Deeper Read – Environmental weeds

A common definition of a weed is ‘a plant growing where it is not wanted’. When we think of bushland or remnant native vegetation, a plant that is not a local original plant is considered out of place. The occasional non-locally native plant growing in remnant vegetation is not a problem, however many of these plants spread and invade remnant vegetation and can be considered environmental weeds. 

Plants can be weeds in terms both of agriculture and remnant vegetation, but many are specific to either one or the other. Some plants are desirable in one situation but a weed in another — Bracken Fern (Pteridium species) belongs in native vegetation but when it moves on to farmland it is considered an agricultural weed. Phalaris is a desirable farm plant, but also an environmental weed when it invades native vegetation. So, it is often the situation or context that determines a plant’s status, not the plant itself. 

Apart from actual clearing of native vegetation, environmental weeds and grazing are the greatest threats to the long-term viability of remnant vegetation in agricultural areas. As Stella Humphries found in the 1991 CSIRO study on environmental weeds, ‘weed invasions pose one of the most serious nature conservation problems in Australia’. 

Environmental weed invasion is an insidious problem, not unlike salinity. Many people see gum trees and assume that all is well in the bush. Few people can identify understorey plants (shrubs, grasses, herbs) and often we are not aware that many of our ‘understorey plants’ are weeds. Some environmental weeds, such as Cape Broom (Genista monspessulana) and Madagascan Fireweed (Senecio madagascariensis), could be mistaken for native plants. 

Native plants can also be environmental weeds, such as Cootamundra Wattle (Acacia baileyana), Western Australian bluebell Creeper (Billardiera heterophylla) and Crimson Passionflower (Passiflora cinnabarina), when they grow outside their home territory.

Environmental weeds take up the physical space where locally native plants would normally grow, hence reducing the numbers of such plants by crowding them out. There are often few natural predators of weed species to keep them in check. Environmental weeds also have many other detrimental effects that may be not so obvious. 

Environmental weeds affect regeneration of locally native plants by changing the environmental conditions — providing more shade and humidity; producing chemicals that inhibit germination of other plants; producing different leaf litter; and using up nutrients and moisture. 

They alter the balance of fauna by favouring some species, particularly introduced animals. When weeds reduce the number of locally native plants, they reduce the food supply for animals that rely on these plants. Animals play a large part in the health and regeneration of native vegetation, for example by keeping insects in check, pollinating plants, spreading seeds, and helping leaf litter to decompose. It is often rare species that are most affected, as they tend to have specific needs and don’t have the capacity to use other food or resources. Many changes are probably occurring on a level that we do not notice. The consequences of such changes are yet to be discovered. It is not easy, for example, to notice what is happening to insect and reptile populations, yet they have a direct relationship to the health of remnant vegetation. 

Weeds can change the way our natural systems work. They can increase the risk of fire or change the temperature of burning, for example wild oats (Avena fatua) and African lovegrass (Eragrostis curvula) may burn hotter than smaller, greener native grasses. Much of the vegetation in this area is closely associated with fire and depends on particular fire regimes for its survival. Weeds can change water flows. For example, changes in creek courses are often caused by willows. 

Weeds simplify ecosystems by reducing their diversity. Diversity of structure (provided by trees, shrubs and groundcovers) and diversity of species is desirable for the health of remnant vegetation. 

Native environmental weeds such as Cootamudra Wattle (Acacia baileyana) [https://weeds.org.au/profiles/cootamundra-wattle/] can hybridise with other local species and cause geneticc contamination in ocal populations. So far this has been seen in Silver Wattle (Acacia dealbata) and Early Black Wattle (Acaia decurrens), but it has potential to hybridise with many more species and subspecies. 

There are various methods through which weeds are spread. Seeds are transported in soil (particularly along roadsides by heavy machinery), in car tyres, and in soil used for road construction). Disturbing the soil encourages seeds to establish. Often invasive plants are planted near remnant vegetation due to lack of understanding of the threats or consequences. Dumping of garden waste spreads seeds, bulbs and cuttings. This method of spreading appears to have increased since garbage tips began charging for rubbish disposal. Approximately 70% of new invasive plants are garden escapees.

Birds have a high potential for spreading weed seeds great distances when they eat the ripe fruits of plants such as privet (Ligustrum spp), Cotoneaster (Cotoneaster spp), Hawthorn (Crataegus monogyna) and Prickly Pear Cacti (Opuntia spp). In addition to this, mammals such as pigs, foxes and deer will eat Blackberries (Rubus fruticosus), Plums (Prunus spp) and Briar Rose (Rosa rubiginosa) fruits. Animals will also spread seeds which attach to their coats such as Chilean Needle Grass (Nassella neesiana), Paspalum (Paspalum sp) and Bathurst Burrs (Xanthium spinosum). 

Any plant with a small or medium sized berry (e.g. Privet, Ligustrum species, Hawthorn (Cretaegus monogyna), and Cotoneaster (Cotoneaster spp) has the potential to spread by birds. Foxes, deer and pigs are known to spread blackberry seed in their droppings after eating the fruit. Seeds are carried on the coats of animals. Water and wind are seed dispersers,  plants in the Asteraceae family such as fleabanes, hawkweeds and dandelions (Taraxacum spp) are spread by wind and Common Thornapple (Datura stramonium) and Caster Oil Plant (Ricinus communis) are frequently spread by water throughout riparian areas.

