Weed eradication is an ambition unlikely to succeed. It’s more about managing their impact. Environmental weeds must be treated as a symptom of a problem and not just the cause; the more fundamental problem is usually habitat disturbance or invasion from neighbouring areas and the most successful approach is to change the conditions under which weeds thrive. In urban areas, weeds must be thought of in the light of the continuing disturbance. Information about the weeds on site drives the planning process.
Brown, K., & Brooks, K., 2002. Bushland Weeds: a practical guide to their management, with case studies from the Swan Coastal Plain and beyond. Environmental Weeds Action Network (Inc), PO Box 380 Greenwood, 6924, Australia.
General characteristics of weeds
Mechanisms which weeds have developed to cope with stresses and exploit the opportunities of disturbance include:
- Recognition of disturbance conditions such as:
- The absence of competing vegetation;
- Either insufficient or excessive levels of major nutrients (nitrogen, phosphorus, and potassium) can give certain weeds a competitive advantage;
- Many weeds grow and thrive in the absence of a healthy soil microbial population.
- Rapid seed germination in response to light, wide fluctuations in soil temperature or moisture, high levels of soluble plant nutrients or other stimuli that indicate recent soil disturbance. [Many small weed seeds react to particular parts of the light spectrum. Direct white light (sun or moon) stimulates germination but a closed canopy which gives a green light will inhibit germination. For many weeds, plant growth is also inhibited under a closed canopy.]
- Relatively rapid growth and formation of mature seeds, vegetative propagules, or both;
- Prolific seed production;
- Seed characteristics that promote wide dispersal, such as small size, ability to pass unharmed through animal digestive tracts, burrs that attach to fur or clothing, and feathery structures for wind dispersal;
- Seed dormancy mechanisms;
- long term survival of soil seed until conditions are favourable for germination;
- Ability to take up and utilize large amounts of soluble nutrients;
- Ability to regrow or reproduce from small fragments of root, rhizome (underground stem), tuber, or other underground structures;
- High tolerance to stresses, such as low or excessive levels of certain nutrient elements in the soil; drought, waterlogging, or temperature extremes; or repeated grazing or mowing.
What weeds are on site?
Identify the weeds present. Map the distribution of major infestations. Correct identification is essential for effective weed control. Many weeds look uncannily like native species so if you are not sure what it is, leave it and monitor it.
You might like to keep a herbarium or photographs of your common weeds and native plants to aid plant identification skills.
Monitor regularly throughout the year and keep records on what weeds emerge at different seasons, and on efficacy of any preventive and control measures taken. This will guide future management.
Determine and map weed abundance and distribution.
- How long do individual plants live?
- With annuals and biennials, prevention of seed formation is important;
- For perennials prevention of sprouting is important. Wandering perennials reproduce by one or more types of vegetative structures such as stolons (prostrate stems along soil surface), rhizomes (below-ground, root-like stems), bulbs, and tubers, as well as by seed. They can be the most difficult to control;
- What is the mode(s) of reproduction? (seed, sucker, vegetative);
- How long before seedlings become reproductive and start to replenish the seed bank?
- What is the seasonal pattern of development and flowering?
- How long will it take for the seedbank to be depleted once adult plants are removed from the site that is how long can the seed remain dormant in the soil?
- How and how far is the seed dispersed?
- What is the frequency and scale of seedling recruitment?
- What is the stage of the plants life cycle which is most vulnerable to control? What are the stresses to which it is sensitive?
Does the weed perform a beneficial ecological role?
Before making a decision on weed control and armed with information on the weed’s ecology, we need to consider whether it performs some beneficial ecological role.
- does it reduce or enhance biodiversity
- does it provide useful structure or have a functional role
- does it disrupt or rehabilitate ecosystem processes
- does it disrupt or enhance soil and erosional processes.
Despite the negative impacts of some weeds, some plants usually thought of as weeds provide benefits. For instance, because weeds usually have very high rates of nutrient uptake they are adapted to take advantage of the brief pulse of nutrient release that accompanies the breakdown of organic matter when soil is disturbed. A very important function therefore is that they keep the nutrients in the system.
Some other beneficial ecosystem services may include:
- soil stabilization;
- help develop soil food web in disturbed soils;
- add organic matter;
- habitat and feed for wildlife;
- nectar for bees and butterflies;
- provide shelter for growing plants;
- aesthetic qualities.
