The eucalyptus is an icon of the Australian native bush garden and is a favourite amongst many Australian landscapers.Every year in late March the National Eucalypt day is celebrated. Events and celebrations take place all around Australia. Some of the celebrations involve making eucalyptus tea and many people in Australia will remember camping trips where a gum leaf was added to the tea for a bit of genuine Aussie flavour.
Eucalyptus is one of the world’s most widely planted hardwood trees
Although its natural habitat is almost exclusively in the southern hemisphere. the Eucalypt, or gum tree, is now one of the world’s most widely planted hardwood trees. Part of popularity is the tree’s characteristics of being fast growing, hardy and adaptable as well as its ability to produce complex oils. Eucalypt trees are also know for their attractive foliage and bark.
The controversy about planting Eucalyptus in some parts of the world.
The spread of the gum tree worldwide is not without its controversies. In India and Spain there has been blamed for lowering water table. In California, where eucalypts have been grown since the mid 1800s, there has been a lot of bad press about the blue gum aggressively spreading from its original plantings. Although the oils in the eucalyptus are flammable, so are the oils in pine trees and some other trees. There is also controversy in the Mehdditearaneanean. On the island of Malta, many people want the gum trees removed and replaced with indigenous species. The apiarists want the eucalypts to stay as it produces great honey.
Using the Eucalyptus in the Egyptian Desert to limit the desert growth and treat waste water.
It is not all bad news for eucalyptus plantations outside Australia. In a plantation project in the Egyptian desert, known as the Serapium forest, gum trees along with local species, are being used to both treat sewerage waste and to stop the growth the desert. In this situation the gum trees can grow plantation timber at four times the rate of a typical European plantation and absorb excess nutrients such as nitrogen that could harm the environment.
The Eucalyptus genome decoded
In 2013 the genome of the Flooded Gum (Eucalyptus grandis) was sequenced. This has given scientists an insight into the specialised metabolites that create the oils the trees use as a chemical defence. The original inhabitants of Australia have been using these oils for tens of thousands of years for medicinal purposes and eucalyptus oil is now widely used by the pharmaceutical industry. The research has shown that there is great diversity in these particular genes and this helps to explain why some species are better at repelling insects and why koalas will only select particular species.
Eucalypt, Eucalyptus, Angophora and Corymbia.
The term Eucalypt includes trees which are amongst the closely related genera of Corymbia and Angophora as well as Eucalyptus. The differences between the three are related to the leaf positioning, the flower stalks and the bark. The term gum tree has become the common term for all three eucalypts whether or not the trunk exudes a sticky gum like substance.
The Eucalyptus Family
Eucalypts belong to the family Myrtaceae and are related to tea trees (Leptospermum), paperbarks (Melaleuca) and bottlebrushes (Callistemon). The name Eucalyptus comes to us from Modern Latin. It was coined in 1788 by French botanist Charles Louis L’héritier de Brutelle from Greek eu “well” + kalyptos “covered”. Anyone who has been hit by a gum nut will know how well covered the seed is.
The world’s tallest flowering plant
Eucalyptus regnans is the worlds tallest flowering plant. Amongst the trees, it is second to the California Redwood in height but grows five time as fast. One specimen in Tasmania’s Arve Valley, nicknamed ‘Centurion’, has reached 99.6m in height. It is the not only world’s tallest flowering plant but also the tallest known hardwood tree.
Eucalyptus regnans occur mostly in north-east Tasmania as well as the Derwent and Huon valleys. It can also be seen in the higher rainfall areas of the eastern highlands of Victoria south of the Great Dividing Range.
The scented gums
The genetic diversity of the chemical defences of the Eucalypts leave a clue in the aroma of the trees. The lemon scented gum Corymbia citriodora has a distinctive lemon smell which is very noticeable after light rain. This tree is common in South East Queensland and residents of the Melbourne will be familiar with the smell. This is the eucalyptus most often used for manufacturing lemon eucalyptus essential oil.
There are other gum trees with a completely different fragrance. These trees have evolved their chemical defences to smell like peppermint. One of these is the South Gippsland peppermint Eucalyptus willisii. Similarly the dark green leaves of the Narrow leaved Peppermint Gum (Eucalyptus Radiata) have a peppermint smell when they are crushed. Another well known scented gum is Tamania’s Eucalyptus coccifera or the Mount Wellington Peppermint Gum. The adult leaves are an attractive blue green colour changing from the hearts shaped juvenile leaves.This tree, like many gums, has grey and white mottled bark.In summer creamy-white flowers appear.
