Weeds In South Australia
There would be very few environments left untouched by weeds in South Australia, from our vast dry deserts to our populated coastlines. Weeds affect everyone from primary producers to conservationists to the backyard gardener. South Australia has approximately 900 exotic plants, 90 proclaimed plants (landholders are legally obliged to control proclaimed plants) and an average of 6 new species naturalising every year. Weeds cost South Australian agriculture approximately $650 million/year and they also have a negative impact on biodiversity the cost of which is more difficult to estimate. The most effective form of managing weeds is to prevent them entering the State in the first place or by eradicating and containing new weed species before they have a chance to spread. You can do your part by ensuring that you do not buy known weed species from nurseries, removing weeds from your garden and disposing of them correctly and by not dumping garden waste in local bushland. For further information about weeds in South Australia read on.
Environmental Weeds in South Australia
Environmental weeds are plants that have a negative impact on our natural landscape, generally because of a reduction in biodiversity. Direct impacts occur when weeds out-compete native vegetation for water, light or soil nutrient. Indirectly, environmental weeds cause:
- loss of hollows or nesting sites.
- loss of food sources for wildlife.
- provision of shelter for feral animals that prey on native wildlife.
- change of soil or water nutrient loads, affecting native plant growth, and animal feeding requirements.
- change of fire intensity and frequency, affecting both native flora and fauna.
Environmental weeds can also disrupt water flow patterns, pollute water with leaves and cause erosion by eliminating grass cover.
Of the 105 species proclaimed as pest plants in SA at least half have a negative impact on the environment. These include:
African Boxthorn, Aleppo pine, Blackberry, Boneseed, Bridal Creeper, Bulbil Watsonia, Cape Broom, Cape Tulip, Dog Rose, Gorse, Olive and Willows.
For a full list of proclaimed plants, go to www.dwlbc.sa.gov.au/biodiversity/pests/weeds/plants_list.html
However many other environmental weeds are not proclaimed, such as:
Buffel Grass, Desert Ash, Erica spp., Fountain Grass, Hawthorn, Buckthorn, Native Bluebell, Polygala, Pussy-tail Grass and Sweet Pittosporum.
Many of the species on this list were introduced as garden plants and can still be bought from nurseries. Be aware when buying garden plants that some species can spread into the bush. If in doubt ask an expert.
Agricultural Weeds in South Australia
Agriculture in South Australia falls into three categories: Field cropping, Horticulture, and Livestock (grazing). All of these types of agriculture are affected by many different weedy species, with several species problematic across two or more types of agriculture.
Within field cropping, annual ryegrass (Lolium rigidum) is the major grass weed, with other examples such as wild oats (Avena fatua), brome (Bromus species) and barley grasses (Hordeum species). Examples of broadleaf weeds in field cropping are wild radish (Raphanus raphanistrum), turnip weed (Rapistrum rugosum) and capeweed (Arctotheca calendula). Other weeds such as thistles (Carduus spp.) and Vulpia species are problems in pastureland, while salvation Jane or paterson’s curse (Echium plantagineum) is a well-known example of a weed that is a problem across all three agricultural types.
Weeds impact both the quantity and quality of agricultural products. They affect quantity through yield loss due to competition, and by acting as alternate hosts to pathogens, viruses and insect pests. The presence of weeds can lead to increased effort needed during harvesting and indirectly cause crop damage with the application of weed control agents. Quality of the commodity is affected through poisoning or injury to stock, smaller size, poorer appearance and contamination of the crop.
The Weeds CRC has published a summary of the economic impact of weeds in Australia. They estimate that the annual cost of weeds to Australian agriculture exceeds $3.4 billion. The two main impacts are yield losses and costs of control to the farmer. These costs are felt across the whole community, with the primary producers bearing 81.4% of the costs and consumers bearing 18.6%. The best approach for long-term management of weeds is to take an integrated approach utilising several different methods. The major weapon against weeds since the 1950s and usually the first to be employed is herbicides. Different herbicides can be used at all stages during the lifecycle of weeds: beginning with inhibiting germination, controlling young seedlings and mature plants, and also applying herbicides at flowering to reduce weed seed set. In field cropping in particular, the intensive use of herbicides coupled with high weed density have led to herbicide resistance developing in several weed species. With the increasing adoption of no-till farming in South Australia, these systems are increasingly reliant upon herbicides for weed control, as most no-till producers have abandoned the primary weed control method of cultivation. Herbicide resistance is also a problem in horticulture where herbicides are often the only means of weed control used eg. under grapevines and in orchards. Physical methods of weed control are also employed in South Australian agricultural systems. Cultivation is very effective for stimulating emergence of weed seedlings which can then be controlled, and this is a tool employed in many horticultural and cropping situations that have not converted to no-till. Other physical methods of weed control include burning and physical cutting of the plants prior to seed set. Crop and pasture rotation is an important tool for disrupting weed lifecycles and allowing for more diversity in the types of control methods used. In the end, management of weeds involves the careful selection of control methods that balance the costs and benefits of their uses for the particular agricultural system they are to be employed in.
