This article will provide a brief overview of why weeds are such a problem in our gardens,
along road verges and in our much-celebrated biodiverse bushland. As members of local
communities, we can learn about weeds, find out where weeds come from, know how they
spread and understand the impact they are having of our native wildlife and biodiverse
bushland. Extensive weed invasion has the potential to impact on our quality of life as our rich
flora is increasingly displaced by weeds making the landscape a bland caricature of its former
self. Such knowledge can help us appreciate that task that lies ahead and we can then formulate
a plan of action to stop the spread. This brief article will provide you with some further
understanding and an easy avenue that can make a difference in halting the degradation of the
unique nature that makes Australia so special.
What are weeds?
Weeds (Figure 1) are plants that can grow in locations where they are not desired and can
have an adverse effect on the environment and economy (DBCA, 2024).
In Australia’s 16%+ of wild plant species are weeds and now about 3,000+ weeds have become
established in Australia with more species invading every year. For example, the number of
known weeds in Western Australia has been steadily increasing since European settlement, with
over 1300+ species currently recorded (e.g. Figure 2).
Where do weeds come from?
Many weeds originate from South Africa and the Mediterranean regions of Europe and succeed
because they find the climatic and soil conditions in Australia similar, but the predators and
disease that affect them in their source environments are absent enabling them to establish and
survive in a new environment. Weeds arrive in Australia because of accidental introductions
(trade), from deliberate release (e. g. pasture grasses), illegal entry in the luggage of travellers
at the Australian border, may be introduced from nurseries and arrive in transported soils and
can escape from gardens. Once introduced into the environment, they can proliferate on cleared
and disturbed land and spread along road verges, waterways and infrastructure corridors
(Figure 3).
The success of weeds: Functional traits
Weeds exhibit several characteristics that facilitate the successful colonization of disturbed
areas and ensure rapid growth and survival. Firstly, many weeds are therophytes which means
that they complete their entire life cycle in one growing season (e.g. Galium aparine). Survival
until the next growing season is usually via seeds that survive a dormancy period in the soil.
Some weeds, however, persist as hemicryptophytes, where the growing tissues survive just
beneath the surface of the soil (e.g. Dandelions/Taraxacum), while others survive as bulbs,
rhizomes and corms, and are classified as geophytes. Geophytic weeds commonly encountered
on Perth include African Cornflag (Gladiolus caryophyllaceous), Freesia (Freesia leichtlnii)
and the notorious Arum Lily (Zantedeschia aethiopica).
Other traits that favour the survival and persistence of weeds include those that have a high
specific leaf area, which means that they can absorb a lot of sunlight which facilitates rapid
growth and an early and lengthy flowering period. Many weeds produce
numerous seeds (see Figure 4) which can germinate under a variety of conditions. Weeds can
tolerate harsh conditions and low nutrient conditions and can grow in the absence of the soil
microbial relationships that so many native plants are dependent upon.
How do weeds spread once established?
Weeds colonize and can dominate disturbed areas via wind transport blowing their seeds
around. Human activities involving the disturbance of ground, movement of soil and rubbish
dumping helps to spread weeds which can capitalize on situations where additional nutrients
are added to disturbed land. Weeds can also move along drains, creeks and rivers via storm
water run-off. Those species with stolons, corms and bulbils can survive on going disturbance
because these essential underground parts are protected, such species send up new shoots when
conditions are favourable and are able to spread along hiking trails and road verges.
Weeds can also spread because of recreational activity, attached to animals and humans, if they
are sticky or have tiny hooks to facilitate transport. Catherine Pickering and colleagues of
Griffith University have done a lot of work on weeds in protected areas and demonstrated that
weeds can be spread into protected areas by hikers and vehicles. Those animals that eat a lot of
weeds (e.g. rabbits) can carry seeds in their gut with weeds germinating out of piles of
droppings around their burrows. Horses that eat forage containing weed seeds and browse
roadside vegetation can spread weeds via their droppings (see Figure 5).
The impact of weeds
Overall, the presence and spread of weeds results in the degradation of wildlife habitats and
biodiversity. But what does this mean? From an ecological point of view, it means that weeds
replace diverse native plant communities with ecologically impoverished weed communities.
