly 60 years before the garden and its trees matured with its special green clothing that provided the shade and became everyone's retreat during our scorching summers. So it is surprising that history is repeating itself in the creation of Melbourne's second major public garden at Cranbourne. Particularly now that climate change is bringing us hotter summers that last longer, our need for refreshment seems greater than ever.

The Black Saturday bushfire in the summer of 2009 that stunned everyone reached 48°C, which was as hot as it gets in the arid centre and some 5-8°C hotter than was predicted at current CO₂ levels. What will that temperature lift too when predicted runaway CO₂ levels reach maximums in 90 years time considering that CO₂ stays as a insulating blanket for about a hundred years?

Which brings us back to the crucial issue of how gardeners can create an oasis of green at home to cool their houses and to provide respite from the worst our climate is going to provide.

I have always admired the Adelaide Botanic Garden because although this is our hottest and driest capital city, this garden is a model of coolness on a hot day. They chose the best shade trees, both native and exotic, more effectively than any other botanic garden.

There are three accepted solutions to cooling your brow on a hot day and it all revolves around the movement of water. Water has an amazing capacity to insulate a garden and it's not just the pleasure of looking at a river, pond or pool on a hot day. Water makes up about 90% of the weight of the fruit and vegetables because plants are conduits for the movement of water. As it gets hotter, plants transpire water through the green leaves of our shade trees or from the grass under foot that we grow in our backyards.

On a 40°C day in our Heronswood garden we took temperature readings and found the temperature of the water in our swimming pool to be 15°C cooler at 25°C on a hot day but it was a surprise to find that our turf grass temperature was the same 25°C as our pool. But if you then compared that with the temperature of our paving stones or the building materials for the house or our bitumen roads you would find it all very much hotter.

Our modern cities are heat banks with bitumen and paving heating up during the day and then releasing that heat overnight so that our cities are usually about 5°C hotter than neighbouring green spaces.

So when gardeners replace lawns with the modern fashion of paving and flax without any trees then it is like putting a heat bank in their backyard. It is important to also cover brick walls or fences with climbers too because they will cool the garden as well as shade trees.

So the message from these soundings is to maximise the green foliage in your garden by having expanses of lawn, minimising the amount of paving, and clothing your house walls with green insulating climbers, but above all to have outstanding shade trees in proportion of at least 20%-50% of your outdoor space depending on the size of your garden and of course the number of really hot days.

The Cranbourne Botanic Garden suffers alarmingly in comparison on a hot day. It has almost no shade trees to escape to so that the blazing hot sun is quite dangerous to visitors on a really hot day.

What few gardeners realise is that Australians are blessed with an amazing collection of native evergreen shade trees that are used to long dry spells but provide really dense and cooling shade.

They grow between the Great Dividing Range and our east coast and there are 100 year old specimens in the botanic gardens of Adelaide, Melbourne and Sydney, but because of funding shortages are still largely absent at Cranbourne just when we need them. These highly effective and desirable rainforest trees dominate the southern edge of Melbourne's botanic garden with magnificent specimens of Black Booyong, White and Red Cedar, Agathis robusta, Moreton Bay Fig and Araucarias.

To this grouping Flindersia australis, Harpullia pendula are really impressive evergreen shade trees in the Adelaide Botanic Garden and at the Waite Institute Arboretum.

THE COOLING EFFECTS OF SHADE AND WATER COMBINE AT THE MELBOURNE BOTANIC GARDEN