WoNS in South Australia
A WoNS is a Weed of National Significance. It is a plant species that is perceived to have sufficiently high invasiveness traits, potential for spread and detrimental economic, environmental and/or social impacts to be of national concern. A list of 20 WoNS was compiled by the National Weeds Strategy Executive Committee and endorsed federally in 1999. Each WoNS has a National Strategy and most have National Coordinators. All WoNS are proclaimed plants under the State's Animal and Plant Control Act (1986). For more go to www.weeds.org.au/natsig.htm
Ten of the 20 WoNS are naturalised in South Australia and another has been recorded here:
- Athel pine or tamarisk (Tamarix aphylla)
- Blackberry (Rubus fruticosus aggregate)
- Boneseed (Chrysanthemoides monilifera ssp. monilifera)
- Bridal Creeper (Asparagus asparagoides)
- Chilean Needle Grass (Nassella neesiana)
- Gorse (Ulex europaeus)
- Mesquite (Prosopis spp.)
- Parkinsonia (Parkinsonia aculeata)
- Prickly acacia (Acacia nilotica)
- Salvinia (Salvinia molesta)
- Willow (Salix spp.)
Athel pine or tamarisk (Tamarix aphylla): Athel pine is a medium sized wispy tree planted in a range of situations from coastal dunes to outback homesteads. In SA it is mainly a problem in the far north where it invades inland river systems. It takes up large volumes of water, drying up waterholes and drops salt-concentrated needle-like leaves, thereby inhibiting germination for other plant species. Environmental Agricultural (return to list)
Blackberry (Rubus fruticosus aggregate): This prickly scrambler is highly invasive in wetter climates and will form dense impenetrable patches in bushland, watercourses, pasture and plantations. It also provides shelter to feral pests such as foxes. The name “Blackberry” actually refers to 14 closely related taxa. Environmental Agricultural (return to list)
Boneseed (Chrysanthemoides monilifera ssp. monilifera): Another garden escapee, Boneseed is a South African shrub daisy that has become weedy in a variety of native vegetation habitats from coastal dunes to mallee. Its seeds can remain viable for decades, germinating prolifically after a fire. Environmental (return to list)
Bridal Creeper (Asparagus asparagoides): Introduced as an ornamental plant, this creeper invades temperate forests, woodlands, native corridors and orchards, forming a dense blanket that out-competes much of the understorey. The dense mat of fleshy tubers formed underground make it impossible for other roots to penetrate and difficult to kill with herbicides. One of SA’s worst environmental weeds, Bridal Creeper is one of 6 Asparagus weeds naturalised in SA. Recently a National Coordinator was appointed, based in SA, to implement the national strategy for this WoNS. Environmental Agricultural (return to list)
Chilean Needle Grass (Nassella neesiana): This grass and the closely related Texas Needle Grass are very similar in appearance to our native Speargrasses (Austrostipa spp.). Originally introduced as a pasture grass, their tussock form tends to dominate pastures. Each plant produces thousands of sharp seeds that irritate and injure livestock. These grasses also invade native grasslands and shade out many herbaceous species. In SA, mostly restricted to the Southern Lofty region. Environmental Agricultural (return to list)
Gorse (Ulex europaeus): Introduced as a hedge plant, this prickly shrub prefers the temperate higher rainfall areas of SA. It invades woodland and forests, reducing biodiversity and providing shelter for feral animals. In pasture it increases fire risk due to its flammability and its dense thickets reduce access and stocking rates. Environmental Agricultural (return to list)
Mesquite (Prosopis spp.): The name Mesquite refers to plants of the genus Prosopis originally from the Americas. Four of these are naturalised in Australia. P. velutina, P. glandulosa and their hybrids are the most likely taxa to establish in SA. These thorny shrubs and trees are spreading from other states into our rangelands. Mesquite forms impenetrable thickets that prevent stock access to watering holes, crowd out grasses valued for pasture and biodiversity, injure and poison livestock and provide refuge for feral pests. Environmental Agricultural (return to list)
