Article by Theresa Malin, natural areas offset support officer, Sunshine Coast Council
In recent years there has been a noticeable increase in the occurrence of the introduced species Asian melastoma (Melastoma candidum) on the Sunshine Coast. The presence of this invasive garden escapee is a growing source of concern, as it is easily confused with the native blue tongue (Melastoma malbathricum), and so often introduced by people who unknowingly plant a weed instead of a native. Gladly there are some clear defining features that once known, make it easier to tell them apart.
What are the benefits of native blue tongue?
Of approximately 70 species of melastoma occurring in India, Southeast Asia, Malaysia and Indonesia, blue tongue is the only melastoma to naturally occur in Australia. They play an important role in the vertical structure of natural areas and provide valuable habitat in the shrub layer. Whilst producing no nectar, the blue tongue’s attractive purple flowers produce large quantities of pollen that provide a food source for bees and other native insects. Their fruit is eaten by native birds, which is the main method of seed dispersal. The roots of the blue tongue have also been found to accumulate aluminium in soil.
A known bush tucker source, the purple fruit of native blue tongue is sweet and turns the tongue a purple-blue colour, hence its name. Different parts of the plant have been used throughout the world for their medicinal properties including remedies for wounds, toothache and stomach ache.
Why is Asian melastoma harmful to our natural areas?
Cultivated as an ornamental shrub and readily spread by birds, Asian melastomas have become a problematic weed in tropical and sub-tropical areas including Australia, Southeast Asia and Hawaii, (where it’s possession or sale is illegal). More robust and readily adaptable than its native counterpart, they are capable of forming dense thickets that outcompete native species to reduce overall species diversity in a variety of habitats including eucalypt forests, open land and wet lands.
Additionally, Asian melastoma have been found to be capable of readily hybridizing with blue tongue, meaning that the seedlings that they produce are cross-breeds. Prolonged periods of hybridization between these two species could result in the Asian melastoma out breeding the blue Tongue and reducing overall genetic diversity of the native.
How can I tell the difference between blue tongue and Asian melastoma?
There are several distinct characteristics that allow us to accurately identify the melastoma, as outlined in the table below.
|Melastoma malbathricum (previously M. affine)
|Melastoma candidum (also known as M. septemnervium)
|Pink (also a white form), 5 petals approx. 20-30mm long
|Deeper purple, five petals, approx. 25-32mm long
|3 longitudinal veins with 2 distinct intramarginal veins (see picture below). Leaf thinner and hairs shorter than introduced.
|5 longitudinal veins with 2 less distinct intramarginal veins (see picture below). Leaf is thicker and hairs are longer than native.
|Purple blue in colour
|Bright raspberry red in colour
|Purple or white with short hairs (see picture below)
|Purple or white with long hairs (see picture below)
|Usually grows to 1-2 metres
|Grows to 2 metres
|Sparse and 'scrappy'
|Robust and dense
Asian melastomas are often unknowingly sold in nurseries as blue tongues, so it is best to check that the plant is in keeping with the diagnostic features listed above. As immature leaves of Asian melastomas sometimes present as those of blue tongues, it is important to examine several mature leaves to confirm the number of veins present. Only the introduced species will have five distinct longitudinal veins. If you are unsure and would like a plant sample positively identified, contact the Queensland Herbarium or your Land for Wildlife officer.
What can be done to control Asian melastoma?
Landowners on the Sunshine Coast can have a great effect on reducing the spread of Asian melastoma before they become uncontrollable, such as in Hawaii where they have resorted to the release of several moth species as a biological control agent. Once you have positively identified an Asian melastoma, it is recommended to pull the younger plants by hand where possible. Techniques for chemical treatment of mature plants should follow the conditions for woody weeds provided in the Australian Pesticides and Veterinary Medicines Authorities permit for use of chemical product for control of environmental weeds.
Whether you plan on removing just one Asian melastoma from your garden or many, it’s important to consider the impact that this will have on surrounding native flora and fauna. Considerations should include what role the melastoma plays in providing habitat and a food source, erosion control, and canopy cover. Aim to progressively replace weed species with local natives that perform the same function, in this case a small fruiting shrub that produces a large amount of pollen. A tool that may help you find the most suitable replacement is GroNATIVE, a free app designed for south-east Queensland that provides a list of local natives tailored specifically to your postcode.