Weeds are introduced plants that reproduce or even proliferate unaided. Most weeds are exotic, however native plants can also be considered weeds if introduced outside of their natural range. In many cases it is not for many years, or even decades after a plant’s introduction that it is considered a weed as they tend to be recognised as such only when they have already spread.

Environmental weeds are plants that represent a threat to the conservation values of natural ecosystems.

Weeds can out-compete native plants for essential resources such as space, light, water and nutrients. As they invade native plant communities they cause a reduction in plant diversity with the loss of threatened species.

This alteration of plant communities affects animals when plants they depend on for food and shelter are replaced. Species that are already threatened are particularly vulnerable to weed invasion. Weeds can change the structure of a habitat, making it unsuitable for threatened animals and native wildlife in general and dense thickets of weeds impede movement of wildlife.

Weeds can alter landscapes by choking rivers and smothering grasslands. They can also alter fire regimes, in most cases increasing the frequency and intensity of fires. This can lead to the death of plants and animals and destroy essential habitat features.

Weeds are encouraged by disturbance of the natural environment, increased nutrient levels and the absence of predators. In tropical savannas, disturbance of the natural environment includes floods, alteration of the natural fire regime, over-grazing, extensive tree-clearing and changes in water availability.

Weeds - Hyptis sp.

Photo: © Gabriel Crowley

Weeds in the Northern Territory

The Northern Territory features some of the most extensive, unmodified natural landscapes in Australia. Of the rich flora of the northern savannas less than 10% is introduced (compared for example to 16 and 24% for New South Wales and Victoria respectively). Despite the comparatively good condition of northern savannas there are some significant weed issues. These include highly invasive introduced pasture grasses, aquatic weeds, prickle bushes and weeds of disturbed land. Environmental weeds in the Northern Territory are discussed briefly below by life form.

Trees

The Northern Territory has relatively few tree weeds, and none are currently deemed a direct problem for any threatened species. However, Athel Pine is a significant transformer weed, meaning it has the capacity to completely change the landscape. Athel Pine has spread through waterways in central Australia, where it interferes with stream flow and replaces River Red Gums. Neem is also a transformer weed which is rapidly dominating riparian zones in the Queensland Gulf and the East Kimberley regions. It is presently found only at scattered locations in the Northern Territory, but is a common environmental weed around Darwin. Neem has been cultivated for centuries for its medicinal and insecticidal properties. Its fruit are readily spread by birds. Both Athel Pine and Neem support a narrower range of wildlife than do the native trees they replace.

Neem tree

Photo: © Peter Duce

The enthusiasm for converting northern Australia into a carbon bank is likely to see increased pressure for more exotic tree plantations. It is probable that at least some of the trees introduced for this purpose will do well enough outside plantations to pose a threat to biodiversity. One of the favoured species for biofuel production, Physic Nut, is already a declared weed in Western Australia and the Northern Territory.

Exotic grasses

A handful of the hundreds of grasses that been introduced into Australia have become a serious problem for biodiversity. Most were chosen for being productive and nutritious for livestock and introduced with the intention of replacing native species. Few people would argue against the usefulness of exotic grasses within the confines of a grazed paddock. If introduced grasses were highly palatable at all stages of their life cycle, then they would be eaten down by livestock and not present a problem. However not all exotic grasses are palatable, or their palatability may fluctuate through their life cycle. So there are times when these grasses can grow out of control. Some problem grasses are not palatable at all - most of these came into the country accidentally or for use in gardens or erosion control.

Problematic introduced grasses also tend to be good breeders: Mission Grass produces prolific seeds; Gamba Grass has robust basal clumps that resprout after rain; Para Grass sends out long tillers that set down new roots in the mud; Buffel Grass has both a perennial root stock and abundant, wind- and animal-dispersed seed. All of these grasses are highly invasive. Due to their capacity to fuel extensive high-intensity fires that weaken and kill trees, these exotic, perennial grasses now constitute one of the most serious environmental issues facing Australia's tropical savannas.

Gamba Grass is a weed

Photo: © John Westaway

Grader Grass is an introduced, unpalatable annual grass that has invaded savanna woodlands and croplands in high rainfall areas, especially in Queensland but also on properties in the Katherine and Darwin regions. It has the ability to smother native plants and should be controlled and not spread further.

Because of their reproductive properties, exotic grasses often escape into the wider environment where they threaten biodiversity values. These grasses also reduce the ability of pastoral properties to conserve wildlife.

Prickle bushes

Another group of significant weeds in the Northern Territory is the thicket-forming prickle bushes. These include Mimosa and Prickly Acacia which are well established in threatened species habitat in the Top End. A number of other prickle bushes, including Mesquite and Parkinsonia, are also widespread and problematic but have not yet encroached on threatened species habitat. These bushes form dense thickets which out-compete native vegetation, use up valuable soil water and shade out native grass species. This results in a loss of ground cover and increased soil erosion.

Aquatic weed cover

Photo: © NRETAS

Aquatic Weeds

A number of aquatic plants have been introduced into Australian waterways, some of which have become invasive weeds. These weeds replace native aquatic plants and choke wetlands and waterways. This in turn prevents birds and other wildlife from using the habitat and reduces fish stocks by depleting oxygen levels in the water.

Examples include Alligator Weed, Water Hyacinth, Cabomba, Salvinia and the grasses Para Grass and Olive Hymenachne. They are aggressive invaders of waterways and swampy areas and most are regarded as Weeds of National Significance.