Parkinsonia (Parkinsonia aculeata): Similar to Mesquite in form, habit, impact, distribution and origin. In SA it is currently limited to isolated infestations in the far north. Environmental Agricultural (return to list)
Prickly acacia (Acacia nilotica): A thorny tree from tropical and subtropical Africa and Asia, this plant prefers warm climates with 350-1500mm rainfall, but scattered infestations have been found in SA. Although introduced as a shade and fodder tree, its negative impacts are similar to Mesquite and Parkinsonia. Environmental Agricultural (return to list)
Salvinia (Salvinia molesta): This aquatic weed is a multi-branched free-floating fern with fronds that pack together in concertina fashion. It can grow rapidly on still or slow-moving water to form thick mats that choke waterways, shades out other aquatic plants and impedes oxygen exchange for aquatic animals. It also blocks irrigation, causes flooding, pollute drinking water, and prevent recreational activities. In SA it is occasionally seen in garden ponds but as yet has not established to any large extent. Environmental Agricultural Social (return to list)
Willow (Salix spp.): Most seeding willows are WoNS. Originally planted along riparian zones for shade and erosion control, they cause major damage to our watercourses by dropping leaves that cause nutrient toxicity to wildlife and disrupting the water flow with their roots, thereby actually causing erosion instead of reducing it. They can spread by seed or fragments of twigs or branches, forming dense thickets that replace native vegetation. Environmental (return to list)
To find out more about these weeds visit www.deh.gov.au/about/publications/list.html#invasive
Around 65% of Australia’s weeds were originally introduced as garden plants. Over time they escaped from gardens, by such means as:
- birds eating fruits and defecating seeds
- dumped garden waste (plant material and soil)
- wind-blown seeds
- seeds or vegetative propagules (e.g. bulbs) washing along drains and waterways
- seeds getting stuck on clothing, animals, tools and vehicles.
Many of South Australia’s most serious weeds, such as bridal creeper (Asparagus asparagoides), salvation Jane (Echium plantagineum), cape broom (Genista monspessulana), cape tulip (Moraea flaccida) and blackberry (Rubus fruticosus agg.) have all originated in SA gardens.
Garden plants with weed potential are still being sold at nurseries, markets and street stalls. Common examples are olives (Olea europaea), mirror bush (Coprosma repens), periwinkle (Vinca major), gazania (Gazania spp.) and fountain grass (Pennisetum setaceum). To help solve the problem you should avoid known weedy species in your area and ask about the weed potential of new species before you buy. Rapid vegetative spread, berries eaten by birds and extreme hardiness are some of the warning signs to be aware of. The Weeds CRC and SA Garden Escapes Committee produced a factsheet titled “Alternatives to invasive garden plants” in 2004. This lists 68 invasive garden plants and suggests less invasive alternatives of similar appearance or function. The factsheet is on the Weeds CRC website at www.weeds.crc.org.au/publications/fact_sheets.html and hard copies are also available.
Native Plants as Weeds
Most of our weeds are introduced from overseas, but some are Australian native plants that are growing outside of their natural or pre-European range. Just like their exotic counterparts, these plants become weedy when they encounter a new environment that lacks the usual herbivores, fungal diseases, insects or climatic constraints that keep them in check.
Many of these have escaped from gardens, but some have spread from ill-conceived revegetation projects, such as windbreaks or shelter belts. In SA the most common group of natives to go weedy is the wattles, but also Hakeas, Grevilleas and Eucalypts.
Examples of weedy native plants in SA include:
Coastal Wattle, Cootamundra Wattle, Golden Wreath Wattle, Native Bluebell, Pincushion Hakea, Rosemary Grevillea, Silky Tea-tree, Sugar Gum, Swamp Sheoak (Casuarina glauca), Sweet Pittosporum and Tasmanian Blue Gum.
Weed Management Initiatives in SA
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