Prickle bushes in the Kimberley can be very hard to properly identify due to their similarities with one another and the fact that some are only found in one or two isolated areas.
To help with their identification we’ve put the spotlight on the significant prickle bush weeds common to the Kimberley. Some are declared in Western Australia and are eligible for our weed subsidy and one is naturalised to the region.
One thing that is apparent is that weed names tend to change from location to location (see table) and also the physiology of the plants tends also to change as a consequence of environmental constraints. As an example, mimosa bush in some parts of the West Kimberley has a structure and spine size that makes it look much like the prickly acacia found in the East Kimberley. Mesquite in one location can have a thorn that will stake the sidewall of a tyre, but elsewhere the thorns can be small or completely absent. It’s no wonder working out what is what and where can be difficult.
This is by far the most common of the two species in the Kimberley. It’s common right across Northern Australia including the Pilbara and down into parts of NSW. It is believed to have been introduced to Australia before European settlement, from Central America (where it is native) via the Philippines.
As it’s so widespread it’s not a declared plant. In thickets it can make mustering difficult so is controlled in some situations but in harsher times it is also considered to be of some value as stock feed. Cattle will strip the plants of bark, leaves and the seedpods. For identification purposes both mimosa bush and prickly acacia look fairly similar, they’ll both grow that vase shaped structure, the yellow flowers are similar and both have thorns. Mimosa bush tends to have the smaller thorns of about 10 - 15mm in length and quite slender but I’ve seen then up to 25mm long and quite thick at their base. The clincher for identification is the seed pods, see images here to note the difference.
Mesquite is both a WoNS and Declared in WA. The DPIRD website tells us that mesquite was planted on many north-west stations in the 1920s for shade and for their nutritious pods. The original plants were spineless and showed little tendency to spread. However, within a few years trees reverted to wild types with spines and weedy tendencies spreading rapidly after a series of wet seasons. They now infest many square kilometres of grazing land, especially in the Pilbara region and other parts of northern Australia.
According to the Pilbara Mesquite Management Committee Mardie Station in the Pilbara houses the single largest infestation of mesquite in Australia, covering 2/3 of the 225,000 hectare pastoral grazing lease. The population is comprised of a hybrid mix of species, which has given it extra robustness in surviving in the semi-arid conditions of the Pilbara.
In the Kimberley there are four areas of known infestations, all of them currently are being controlled and density levels as a consequence are low. These areas range from the coastal plains south of Broome to the NT/WA border east of Halls Creek. The Fitzroy River has two of these infestations along its length. The East Kimberley infestation appears to be a different species to the remaining locations, all tend to be moderately invasive at this point in time.
Identifying mesquite is a mixed affair, easy to spot from the air when mixed in with native melaleucas but extremely difficult when mixed in with similar looking plants such as mimosa bush. Fortunately, at least in some locations a biological control named Evippe spp. (leaf-tying moth) that causes the defoliation of the leaves and can prevent flowering is present.Its presence makes it relatively easy to distinguish mesquite from the mimosa bush it grows amongst, at least when the moth is actively feeding on the plant. The moth has existed in the East Kimberley infestation for a number of years but was also found a couple of months ago in the infestation at the mouth of the Fitzroy River close to Derby
Three tell tail differences that seem consistent are the size and shape of the seed pod, the flowers (see images), additionally mesquite seems to have more slender upper and outer branches that are influenced by wind or a chopper downdraft to a greater extent. The latter is not a lot to go on but if it makes you look twice other differences dependant on local variations in either the mesquite or mimosa bush hopefully will become noticeable.
Control is nothing special. We have used a D6 to good effect on large trees in the past, pushing them over and raking out the roots. For chemical control basal bark spraying or the application of residual herbicide granules are both pretty effective provided in terms of the latter you get the rainfall required to wash the chemical into the soil profile.
If you’re unsure about the identification of anything you might suspect of being mesquite take a photo of the plant, if there are flower or fruit get a shot of them as well and send them though to us at dick@krbg.com.au or your local DPIRD biosecurity officer, all are pretty active in the business of identifying and killing weeds.
From a ground perspective it also can be somewhat deceptive until you get your eye in particularly in amongst mimosa bush. The larger the tree the easier it is to get a feel for the differences. Spine size and configuration is often slightly different and of course both flowers and fruit if you’re lucky enough to find them are very different between the two species of plants.
Prickly acacia is native to the tropics and subtropics of Africa and Asia. It was introduced into Queensland for shade and as a fodder crop. Infestations favour water courses where trees can spread out onto adjacent grasslands (see image). As a tree increases in size it outcompetes pasture for moisture, its thorny thickets interfere with mustering, movement of stock and access to water.
Prickly acacia was first found in the Kimberley in 2003. The infestation covers about 10,000ha and has been the subject of control for about 15 years with the KRBA becoming involved financially in 2009 and operationally in 2017. This is the only known active infestation in WA.
As mentioned overpage seed pods are the only really reliable to way to tell the difference between prickly acacia and mimosa bush on every occasion, however thorn length can be a good indicator in some circumstances.
From the images below and you can see that whilst the double thorn emanating from one growing point is common across both species the thorn size when compared can (but not always) be very different. The acacia thorn is usually between 25 and 40mm long, the one pictured below is actually 50mm. The longest mimosa thorn I’ve seen has been only about half that length.
Control is simple, small ones can be grubbed out and chemicals can be applied by basal bark spraying or spreading residual herbicide granules. All are very effective provided in the terms of the latter you get the rainfall required to wash the chemical into the soil.
Whilst control is routine a major threat the plant presents is its seed longevity. If a plant seeds you could be revisiting the site regularly for at least 15 years to deal with the subsequent germinations. Miss a year at the wrong time and you reset the clock back to zero as plants can produce seed in under two years.
We collect and destroy any seed pods found on site to reduce regrowth and reduce the spread by cattle as the seed is palatable to stock, though somewhat bitter.
On that basis we’re keen to hear from anyone who thinks they may have prickly acacia on their property. If you’re unsure about the identification of anything you suspect of being acacia, take a photo of the plant, if there is fruit get a shot of that as well and send it through to us at dick@krbg.com.au or your local DPIRD biosecurity officer, all are pretty active in the business of identifying of killing weeds.
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