Weed Information

What Is A Weed?


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Some Background on Weeds

It is widely accepted, that a weed is a plant that is growing out of place. They may be in a location or at a time that man considers to be inappropriate. Waldo Emerson (1964), wrote,    “A weed is a plant whose virtues have not yet been discovered”. The definition of a weed means different things to different people, however, not all weeds are noxious:-

Weeds are hard to eradicate, spread rapidly and may have toxic or harmful properties.

Weeds cost the Australian economy in excess of $3 billion annually. This figure does not include any costs weeds cause to human health, or the environment, both of which have recently been recognized as being greatly affected by weeds.

Most of the serious weeds in Australia have been introduced from overseas. Some have found their way in as contamination in produce or packing material such as straw. Others have been smuggled in or introduced legitimately at first, for use as ornamental plants. Many of these plants have escaped from gardens and thrive in our conditions because none of their natural predators or diseases exist in Australia.

Plants such as rhus trees, pellitory and noogoora burr can cause serious health problems including asthma, allergies, dermatitis and conjunctivitis. Blackberry, pampas grass and aquatic plants such as water hyacinth and salvinia are ferocious invaders. In certain conditions aquatic weeds can grow so vigorously, that they can reduce oxygen levels in the water and prevent the entry of sunlight.

Aquatic weeds stifle the growth of native trees and shrubs, block waterways and dams, are harmful to domesticated animals and indigenous wildlife and provide a haven for insects and vermin. Alligator weed has the potential to destroy the turf farming, extractive industries and water sports.

Although attractive when small, many of these species either quickly outgrow their space needing constant care or pruning – or produce large quantities of berries or seeds. They spread uncontrolled into neighbouring land, carried by birds or the wind, where the plants quickly establish at the expense of existing vegetation.

The majority of our troublesome weeds were introduced into Australia for their aesthetic value, as aquarium plants or due to previously inadequate quarantine restrictions. Once alienated from their climatic restrictions and free from predation that kept them under control in their natural environment, these plants were able to establish, thrive and become naturalised. With the urban development that followed, exotic plants were further encouraged by the increase in nutrients available to them.

Many exotic plants have become established after being accidentally released, however due to mans indifference to his environment, most infestations have occurred due to the neglect or thoughtlessness of man.

The urban sprawl continues to diminish natural buffer zones along the waterways of the catchment area. These buffer zones are essential in removing a certain amount of nutrients from surface water before it reaches the waterways and reduce the amount of silt carried away during heavy rain.

There are many ways to control unwanted weeds, and total eradication usually requires a concerted effort over a long period of time. Methods of control include hand weeding, burning, herbicides and in warmer northern areas, biological control agents. Once the weeds have been controlled it is essential to replace them with desirable species.

Why Are Weeds A Problem?

Weeds may cause problems in a variety of ways. Characteristics which result in weeds becoming a problem are:

CompetitionWeeds can grow faster, out-compete desirable plants and  use up precious water, nutrients, sunlight and space. For example, paterson’s  curse in pastures.

Stock Injury Certain weeds can cause injury to livestock. For example,  the sharp spines of the boxthorn.

Poisoning Some plants are poisonous to livestock and humans. For example, green cestrum can be lethal to both humans and livestock. st. john’s wort contains hypericin which makes animals sensitive to light.

Acting As Hosts Or Shelter For Pests And Diseases Blackberry can provide shelter for rabbits and other pest animals. Other weeds may carry plant diseases. For example, ox-eye daisy can host a virus which damages potatoes.

Tainting And Contamination Wool can be contaminated by the seeds of many a weed species such as bathurst burr. The presence of burr and vegetable matter leads to a downgrading of the fleece. When animals eat wild garlic, their milk and meat becomes tainted. This subsequently lowers the values of these products, even making them unsaleable.

AllergiesHumans can be allergic to different types of weeds. Some people develop skin reactions after being in contact with st. johns’ wort or rhus tree. Parthenium weed is known to cause dermatitis problems in humans.

Increasing Fire Hazards Some plants, such as pampas grass, are serious fire hazards particularly when dried out.

Interfering With Agricultural PractisesSome weeds make cultivation difficult by becoming entangled in machinery and preventing efficient harvesting. Blackberry and privet can form impenetrable barriers to stock.

Soil Erosion Some weeds are very good competitors and shade other plants. The problem occurs when the weed dies off over summer, leaving the ground bare and prone to erosion eg. patterson’s’ curse.