We must therefore take a more holistic approach and consider those characteristics which make a weed useful in restoration processes.
Classify and prioritise weeds on site
There is no doubt that many weeds (particularly pasture species) develop an environmentally damaging climax community which will inhibit the recovery of local natural biological diversity. To lessen the cost of weed control while reducing weed impact, on the basis of the information about site weeds, they can be classified as:
The species has substantial effects on native ecosystems or on native populations OR the species has substantial economic effects in managed ecosystems.
Some of these weeds have been called ‘transformer weeds’ – those which can change the character, condition, form, or nature of a natural ecosystem over a substantial area. They often form monocultures [monocultures can also indicate a particular stage of regeneration in a long-term process (eg. Lantana)].
Sheppard et al. maintain that “the long-term changes to ecosystem function and processes that they cause are often impossible or extremely difficult to reverse. Beyond smothering the existing flora, transformer weeds can also alter many abiotic conditions. Richardson et al. (2000) identified eight categories of transformer: (1) excessive users of resources; (2) donors of limiting resources, (3) fire promoters/suppressors, (4) sand stabilisers, (5) erosion promoters, (6) colonisers of intertidal mudflats/sediment stabilisers, (7) litter accumulators and (8) salt accumulators/redistributors. Thirteen of Australia’s 20 Weeds of National Significance (WoNS, Thorp and Lynch 2000) are transformer weeds.” (1)
Weeds may also be prioritise if they cause health issues in humans or other animals.
The species may have some effects on native ecosystems or populations, but the effects where they occur are relatively minor OR the economic impacts of this species are present but minor.
Not a pest
The species has beneficial effects or does not seem to have measurable negative impacts on native ecosystems or populations where it occurs.
1. Sheppard, A.W., Catford, J.A., Daehler, C.C., Hardesty, D., Murphy, H.T., Pergl, J., Rejmanek, M., Westcott, D.A., and Bellingham, P.J., 2010. Are transformer weeds ecological rule breakers? Seventeenth Australasian Weeds Conference, Christchurch, N.Z.
What Conditions Favour Weeds? The Role of Disturbance
“Attempts to control weeds without addressing the causes of the invasion are doomed because they treat symptoms rather than causes.” – Hobbs and Humphries (1995).
Most weeds are pioneers which adapt them to thrive in recently disturbed or continually disturbed habitats. They are generally preferentially favoured over native species by disturbance so the best management seeks to mitigate this. The first thing to do is to identify and remove threatening pressures and causes of disturbance, we are aiming to develop conditions which do not favour weeds.
Additionally the disturbance and its aftermath may have changed environmental conditions. Some of the changes we need to consider are:
- Fire regime – the frequency and intensity;
- Soil changes – erosion or pollutants;
- Hydrology including storm water management;
- Air pollution;
- Human usage including tracks, roads and trails, dumping of rubbish;
- Climate change.
For each kind of disturbance determine the stresses which disadvantage native plants (temperature extremes, exposed subsoil, poor retention of moisture) and the opportunities which favour weeds (removal of shade and competing vegetation, release of soluble plant nutrients). Work towards changing these conditions. As well as the processes that promote weed invasion, consider the mechanisms in the natural community which facilitates resistance to invasion and promote these mechanisms.
What legislative requirements need to be considered?
“Thirty two Weeds of National Significance (WoNS) have been agreed by Australian governments based on an assessment process that prioritised these weeds based on their invasiveness, potential for spread and environmental, social and economic impacts. Consideration was also given to their ability to be successfully managed. Landowners and land managers at all levels are responsible for managing WoNS. State and territory governments are responsible for legislation, regulation and administration of weeds.” There is a list of WoNS here.
Other weeds of national interest include National Environmental alert, sleeper weeds, species targeted for eradication, species targeted for biological control. Lists are available here.
In addition to the national lists above, state and territory governments have their own lists of noxious weeds. Noxious weeds are weedy plant species that are controlled and or managed under state or territory legislation. Legislative classification differs from state to state. The legislation prescribes a variety of management options from prohibition of sale and trade to enforced control. The legislation regulations may affect all or part of a jurisdiction. For instance, the following weed control classes may be applied to a plant by a weed control order:
a. Class 1, State Prohibited Weeds,
b. Class 2, Regionally Prohibited Weeds,
c. Class 3, Regionally Controlled Weeds,
d. Class 4, Locally Controlled Weeds,
e. Class 5, Restricted Weeds.