The bright red flowers at the ends of the branches of the red flowering gum are spectacular, as everyone would agree. The tree in question is a Corymbia cultivar, a hybrid with one parent Corymbia ficifolia, which is native to an area in south-west Western Australia. This parent species, which can produce flowers that are white, pink, orange or red, has been a popular garden plant for over a century. Red-flowering plants have been the favourite, but the history of trying to grow this type of plant with flowers guaranteed to be red is long and winding. A huge amount of effort and a lot of botanical science has gone into producing the Corymbia varieties available now, with hopefully reliable flower colours (e.g. ‘Summer Red’, ‘Summer Beauty’, ‘Orange Splendour’).
Growing a Corymbia ficifolia from a seed could give you a plant with flowers of one of a number of colours because of the shared genetic material from the parents producing that seed. As a result of this genetic variability in seed, getting a red-flowering plant by growing from seed means looking at the seedlings you’ve grown and picking out from them the plants with red flowers. This is not always an attractive process for commercial growers!

Greg Moore, an engaging and knowledgeable Melbournian speaker on trees, has given a brief story about how growers have produced Corymbias with reliable colours, a story beginning in the late 1800s. (Greg gave an engrossing talk at last year’s APS Melbourne conference, and an interesting outline has recently been posted by Jennifer Farrer).
In the late 19th century, seed was harvested from those trees with the best-coloured flowers, on the assumption that these seeds would produce many plants with at least “good” colour because of their parents. At this time, people also tried to grow the plants from cuttings but were largely unsuccessful. It has become apparent that eucalypts are very difficult to grow from cuttings, but not impossible, and Greg goes on to describe how this challenge was overcome.
Up to and into the 1950s, people tried using a variety of plant material for cuttings, including juvenile material, shoot tips, epicormic shoots and lignotuberous shoots. However, their efforts didn’t pay dividends high enough for any of these techniques to be commercially viable.
Plant hormones have been known since the 1930s and have been a game-changer in plant propagation. These are chemicals that are produced by plants and regulate all aspects of their growth and development. Some have been synthesised for use in plant propagation and are known by various names, such as plant growth regulators (PGRs) and rooting hormones.
Rooting hormones began to be used to propagate Corymbias and did result in a higher success rate. Small steps forward were being made! In vitro (or test tube or tissue culture) methods of plant propagation, together with the use of plant hormones, came into use for plant propagation in the 1960s. It was this combination of plant hormones and lab methods that took the quest for obtaining reliable flower colours in Corymbias another step forward. Still, by the 1990s, propagation techniques attractive enough to commercial growers had not yet been developed. The Corymbia varieties we know today were still not on the nursery shelves.
Another tack being taken for propagation with cuttings was to graft the cuttings onto rootstock of different species. The previous knowledge and experience from work on cuttings combined with insights from tissue culture provided guidance on how to successfully graft cuttings onto different rootstock. With a lot more perseverance, another major hurdle was jumped, and around 2000 a propagation technique was developed that horticulturists judged was worth using for commercial production.
This finally brought Corymbias with brightly coloured flowers into gardens and streetscapes. Today, we take them for granted, but a colossal amount of effort, time, money and good science went into creating them!
All this good science, however, wasn’t purely directed at producing commercially viable and attractive Corymbias! Plant propagation and conservation are closely intertwined. Tissue culture methods can be applied to conserving eucalypt plant material but apparently have not yet been widely taken up in this way. They may be of value in the face of climate change and be taken up to preserve plant material.

Another example of applying good science – and lots of effort – to plant propagation is some recent work done at the Botanic Gardens of Sydney on seed germination, (Branch Out podcast Episode 46, Predicting seed germination under climate change – Branch Out is a great series of podcasts for anyone interested in plants! The Botanic Gardens’ video series, What the flora?, is great too!). Temperature, along with moisture and oxygen availability, is one of the major factors that influence seed germination. Botanists are very concerned with how plants will cope with the predicted rise in global temperatures over the next few decades. They predict there will be changes in the timing of seed germination and the proportion of seeds that germinate. This will affect the structure of vegetation communities and, not surprisingly, have knock-on effects throughout ecosystems.
This work, being done by Nathan Emery and others involved in conservation work at the Botanic Gardens of Sydney, is about predicting seed germination, particularly of threatened species, in an environment with more extreme temperatures, both hotter and colder. They subjected seeds to a range of temperatures, both warmer during the day and cooler at night, and measured the timing and rate of germination. If results from these trials for a particular species predict germination success is going to change in different temperature ranges, then that species might need to be closely monitored in the wild to see whether that predicted change is occurring. If the predicted change does occur, action will need to be taken to try to ensure the species survives. This method can provide an important advance warning of a potential threat to a species’ survival. One important value of this work is that it can be applied globally to any species. It’s another example of cutting-edge work done by Australian plant scientists, work that needs to be supported and maintained.
Background article:
Gregory Moore, The red flowering gum: a perennial favourite and a summer stunner, The Conversation, 21 January, 2025
Botanic Gardens of Sydney, Predicting seed germination under climate change, Branch Out podcast, Episode 46, 12 August, 2022