Aesthetics Plants which you simply don’t like or want can be regarded as weeds. They may be interrupting a view, crowding bushland or other  desirable species, have an offensive odour or be generally causing problems  where they exist, for example water hyacinth, salvinia or blackberry.

How Are Weeds Spread?

Weeds reproduce and spread by many methods and may have special adaptions to assist their dispersal. These include seeds, spores, runners and separated root and shoot fragments. Nature plays a big part in spreading weeds over small distances in wind and water. Humans, unfortunately, are by far the worst offenders at spreading weeds, particularly on dirty tools, machinery, vehicles, clothing and transported animals. Some of the more common methods of weed spread and methods used to prevent this are discussed as follows:

Stock FeedContamination of hay and grains with wet seeds is one of the most common means by which weeds are spread. Feeding animals in a confined area or in one paddock reduces the risk of weeds invading the rest of the property.

Stock Weed seeds ingested by stock can remain viable after passing through the digestive tract. New stock should therefore be confined to one paddock for a week after arrival. This allows time for any viable seeds that have been ingested by the stock to be expelled. Seeds which are sticky or spiny can spread on the animals, for example in sheep fleece.

Machinery After using machinery in weed-infested areas ensure they are thoroughly cleaned. Weed seed can be transported in tyres and in other road materials.

Soil Disturbance Minimise the amount of soil and vegetation disturbance when carrying out work. Disturbed ground creates an ideal seed bed for both existing and introduced weed seeds to germinate.

Humans And AnimalsCheck your own clothing, socks, cuffs, jumpers, boots etc. after walking through weed-infested areas. Remove and destroy and seeds you find. Dogs and cats can also disturb seeds in their coats, as can wild animals, particularly vermin such as foxes and rabbits. Birds also transport seeds when they feed on wet fruits and seeds such as blackberry and cotoneaster.

Garden EscapeesMany weeds were introduced to Australia as ornamental plants or for herbal medicine, which have since “escaped” from our gardens and become wild. For example – pampas grass, broom, Spanish heath and cotoneaster. It is best to avoid “weedy” species when choosing plants for your garden.

Water And WindWet seeds entering waterways or drains can be spread to new areas downstream. On windy days when plants are seeding, the wind can easily disperse the seed quite some distance. Many weed species have seeds especially adapted to be carried by the wind.

Explosive ProjectionMany weeds such as gorse disperse their seeds through explosive projection. The seeds are encased in pods, which can be thrown up to several metres from the parent bush.

How Are Weeds Controlled?

Weeds can be managed using many different methods. The most effective management of weeds is usually achieved by a combination of methods termed “Integrated Weed Management”. It is vital that weeds be correctly identified before a control program is implemented. Prevention is the best method of control. If you intend to buy a property find out its’ weed history first. Here are a few other methods used to control weeds:

Plant Competition – It is important to grow vigorous plants that out-compete the weeds. A healthy pasture or crop can often suppress weed growth.

Mulching – A layer of material is placed on the ground which weeds cannot penetrate. The weed seeds are denied access to light and some are unable to germinate. Mulching also helps to preserve moisture in the soil for the plants which you do want to grow.

BurningBurning can be used to control weeds by killing the mature weed population and stimulating soil stored weed seeds to germinate so that they can be controlled while in the more susceptible seedling stage.  Burning is used in control programs for weeds such as gore and boneseed.

Pasture And Grazing Management – Grazing at different stocking rates can be useful for  weed control. Heavy stocking rates force animals to eat the less desirable plants (but should not be used where the weed is toxic to stock). Lighter stocking rates can lead to selective grazing leaving the weedy and less palatable plants. This gives the weeds a competitive advantage allowing them to increase in density.

Physical – Small areas can be successfully managed by simple hand-pulling or chipping with a hoe.

Mechanical – A sustainable method for large areas of aquatic weeds that have access for the necessary machinery.

Chemical – There are many chemical herbicides available for the control of weeds. The herbicide label will indicate which plant species are susceptible to the chemical and the method of application which can be used. Care should be taken in the storage and handling of herbicides. The instructions on the label should always be read before use.

Biological Control – Biological control involves the use of one living species  (the agent) to control an unwanted species (the target). The aim of biological  control is to restore the natural balance between the pest and its’ environment  by the introduction of natural enemies which can help control the pest to a  level where it is no longer considered a problem.