National Eucalypt Day is on Friday 23 March 2017. Events and celebrations will be taking place around Australia. 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 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. The bark strips dropped by the blue gums are flammable, which can lead to intense fires. Although the oils in the eucalyptus are flammable, so are the oils in pine trees. 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.
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. 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 Gold Coast will be familiar with the smell. This tree 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.
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.
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. Eucalypts are evergreen trees, but tend to shed leaves, and sometimes 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 blog.
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