ScorchWatch Intro: Degrees

Some like it hot and some sweat when the heat is on
Some feel the heat and decide that they can't go on

So go the words of a great sage (Palmer, R 1985). And while he may not have been referring to plants the words are just as spot-on.

With another summer gone it's time to reflect on how our gardens coped with the heat, but in more ways than one. 'Heat tolerance' and 'heat loving' – we hear these terms all the time with plants. However the way they're used, and what we think they mean, are pretty one-dimensional.

I've fallen into this trap – I've needed plants for a hot spot and placed faith in the words on the plant label, only to be disappointed.

What I've learned the hard way is there are degrees of heat and tolerance.

Most people when assessing the 'heat' of their own climate look to general descriptors - the kinds found in garden books and on the back of seed packets ('cool temperate', 'Mediterranean', 'subtropical' etc). Or they may consider monthly maximum temperatures in their area. Or if they're a bit fancier they might go to the Köppen Classification System (Adelaide's is Csb: 'Hot-summer Mediterranean').

These are all OK, but the problem is they're based on averages and don't always reflect extremes. They also don't take into account other variables like humidity, wind, diurnal range (difference between day and night temperatures), interaction with rainfall, and day-to-day temperature fluctuations.

This is all important. The Adelaide Plains average about 28-31 degrees maximum in January and February, the hottest months. So do many parts of Victoria and eastern NSW. So do Brisbane, Cairns and Darwin. But these are very very different places.

Summer temperatures in southern South Australia have two main influences: the very hot and dry inland to the north and east, and cold ocean currents to the south and west. It means summer temps can swing dramatically from the low 20s to low 40s or vice versa in a single wind change. Inland and more northern and eastern areas don't get this as much. Darwin and Cairns barely change from day to day.

The high-end extreme is crucial as well. Cairns & Darwin almost never get above 35 in any summer. Brisbane cracked 40 last year and this made the news. Many southern and eastern areas (eg Western Sydney) reached the mid-40s – several times. Adelaide has seen 45-46 degrees a few times in recent years. It was mostly spared last summer, but this year has had some hideous scorchers in November & December like the rest of southern Australia.

Adelaide heat is usually very dry. Relative humidity on days over 40 degrees is usually below 20% and often below 10%. The wind is often strong & consistent from the north and northeast – wind that can take away 20+ mm in a day in evaporation, almost stripping the moisture out of your throat as you talk.

Some places in the Far North of the State average high-30s in summer maximum temperature. By contrast on most of Kangaroo Island – surrounded by cold Southern Ocean currents – summer average maximum temperatures are low-to-mid-20s. I remember one time I visited KI in the 90s in the middle of an Adelaide heatwave. It was below 20 on KI and I spent most of the time praying I could find a jacket somewhere.

In light of all this we have to ask: what does 'heat' mean, and what on earth do we mean by 'tolerance'? I will be exploring this in future posts.