Penrith is the hot spot of the Sydney basin. Partly that has to do with geography, but at least as important is land use and urban design. Thermal imaging of the city taken on a February morning showed land surface temperatures ranging from 40-49 degrees with localised hotspots another five degrees hotter than that. You could melt the soles of your shoes walking across the road!
Penrith recently released its Cooling the City Strategy, which aims to make summers more pleasant by reducing the city’s Heat Island Effect. The Heat Island Effect describes the way hard surfaces prevent water soaking into the ground and evaporating to cool the air; how non-porous, non-reflective surfaces on rooves and at ground level soak up heat and disperse it all night; and how warm air from cars and air conditioners increases the heat, causing more people to turn on their air conditioners in a cycle that just gets hotter.
What’s this got to do with gardening? As anyone who has sat under a tree in the summer knows, gardens are part of the answer. Those thermal maps of Penrith showed that areas of vegetation were up 20 degrees cooler than the hot spots. Penrith’s strategy involves increasing reflective surfaces, making ground surfaces permeable wherever possible and making the best use of the urban water cycle, but it also stresses the need to look after and increase what planners refer to as green infrastructure, and the rest of us call plants.
With a monster El Nino maturing in the tropics, now is the time to institute your own strategy for city cool. First off, reduce hard surfaces. Bring back the lawn; choose stepping stones rather than hard paths, or compressed gravel rather than concrete. Grow plants up walls and over fences. Make sure the water tank is operational and the pump works. If you don’t yet have a tank, choose from myriad space-saving options, from bladders that sit under the deck, to tanks that are literally the fence. Next, add shade. The best choice is a tree, but only if you choose one that suits your space and conditions.
New to the glamorous tree boutique are the living pergolas grown by Warners Nurseries in Melbourne, and showcased in the gold medal-winning garden designed by Nathan Burkett for Melbourne International Flower and Garden Show this year. The trees are deciduous plane trees, grown to 2.5 metres high, so that there is plenty of room to sit or stand beneath them. The growth above 2.5m is trained to lie flat across a bamboo frame or is trimmed off, so that the tree becomes a flat plane of dense hand-shaped foliage, atop slender pillars of mottled grey. Put four together and you have a living pergola. It’s not a no-maintenance option; new growth will continue to attempt to grow vertically and need to be trimmed back all through the growing season. But in a formal garden this is a really elegant way of combining architectural form and organic life. And it is cool.