Wood from Eucalyptus trees.
The timber from eucalyptus trees has been used for a long time in building construction, flooring, and furniture. Species such as the Sugar Gum (Eucalyptus cladocalyx) with its pale yellow brown heartwood and Spotted Gum (Corymbia maculata) with its wavy grain and brown heartwood can be used to create furniture. The species of Ash such as Mountain Ash (Eucalyptus regnans), Alpine Ash (Eucalyptus delegatenis) and Shining gum (Eucalyptus nitens) are often used for dining tables, staircases and flooring. These hardwood timbers are also great structural timbers due to their strength.
Eucalyptus Essential Oils
The original inhabitants of Australia have been using these oils for tens of thousands of years for medicinal purposes and eucalyptus oil is now widely used by the pharmaceutical industry. The species often used for commercial essential oil production is the Tasmanian Blue Gum Eucalyptus globulus and the black peppermint tree E. salicifolia. The oil is often the active ingredient in inhalers and expectorants.
The lemon scented gum Corymbia citriodora is the source of the lemon eucalyptus essential oil. Typically this essential oil contains citronella, citronellol, geraniol and isopulegol. Lemon eucalyptus essential oil, which is produced by distilling oil from the twigs and leaves, is well known for its ability to repel insects. One eucalypt in particular has been know to alter its chemical defence in order to protect itself. In 2013, biologists discovered that a Yellow Box tree Eucalyptus melliodora in a sheep paddock in Yeoval New South Wales could change the smell of its leaves from one side to the other to protect itself against attack.
The Red Flowering Gum Tree (Corymbia ficifolia)
The Red flowering gum tree is a great street tree for your Melbourne garden. Avoid planting them under powerlines as some hybrids can eventually grow to 5 metres or more very rapidly. Fortunately there is a dwarf variety suitable for smaller gardens. Corymbia ficifolia ‘Baby Crimson’ will grow to around 3 metres tall. Insects, and birds like the rainbow lorikeet, will be attracted to the nectar rich flowers of al of the flowering gums. The eyes of the rainbow lorikeet are particularly sensitive to orange and red colours.
The colours of the Corymbia ficifolia vary from reds to oranges and are unpredictable if grown from seed. If you are looking for a particular colour ask your local plant nursery for a grafted plant that will guarantee the colour you want.
The flowering gum is also a great tree for attracting bees and other beneficial insects. Most honey in Australia is produced from the nectar of gum trees. The Corymbia ficifolia has been described as the greatest producer of honey known.
For your larger Melbourne garden take a look at the grafted hybrid varieties Corymbia Summer Beauty and Corymbia Summer Beauty. These plants are specially bred for your home garden by crossing the Corymbia ficifolia with the swamp bloodwood Eucalyptus ptychocarpa. The result is a tree better suited to the wet and humid Melbourne summers.
Landscaping with Gum Trees
Not all gum trees are forest giants. Varieties such as Eucalyptus gunnii “little boy blue” will fit in the smallest landscape garden. There are also some great dwarf varieties available for the smaller garden.
Eucalypts are evergreen trees, but tend to shed leaves, and also bark, continuously during year. For this reason it is best not to place them too close to your swimming pool when designing your poolside garden. Take a look at some screening plants instead.
Care of your Gum trees
There are no longer active volcanoes on mainland Australia. This ancient geology means that many native plants have evolved to survive in low phosphorus soils. The actual tolerance to phosphorus will depend on the part of Australia the plant originally comes from. To be on the safe side, use a fertiliser like Neutrog Bush Tucker for your Australian Native plants. For more care tips take a look at our Tree Planting advice.
The hardy and versatile Eucalyptus is widely used in commercial landscaping. Most species are very drought tolerant and require little maintenance. Growing indigenous species will help the local wildlife and avoid hybridising the local plants.
In commercial landscaping the large gum tree also provides shade for people enjoying the public space and for cars in the carpark.
Related Landscaping ideas from Red’s Landscaping and Design
For more information on Eucalypts