WHAT IS A WEED?

There are many definitions of a weed. In a broad sense a weed is a plant growing where it is not wanted. Classifying a plant as a weed can therefore vary with the situation and person involved. Weeds that are a problem in pastures and crops are known as agricultural weeds, weeds that invade native bushland areas are known as environmental weeds and unwanted plants in our gardens are gardens weeds.

Almost any plant can be a weed - some species like ragwort and gorse are well-known weeds in Tasmania. Other plants can be regarded as valuable species in the situations in which they usually occur, however, in other situations, the same species may be regarded as weeds. Examples include pasture plants such as white clover and cocksfoot (a grass) that can be serious weeds in orchards. Radiata pine, a valuable species for forestry, can be an invasive weed of bushland. Rice grass has been used to prevent silt building up in shipping channels, however, at some of these sites it has spread into the channel and degraded the waterway.

WHY ARE WEEDS A PROBLEM?

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

Competition
Weeds can grow faster, out-compete desirable plants and use up precious water, nutrients, sunlight and space. For example, slender thistle and californian thistle in pastures.

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

Poisoning
Some Plants are poisonous to livestock and humans. For example hemlock can be lethal to both humans and livestock. Ragwort contains alkaloids that damage the liver and St. John's wort contains hypercin which makes animals sensitive to light.

Acting as hosts or shelter for pests and diseases
Blackberry and gorse 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 weed species such as horehound. The presence of burr and vegetable matter leads to a down grading of the fleece. When animals eat wild garlic their milk and meat becomes tainted. This subsequently lowers the value of these products, even making them unsaleable.

Allergies
Humans can be allergic to different types of weeds. Some people develop skin reactions after being in contact with St John's wort or Bathurst burr. Parthenium weed is known to cause dermatitis problems in humans.

Increasing fire hazards
Some plants, such as gorse, are serious fire hazards particularly when dry out.

Interfering with agricultural practices
Some weeds make cultivation difficult by becoming entangled in machinery and preventing efficient harvesting. Blackberry and gorse can form impenetrable barriers to stock.

Soil erosion
Some weeds are very good competitors and shade out other plants. The problem occurs when the weed dies off over summer, leaving the ground bare and prone to erosion eg. Paterson'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 dandelion, blackberry or arum lily

The Meander Valley Weed Strategy goal is for the:

CATEGORISING WEEDS UNDER THE NOXIOUS WEEDS ACT 1964

Under the Noxious Weeds Act 1964, the Tasmanian Government may: prohibit the introduction into State of designated weeds.

• undertake the eradication of weed species
  
  
• take action aimed at preventing the spread of weeds within the State
  
  
• require that action be taken against weed species where it is necessary to alleviate or prevent a particular problem

Plants may be declared Noxious, Secondary or Prohibited weeds for the whole or any part of the State.

Noxious Weeds
These are species known or believed to be capable of causing severe economic losses or presenting a serious hazard to public health, should they become established or wide spread in the State.

Eradication campaigns are in progress against all declared Noxious Weeds already present in Tasmania. These include serrated tussock, nodding thistle and African feathergrass.

A species will be included in this category only if eradication is practicable. In most cases, this requires that the species is present at only a small number of sites and that infestations at these sites are limited in area.

Secondary Weeds
These species known or believed to be capable of causing economic losses or of interfering with the utilisation of a resource or of being a potential hazard to health.

Their declaration as Secondary Weeds enables action to be taken against them appropriate to the particular situation. Such action may include boundary protection, strategic control, containment of infestations and eradication from specific areas within the State. Examples of weeds in this category are ragwort and gorse.

Prohibited Weeds
This group includes the Noxious Weeds and a number of Secondary Weeds.

Prohibited Weeds may not be brought into Tasmania nor may they be moved from one place to another withing the State.

WHAT DO WEEDS COST TASMANIA?

It has been estimated that weeds cost the Tasmanian economy over $33 million per annum.

This figure includes both the cost of weed control and the loss of primary production caused by weeds. It does not include the less tangible costs such as the losses of aesthetic, conservation and recreational values due to weeds, the impact of weeds on human and animal health, for weed damage to water resources, or for weeds as fire hazards, shelter for vermin or hosts for crop pest and diseases.

The cost of these effects are far more difficult to calculate as the results are often indirect and not obvious, but they may be of greater significance in the long term.

The full costs of weeds to the urban environment, amenity and health values, tourism and the conservation of biodiversity and natural ecosystems has never been calculated. Some biologists place it in the same order as that given above for the losses to primary production. Even if estimates of monetary costs were available, it is unlikely that they would indicate the true dimension of the weed problem.

All Tasmanians contribute to the payment of these costs. This may be through their responsibilities for weed control as land owners or through rates and taxes to finance the weed control activities undertaken by State and local government agencies.

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, unforntunately, 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 feed
Contamination of hay and grains with weed 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 animals
Check your own clothing, socks, cuffs, jumpers and boots etc. after walking through weed infested areas. Remove and destroy any seed you find. Dogs and cats can also distribute seeds in their coats, as can wild animals, particularly vermin such as foxes and rabbits. Birds also transport seeds when they feed on weed fruits and seeds such as blackberry and cotoneaster.

Garden escapees
Many 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 wind
Weed seeds entering waterways or drains can be spread to new areas downstream. On windy day 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 ejection
Many weeds such as gorse disperse their seeds through explosive ejection. 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 acheived 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 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.

Burning
Burning can be used to control weeds by killing the mature weed population and stimulating soil stored weed seed 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 gorse 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 when 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.

Chemical
There are many chemical herbicides available for the control of weeds. The herbicide label will indicate which plant species are susceptible to 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.

Information contained in this page was compiled from material supplied by the Department of Primary Industry and Fisheries.