About Weeds

What is a weed?

A picture of Pampas Grass, Cortaderia jubata in flower.

Pampas Grass is a weed,
© Karen Stewart, DPIPWE

A common definition of "weed" is a plant in the wrong place. But what makes it the "wrong place"? Typically we call a plant a weed because it interferes with the way we want our land and landscape to be. Thistles in the lawn, sticky weed in the garden, or any plant other than the vegetables we want to grow in our vegetable garden are weeds that we readily identify and seek to control. We recognise their weediness because they are prickly, or irritating, or because they directly compete with plants we want to grow.

There are many plants that threaten the economic value of land, perhaps by replacing pasture or poisoning livestock. They may also have the less obvious impact of damaging machinery, increasing fire risk and providing a home for feral animals.

There are also many plants that have the same impact on our native bushland. Although the process may be less obvious, the effect can be hugely damaging - native plants and the animals they support disappear and are replaced by pest species. Once the process starts it gains momentum and large areas can be quickly destroyed. In no time, the things we love about where we live - birds, trees and other wildlife - are gone.

When environmental weeds invade bushland, they threaten native plants, usually by out-competing them for light, nutrients and water. The result is the death of these native plants and often the animals, birds, insects and other creatures that depend upon them. This loss can in turn cause further declines in local biodiversity - the unique variety of living things around us, and upon which we depend. In addition, some environmental weeds are poisonous to people and animals, while others provide shelter for animal pests and diseases. Weeds can also alter the way water moves through the ecosystem, and increase fire risk. All our natural environments - from the mountains to the coast - and the beautiful things they contain are at risk from environmental weeds.

Where did they come from?

Environmental weeds come from a variety of places. Most are from other parts of the world that have similar climates: capeweed from southern Africa, Spanish heath from the Mediterranean, and mirror bush from New Zealand. Typically there are factors in their place of origin that limit their weediness: climate, insect pests, soil type etc. In Australia these factors may not occur, and there is nothing stopping the spread of these plants. An increasing number of environmental weeds are actually native Australian plants that have been taken out of the areas where they grow naturally. These can be just as invasive as plants from overseas. In addition, they have the capacity to interbreed, and so change the nature of the local gene pool.

An Australian native plant growing out of its natural range may flower earlier or more prolifically than a similar local species. If the insects or birds feed on the new arrival, they may not pollinate local species, and without pollination the local plants will not be able to reproduce. Eventually they might die out altogether.

How did they get here?

Some weeds have been accidentally introduced when they have arrived as seeds in clothing or baggage, or as contaminants of desirable seeds, like wheat. Others have been deliberately introduced as crops or food sources, but they have escaped cultivation and gone wild. In many cases we have no idea how these plants have entered Australia.

In the past 25 years, 65% of all plants that have become environmental weeds in Australia have been deliberately introduced as ornamental or garden plants.

Not all exotic plants are weeds, and not all Australian plants are non-weedy.

How do they spread?

Plants are equipped to spread themselves very effectively. Some can do this under their own steam: members of the daisy family, including ragwort and thistles, have light, fluffy seeds that can blow and parachute themselves many kilometres from the parent plant. Willow seeds are also dispersed in this way.

Brooms have seed pods that explode when they dry out, ejecting seeds several metres from the parent.

Other plants, like boneseed and hawthorn, produce fruits that are eaten by birds and animals, which then excrete intact seeds. In many cases the digestion process is necessary to break open the hard coat around the seed. Elisha's tears and cotoneaster spread this way, and can so become established a long way off. Other seeds may be hooked or sticky in order to cling to the fur (and socks) of passing animals. Ants may also carry seeds.

Many seeds are dispersed by water, and in the soil and mud that sticks to shoe soles, tyres and machinery.

Not all plants can grow, or are restricted to growing, from seed. Many will grow from a piece, or will send out stems and suckers that take root and form new plants.

Weeds cost Australia an estimated $66 billion a year. This cost may be obvious when we see spray contractors undertaking weed control work in order to keep our roadsides safe and are recreational facilities maintained; but it is also a factor in our grocery prices, and in the cost of a pair of cotton socks. Weeds don't just damage the bush, or create health and fire risks - they cost us all money, and part of our rates and taxes goes to combating the weed problem.

You can help!

To tackle the problem of environmental weeds you need to know about prevention and control.


Invasive Species Branch