This occurs through competition for resources such as water, nutrients, light and space. Native
plants are highly adapted to infertile soils and difficult climatic conditions such as long dry
spells, sometimes flooding, extreme heat, sometimes cold, fires and herbivory. A weed can
grow fast under favourable seasonal conditions, shade native plants, disrupt soil chemical
conditions, does not require specialised fungi to grow, actively compete for water and nutrients
and complete its life cycle in less than a year. Weeds are then able to inhibit the growth of
native species, displace species and supress regeneration. Such a change in vegetation cover
and conditions ultimately impacts on fauna. Less plant diversity means less insect diversity and
there is less scope for complex arthropod food chains that support native reptiles, birds and
mammals. Also, when grassy weeds such as Fountain Grass (Figures 4 and 6) take over large
areas, it enhances the fire hazard under dry windy conditions as the dead grass and dry stems
constitute significant fuel and is very flammable.
From a social perspective degradation and loss of biodiversity means lost natural heritage,
impoverished faunal habitat, compromised recreational, educational, scientific and tourism
related values. There is reduced opportunity for being amazed about the uniqueness of our
natural heritage, less scope for inspiration and a sense of wonder about the beauty of nature.
Synergistic impacts – where do weeds fit in?
Our flora and biodiversity remain under threat from clearing, over-zealous prescribed burning,
natural fire events caused by lightning and an ever-increasing number of deliberately lit fires.
Add to this our drying climate, decreased winter rainfall, increasing heatwaves, longer periods
of high temperatures and an extended fire season putting our native vegetation under ever
increasing stress. Frequent hot fires can kill slow moving wildlife, displace animals from
preferred habitats and result in the loss of tree hollows and hiding places. Frequent and on
gong fires in the same area changes the flora as many plants don’t get the chance to regrow,
flower and set seed before the next fire comes along. If burnt areas are close to patches of
weeds or weed invaded trails, weeds can readily invade. Weeds are thus tied to the on-going
degradation of our vegetation. While we may not be able to do much about climate change
related impacts one of the things we can do is tackle the weed problem.
Concluding remarks and what you can do
Our Federal and Local Governments have recognized and responded to the weed problem.
Decision makers have acknowledged that weeds present a problem for urban bushland and
protected areas across Australia. Governments and NGO’s have developed policy and strategy
to tackle the weed problem so of which include include the Australian Weeds Strategy, CSIRO
National Weeds Programme and the AWC National Weed Management Strategy. The Shire of
Mundaring has been very proactive and operates an excellent environmental sustainability
programme that involves public education and advice on weed management. The weed
problem is so big, however, that Government can’t do it alone. You can help too by doing the
following:
Celebrate your natural heritage and learn about wildflowers and our unique ecosystems
Learn how to identify weeds
Know about the impact of weeds on our natural heritage
Support Government initiatives
Manage your verge and garden
Join a friends/weeding group. For example: friendsofjohnforrestnp@gmail.com
Happy Perth Springtime
Written by:
David Newsome, PhD
Emeritus A/Professor
Environment and Conservation
Murdoch University
Western Australia, 6150
David Newsome – Google Scholar
The Friends of John Forrest National Park
The Friends of John Forrest National Park (FoJFNP) is a volunteer group that works alongside
park rangers to manage the invasive plants (i.e. weeds) that are threatening the natural
ecosystems in many areas of the park. From April to November the group meets every Saturday
morning from 8:30 to 10:30, to remove weeds and inspect the return of native flora, finishing
each morning with a friendly morning tea. Of the 30 active Bushcare volunteers 3 to 16 come
on Saturday mornings to work, with some meeting to do other tasks mid-week. Several
members are experienced natural scientists, but most are enthusiastic amateurs, who love
learning about the bush together.
The group targets weeds throughout the National Park in around 50 project areas. Many weed
control methods are used, and weekly activities keep the team connected to the landscapes in
this amazing national park while walking through the bush spotting and removing weeds and
enjoying the local wildflowers as they burst into flower.
The regular weekend activities, complimented by additional midweek work have attracted
increasing recognition, and the FoJFNP’s profile has continued to grow. Current members
come from all over the Mundaring Shire and beyond, as far as the northwestern suburbs.
Members are Bushcare volunteers who meet to learn from each other, share common
passions and protect areas of natural bush from the weed threat.