A plant can be desirable in the farm or garden but become an environmental weed in native vegetation nearby. Phalaris is a serious environmental weed of remnant vegetation on roadsides and creeks, due to its invasive nature, but will continue to be planted because of its qualities as a pasture grass.  We need to manage these types of situations for the benefit of both farmland and native vegetation. If a Phalaris paddock is next to remnant vegetation, a buffer area should be left. This could be planted with a shelter belt or farm forestry and managed so that the Phalaris does not invade. Buffer zones have protection value, as well as many other benefits, such as providing habitat, preventing erosion and keeping chemicals out of the water.

Tree Lucerne (Tagasaste) is another plant promoted for farm planting but is a serious weed when it invades native vegetation. If thought is given to this in the planning stage, Tree Lucerne can be planted away from remnant vegetation. Pruning before seed set will reduce the chances of it becoming invasive and will also keep the plants in a useful form for grazing. Willows (Salix species), widely planted along watercourses for stabilising banks, have become the dominant feature of many watercourses at the expense of native vegetation.

While the environmental weeds problem was earlier compared to salinity, it is much easier to rectify than salinity if environmental weeds are tackled while there is still remnant vegetation. However, the sooner environmental weeds are tackled, the better the chances of survival for native vegetation, as the remnant patches continue to degrade at an ever-increasing rate. 

Because our native vegetation has developed in a land of disturbances and unpredictability, particularly fire and drought, it has a significant capacity to regenerate naturally. Given some assistance, particularly the removal of environmental weeds, native vegetation tends to regenerate. There are a number of other forms of assistance that are used successfully, such as fire, direct seeding and channelling of water.

Weeds tend to invade native vegetation when it is disturbed, so avoid disturbing it — keep to tracks, keep heavy machinery use to a minimum, don’t overgraze, and don’t over-burn. Use weed removal techniques that require the least disturbance (e.g. cutting and poisoning and injecting trees rather than bulldozing — see Figure 2). 

.Remove environmental weeds from native vegetation before they have become established. If they have set and dropped seed, more seedlings will germinate, requiring removal. If weeds can be removed before they drop seed, the work needed to restore the native vegetation will be greatly reduced. 

Remove environmental weeds from gardens or farm plantings. 

Find out about bush regeneration, this is particularly important if an area is heavily infested with weeds. 

Become more familiar with locally native plants so that weeds will be recognised.

When planting windbreaks and shelter belts check that you are not planting environmental weeds. Planting locally native species is safest and has many other benefits. 

When planting near bush use locally native species from locally collected seed, to avoid altering the gene pool (i.e. ‘genetic pollution’). 

Weed invasion is usually worst around the edges of remnant vegetation. Try to minimise the length of the edges of areas of remnant vegetation (blocks have less edge than strips), and join isolated patches. 

This list is not exhaustive and any non-local native plant showing invasive tendencies in remnant vegetation should be considered an environmental weed for the particular area. Some plants have shown very invasive tendencies in other parts of Australia and although they are not a problem in this region, it is thought that their potential is such that they should not be planted here. Athel Pine (Tamarix aphylla) is one such plant.

Phalaris Phalaris aquatica 

Paspalum Paspalum dilatatum 

Wild Oats Avena fatua 

Bridal Creeper Myrsiphyllum asparagoides

Paterson’s Curse Echium plantagineum

Horehound Marrubium vulgare 

St John’s Wort Hypericum perforatum 

Shell grass Briza maxima 

Silver Grass Vulpia species

Barley Grass Hordeum leporinum

Bathurst Burr Xanthium spinosum

Brome Bromus species 

Find out more…

https://www.lls.nsw.gov.au/regions/murray/articles,-plans-and-publications/priority-weeds-of-the-murray-and-riverina-regions-identification-guide

The Regional Weed committees for the Murray and Riverina LLS regions determine the priority weeds, weed status and control strategies for  weeds for the region and can be accessed here:

https://www.lls.nsw.gov.au/__data/assets/pdf_file/0004/722632/Murray-Regional-Strategic-Weed-Management-Plan-2023-2027.pdf

https://www.lls.nsw.gov.au/__data/assets/pdf_file/0007/722446/Riverina-Regional-Strategic-Weed-Management-Plan-2023-2027.pdf

https://weeds.dpi.nsw.gov.au/

https://www.dpi.nsw.gov.au/biosecurity/weeds/nsw-weedwise-app

Buchanan, R. 1989, Bush Regeneration — Recovering Australian Landscapes, TAFE Sydney, NSW. 

Monument Hill Parklands Association Inc., Albury Wodonga Field Naturalists Club Inc. 1997, Along the Bush Tracks — Albury Wodonga, Albury Wodonga Regional Parklands, Albury. 

Humphries, S.E., Groves, R.H. & Mitchell, D.S. 1991, ‘Plant invasions of Australian ecosystems, a status review and management directions’, Kowari vol.2, pp 1-134. 

Lamp, C. & Collett, F. 1979, A Field Guide to Weeds in Australia, Inkata Press, Melbourne. 

Wright, P. 1991, Bush Regenerator’s Handbook, National Trust, Sydney. 

(Groves, R H, Boden, R, Lonsdale, W M. (2005) Jumping the garden fence: Invasive Garden Plants in Australia and their environmental and agricultural impacts. WWF-Australia.)