 Shrubs and herbs

There are also a number of herbaceous shrubs and sub-shrubs that can transform large areas of habitat. Primary among these is Noogoora Burr which occurs along parts of the Victoria River. Others to watch out for are Sicklepod, Malachra and Lion’s Tail, all of which have markedly expanded their distributions in northern Australia in recent decades.

This group includes a number of weeds, such as Hyptis, Rubber Bush, Snakeweed and Sida, which tend to be more of a problem in overgrazed or heavily disturbed areas. These weeds may be more an indicator than a cause of environmental degradation.

Siam weed

Photo: © Darryl Evans

Also within this group are species on the Alert List for Environmental Weeds, a list of 28 non-native plants that threaten biodiversity and cause other environmental damage. These include Barleria, also known as Porcupine Flower, and Siam Weed.

Barleria is an erect, prickly shrub that can form dense thickets in open woodlands due to its persistent and invasive nature. It has been found around townships in the Northern Territory including Darwin, Berry Springs, Katherine, Mataranka and in the Victoria River district. Siam Weed is another highly invasive plant. It is recognised as one of the world’s worst tropical weeds and is currently present in a few small infestations in far north Queensland. Although only in the early stages of establishment, these weeds have the potential to seriously degrade Australia’s ecosystems. Preventing spread of such species will protect savannas and riverine habitat of northern Australia.

Impacts on wildlife conservation

Wetlands and floodplains are particularly prone to weed invasion because of the regular disturbance caused by floodwaters, combined with the nutrients washed in from across the catchment. Weeds taking advantage of these conditions include a number of exotic grasses, prickly bushes, herbs, shrubs and trees. Couch Grass, Buffel Grass, Para Grass, Prickly Mimosa, Prickly Acacia and Noogoora Burr are identified problems for one or more of the Northern Territory's threatened species found in these environments.

There are also a number of threatened species for which weeds are a potential threat (particularly in association with pest animal disturbance), but for which no single weed menace has been identified. Native ferns found along moist sheltered streams in the Top End are particularly at risk of such weed invasions, either through direct replacement or by resulting changes to fire regimes.

General weediness can also cause the deterioration of animal habitat and reduce the abundance of many small animals, including insects. Even where this decline is not significant enough to class the affected animals as threatened, both reduced prey abundance and reduced visibility of prey could have a flow-on effect to threatened predators. These predators include the Arnhem Leaf-nosed Bat, Bare-rumped Sheath-tailed Bat, Red Goshawk and Masked Owl. For this reason, it is important to control any significant weed infestation, whether or not it has a known impact on a threatened species.

Control

Legislation in the Northern Territory places responsibility for weed control on the landholder or occupier. In addition some weed species are covered by an approved statutory Weed Management Plan (Click here). Weed Management Plans establish the management requirements that MUST be undertaken by land managers with respect to declared weeds. Species with Weed Management Plans in the Northern Territory are Prickly Acacia, Mesquite, Chinee Apple, Cabomba, Mimosa, Bellyache Bush and Gamba Grass.

Much effort can be invested in weed control but there are few demonstrated cases of a problem weed being entirely eliminated. So it is extremely important that a strategic approach is adopted to maximise the effectiveness of control efforts.

The first thing to do is to work with your neighbours and the wider community so that weeds removed from your country are not simply replaced through seeds washed or blown in from nearby infestations. Working collaboratively may provide access to financial support or weed management teams. So find out if there is a local Landcare or ranger group.

Buffel grass weed cover

Photo: © Chris Brock

Concentrate your efforts on priority weeds, based on severity of impact and ease of control. To benefit threatened species, focus your control on the weeds listed in this document. Remove weeds at the edge of any infestation, especially any outliers. Don't wait until the problem is too big to handle. Control of new weeds before they become a problem is particularly important.

There is no point in clearing up a patch of weeds once or twice, only to let it regrow. So get to know your weeds, where they grow, when they produce seed, and when they are most susceptible to pesticides. Plan a follow-up control program, regularly returning to check areas that you may think are now weed free.

Constantly assess what you have achieved. Map weeds or record their locations so that you know if they are expanding or contracting. Even recording areas that are currently weed-free can be useful down the track. Record the control measures that you have used, so that you can assess which is the most effective. If one approach is not working, try another.

The best control is prevention. So when choosing plants for your garden or for pasture forage, check whether they have weedy characteristics. They may already be on a list of prohibited or undesirable species. But there are signs to watch out for once you plant them. Check whether they spread uncontrollably from seeds or suckers. Do the birds feed on and spread the seeds? If it is hard to control in your own garden, especially if you are not pampering them with ample water or fertilizers, it is even more likely to become a weed in the bush.

Look out for plants coming up outside your property, focus on areas that are downstream and downwind. Even if they aren't in your area yet, familiarize yourself with Rubber Vine and Pond Apple which have had serious impacts on native riparian vegetation in Queensland. Look out for strange plants that could become a menace, and send them to the Northern Territory Herbarium for identification. Avoid use of hay from weed-infested areas.

 

Rubber vine

Photo: © Gabriel Crowley

Don't bring in trouble accidentally. Practising good weed hygiene is essential to avoid the spread of weeds, for example aquatic weeds such as Salvinia, Cabomba and Olive Hymenachne that are potentially devastating to wetlands. So when retrieving boats and trailers from waterways, take care that weed plants and seeds are not transported to other catchments. Wash-down your vehicle if you think you have been driving through areas where it might have collected weed seeds. When controlling weeds, ensure pesticides do not enter the waterway and avoid disturbing the river banks.

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