A newspaper article prompted some thinking the other day; it was about the ‘heat island’ effect that is experienced in cities due to the amount of tarmac, concrete and other surfaces found in urban environments. These can increase the temperature in a city by as much as three degrees in summer. That doesn’t sound much, but in a tropical city like Darwin this can literally mean the difference between life and death. With a hot summer expected this year, temperatures on their main street may get up to 70C.
While we may not be so extreme, we’re still expecting a hot summer this year. One of the best ways to counter heat is, not surprisingly, by planting trees. Big, spreading trees are best. They cast the most shade and further cool the surroundings through transpiration.
However, these same trees can cause an already damp and gloomy Kiwi winter to be even wetter and darker. Not to mention that your house and your neighbours may become dwarfed by a forest giant.
I’ve found a useful solution to these problems; I’ve become enamoured of tropical and subtropical deciduous trees. Unlike northern hemisphere trees that drop their leaves in autumn and leaf up at the start of spring, many of these trees have evolved to drop their leaves near the end of the dry season, which is our late winter, then sprout new growth in late spring or early summer in expectation of monsoonal rains.
So just when our gardens are at the wettest and most miserable, these trees obligingly let more light in, covering up again only when our weather is starting to get hot and dry. Not only that, but they tend to grow much slower here than in the tropics, often with a naturally spreading habit – easily shaped and controlled if necessary.
Some of my favourites include the trusty albizzia with its ferny foliage and pink brush-like blooms; jacaranda, also with ferny foliage but with a magnificent display of purple in spring; its relative, tabebuia, produces masses of yellow trumpet flowers at the same time; the orchid tree, bauhinia, has butterfly shaped leaves and orchid-like flowers; chorisia, the silk floss tree has spectacular pink flower and an imposing spiked trunk; lagerstroemia, the crepe myrtle, is another exceptionally spectacular small tree in flower.
There are many more, but these are likely to be the easiest to locate (although some phoning around nurseries may be needed) and the easiest for us to grow. Others, like the astoundingly beautiful Royal Poinciana, are decidedly marginal in our conditions and even bauhinia can be touchy in a cold year. If you are a committed plantaholic though, give them a go!
One point to keep in mind is that these trees won’t flower as well for us here in New Zealand as in Australia, South Asia, South Africa or South America; not because we’re too cold, but because we’re too wet in winter. Never mind, we can’t have everything in our favour in the world’s luckiest country.