Weed management

Weeds are considered to be any plants, native or exotic, that are growing where they are not wanted. However, there are also specific regulations around specified invasive plants as they may be prohibited or restricted in Queensland. In terms of aquatic ecosystem management, vegetation has many benefits, such as providing habitat and increasing the strength of the substrate. There are situations where vegetation has negative implications. Examples include where vegetation has grown more abundant compared to its reference condition or its establishment has changed the boundary conditions such as the strength or roughness of the river channel. The consequences of these can be that native vegetation is outcompeted or that the rates and location of erosion and deposition are altered. All these impacts can result in a loss of habitat.

The Queensland Government provides information on a range of weeds in Queensland and fact sheets on selected weeds species.

Other general information on weeds and their management is available from sources such as the Commonwealth Government's list of Weeds of National Significance.

Landowners are responsible for taking all reasonable and practical steps to minimise the risks associated with invasive plants under their control. This is known as the general biosecurity obligation (GBO). Your local government has a biosecurity plan that outlines everyone’s role and responsibilities in managing specified invasive plants.

Abundant growth of native or invasive species can often occur in situations where the nutrient status or flow regime of the water has changed. The hydraulic roughness characteristics of the vegetation need to be considered as well as its spatial arrangement. Vegetation that becomes prone during floods may have less of an effect on the flow and sediment than vegetation that remains upright. Equally, patchy vegetation along or in the channel may be less of an issue compared to growth across the whole channel cross-sectional extent.

The growth form of weeds is also important and may impact beneficiaries of the river. Submerged weeds can impact pump infrastructure whilst floating or emergent weeds can impact recreational use and amenity. The complete life cycle of the vegetation, and its succession, needs to be considered. For example, an increase in leaf area cover can alter the amount of sunlight reaching the water, and its subsequent death can increase organic matter in the water changing the water quality.

Examples of different weeds that have very different locations, forms and subsequent effects are:

• Water hyacinth  (Eichhornia crassipes) infestations that can change the instream conditions. It can spread extremely rapidly, with an infestation doubling in size every week where growth conditions are optimal. Its effects are worst in stagnant water where it increases water loss by transpiration and reduces instream oxygen, eventually killing native fish and other wildlife. It has social implications where swimming, boating and fishing may all be impeded.
• Pond apple  (Annona glabra) is a weed of national significance that occurs in north Queensland creating dense thickets in riparian areas and mangroves. It is an example of a woody pioneer species that can establish after flooding.
• Invasive Aquatic Pasture Grasses, notably olive hymenachne (Hymenachne amplexicaulis), para grass (Urochloa mutica), and Aleman grass (Echinochloa polystachya) can drastically alter the structure, composition and ecological processes of wetlands and riparian zones and require ongoing management of the pressures to natural wetlands.

Weed control can be conducted within all zones of a river system including terrestrial riparian areas adjoining river channels, exposed streambank and streambed within the channel and instream aquatic areas. The channel morphology and weed location can help guide the appropriate weed control actions. Aquatic weeds can include submerged or floating plants and emergent plants on the wet channel margin. Aquatic weed control is generally quite distinctive to terrestrial weed control. There may be greater sensitivity to issues of water quality and biota impacts, chemical use, access constraints, and reinfestation potential associated with water borne propagules from upstream source areas.

The main methods of weed control include:

• manual control using hands and hand tools
• mechanical control using various forms of machinery
• shading and other forms of exclusion such as matting
• chemical control using a range of potential herbicide application methods such as injection or spraying
• biological control using plant pathogens or insects.

Fire, such as controlled burns, can be used to kill both the weed and its seeds, reducing reinfestation. Most species of invasive aquatic grass weeds, including Hymenachne and Para Grass, are fire sensitive. In seasonally dry climates the introduction of controlled burns in riparian areas can help to both control grass weed infestation levels and promote native macrophyte species. These burns require careful management so they do not end up out of control.

Crash grazing, where stock are allowed onto the site for a short timespan to limit trampling and other impacts, may be a control strategy. Grazing strategies can be put into a farm management plan so that assisted revegetation is not adversely impacted. Environmental flows may be used to change the water regime and water quality to remove/reduce weeds and promote native vegetation.

Removing vegetation from within a river channel presents a range of potential impact risks including generating channel or bed instability, water quality degradation and habitat loss. These risks need to be assessed alongside the possible reinfestation of the site. If the cause of the infestation is external to the site, such as a changed flow or sediment regime, then this needs to be addressed.

Potential benefits from this intervention:

• Improves habitat values, environmental quality and natural ecosystem function of rehabilitated site.
• Site works can generate benefits beyond the site (i.e. reach or catchment scales).
• Can increase dissolved oxygen levels, native predator abundance, aquatic habitat connectivity and instream macrophyte establishment.
• Improves site amenity including aesthetic and recreational values.

Potential negative implications from this intervention:

• Can cause site impacts including bank instability/exposure to soil erosion, reduced vigour or death of non-target species, fire risks, water quality degradation and habitat loss.
• Native vegetation, either naturally regenerated or planted, will take time to establish and so may leave the site depauperate in certain habitats.
• May create opportunities for secondary weed infestations including additional weed species.
• Longer term site maintenance needs.
• Site amenity including aesthetic values may be impacted.
• Chemical/herbicide use has both environmental and social impacts that must be considered and addressed.

Intervention considerations:

• Seek appropriate specialist advice and check legal obligations (e.g. permits).
• Consider if the weed infestation is significant enough to warrant control. For instance, determine if it is impacting site rehabilitation or if there is sufficient regenerative capacity within the remnant vegetation or other active natural processes (e.g. flood, fire) that are operating to independently reduce the weed infestation.
• Consider if weed infestations are providing valuable ecosystem roles (e.g. fauna habitat, erosion protection, water quality regulation).
• Ensure the weed control complements other rehabilitation activities. For instance use of broad-spectrum herbicides for grass control can also kill young trees and shrubs, so selective herbicides can help in passive revegetation.
• Consider if there is merit in staging the extent or scale of vegetation removal over an extended time period.
• Development of a site weed control program should occur, which should assess:
  • the weeds and areas within a site which are the priority for control
  • the scale at which control should be implemented
  • the timing and number of control interventions
  • site maintenance needs
  • successful control condition objective
  • required management resources
• Determine the environmental sensitivities at the site and how can potential site impacts associated with weed control can be managed.
• Consider seasonal climatic risks and/or potential benefits (e.g. rainfall, flood, dry season fire) that may impact the success of weed control.
• Determine the reinfestation potential and reinfestation source areas at the site, and how these can be managed.
• A selective removal approach with a long-term plan or rolling program may be appropriate.
• Safety of volunteers and employees including seasonal exposures (e.g. heat) and high risk areas (e.g. crocodile presence in waterways).

Additional information

Publications:

Chenoweth EPLA and Bushland Restoration Services. 2012. South East Queensland Ecological Restoration Framework: Manual. Prepared on behalf of SEQ Catchments and South East Queensland Local Governments. Brisbane.

Links:

Queensland Biosecurity Strategy 2024–2029

Wetland weeds

Prohibited invasive plants

Queensland Agriculture Using trees to control weeds- passive revegetation - YouTube

Queensland Government Weeds and diseases information

Weeds Australia

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Last updated: 28 June 2022