In the last two years Eastern Australia has seen a spate of wet weather that is in stark contrast with the preceding ten years of drought. Melbourne has been positively luscious of late, with plants of all kinds booming in the idyllic conditions. With the months of lush, colourful flowering now well and truly behind us, autumn is a time to admire those trees and shrubs that brighten up the chilly mornings with their fiery displays of yellow and red. Like flowering perennials and annuals that have been a dream to grow lately, so too has autumn colour benefited from the extra rain. Here in Melbourne we’re having one of the most stunning autumns in a long time, even better than last year’s wow-fest. The intensity of colours we are seeing in many of the deciduous species around town is breathtaking. Reds, yellows, oranges and even purples are busily setting gardens ablaze throughout the suburbs, adding a warming glow to the slowly chilling streets. The processes by which plants achieve such intense colouring are fascinating and, I think, as beautiful as the colours they result in.
The summer equinox marks the point at which the days begin getting shorter, the sun gets lower in the sky and the hot summer temperatures begin to wane. Deciduous trees then soon begin to prepare for winter by drawing nutrients in from the leaves to the branches and trunk. Most of the nutrients that are broken down are the constituents of chlorophyll, the chemical compound that gives plants their green colour. As these compounds are sequestered from the leaves into the branches, the leaves begin to change colour. What colour autumnal leaves turn depends on the pigments left behind in the leaves. Basically, there are two pigments that determine autumn leaf colour – yellow and red. A plant may have either or both pigments present, it depends very much on the species. Lots of plants have both, which is why you don’t just see either yellow or red during autumn but a whole spectrum of shades in between.
The yellow pigments (carotenoids) are present in the leaves all year round, but they don’t come through because of the high concentration of chlorophyll in the leaves. Red pigments (anthocyanins), on the other hand, are only manufactured by the tree towards the end of the season and are not found in great abundance in the tree throughout the year. When chlorophyll and other compounds are broken down and absorbed by the plant during autumn, the presence of the yellow and red pigments in the leaf tissue begins to be revealed. This is what we know to be autumn colour. When conditions are right higher concentrations of these pigments are made by the tree, which is why, in good years, autumn colour is much more intense.
Temperature and sunlight in early autumn also play a role in colour intensity. More red pigments are produced in the leaves if conditions are not only cooler during the night time, but there is lots of sunshine during the day too. Colder temperatures don’t have to be consistent in order to get good colour – a few days or a week of crisp, cool nights is enough to get the ball rolling. In Melbourne this year we had a cold start to autumn before the temperatures again climbed to the high twenties. This has happened a couple of times over the previous month. The clear, warm days amongst cold spells creates perfect conditions for the red pigments to accumulate rapidly in the leaf tissues.
Some of my favourite autumn trees and shrubs this year have not been remarkable performers during past autumns. Others are consistent performers. One of trees to make an impression on me this year is a cultivar of ornamental pear (Pyrus calleryana). The pear was an existing tree when we moved into the house and was one I had considered removing. I’m glad to say I thought better of it because this year its autumn colour has been intense. Last year we only got yellow coming through in the autumn, whereas this year we are getting reddy-oranges too. In the afternoon light the tree looks like it’s on fire. The colour is so intense it almost hurts my eyes!
Another surprise this year was the ornamental grape (Vitis vinifera) we have growing over our back arbour. When we moved in there were originally four plants that covered the arbour. Anyone who is familiar with Vitis vinifera will know well how vigorous it is. Four plants to cover a 10 x 3 meter pergola is a bit over the top, to put it lightly. The first year of pruning it took me three weekends of ladder and roof work, and the sheer mass of canes leftover once the job was done was staggering. I ended up cutting three of the plants out completely the following winter, and retraining the remaining one. This year’s good conditions saw the almost naked arbour flush out to a full and healthy cover in no time. Opening the arbour up by removing excess plants has kept the grape fungus-free all year. Consequently, the show the vine has put on this year far exceeds the others it has in the past. The crimson red colour it turned this autumn took me completely by surprise as I had considered the colour of it in previous years as detracting rather that beautiful. I think the lack of disease in the leaves played the biggest role in this year’s stunning display. In previous years botrytis had made the autumn colour on the leaves patchy and sickly looking. The lack of disease in the summer time has resulted in consistent colour throughout the vine this autumn. This is a good example of how cultural controls (pruning) can be as good or even more effective than chemical controls (spraying with fungicide) when it comes to dealing with plant diseases.
Another genus of shrubs putting on a great autumn display at this time of year are the sumacs (Rhus spp.)***. Autumn colour tends to make genera such as maples, liquidambars, elms and oaks spring to mind, but big landscape trees are not the sole bastion of pretty autumn tones. Smaller trees and shrubs abound with great autumn displays, the sumacs being amongst the most spectacular and reliable. They are perfect for the city suburbs because most of them don’t get too large. They are also good in areas with milder winters because they don’t seem to need very cold snaps to produce great colour. The sumacs are sadly underrepresented in Australian gardens, but with the warming climate we are set to face in the future perhaps their stock amongst gardeners will rise with the temperature.
There are still a few weeks of autumn colour left out there to see, so my advice is to go out and see as much of it as you can! I read recently that the global climate pattern is showing signs of shifting back to El Niño once again, which results in drought in many parts of Australia. So this might be one of the last years in a while when the colours of autumn will be this beautiful. If you want to see particularly good colour then pack up the car and head for the hills – Mount Macedon and the suburbs around Mount Dandenong are putting on a wonderful show at present.
Until next time, happy gardening.
[***Editors note for Australian readers - Rhus spp should not be confused with trees often called "rhus", which are actually Toxicodendron succedaneum. Another example of the problem of using common names to identify plants! These trees are declared noxious weeds in most of Australia. The true Rhus genus has many beautiful small deciduous trees from China & North America, grown throughout the cooler areas of the world and which you can also see in Melbourne Botanic Gardens, as well as Mt Tomah Botanic Gardens, west of Sydney.]