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Weeds |
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What is a weed?Weeds are amazing hitchhikers. They can spread in many ways – by wind and water, on animal fur and droppings, in soil on tyres and our shoes, and even on our socks and picnic blankets! There are several ways in which plants can become weeds: • Exotic plants introduced from overseas which are adapted to Australian conditions • Native plants which have spread outside of their natural habitat range • Native plants that have become ‘out-of-balance' within their natural habitat. Disturbances - upsetting the balance In natural ecosystems each plant population is controlled by factors such as soil, water, climate, animals and other plants. Disturbances, even small ones such as a walking track or a campsite, can create opportunities for weeds to invade natural ecosystems. Although weed invasions are usually caused by human disturbance, weeds do not always require disturbance to invade natural ecosystems. Even remote wilderness areas are not safe from weeds! Introduced Ivy (Hedera helix) can tolerate shade - this adaptation has allowed Ivy to invade undisturbed forests of the Otways Ranges . Once a weed invades a natural ecosystem it can create negative changes which ripple through the food chain affecting native plants and wildlife. What's so bad about Weeds?Even though some weeds look nice and some like blackberries are delicious, many weeds are bad news for the health of our native plants and wildlife as well as to humans! Along with land clearing, weed invasion is considered to be the most serious threat to global biodiversity and nature conservation. Weed invasion can lead to the following problems: • Reduced biological diversity Weeds threaten natural ecosystems, by out-competing native plants for moisture, nutrients and light. Left unmanaged, weeds can result in extinction of flora and fauna. • Increased soil erosion Annual weeds such as Paterson 's curse ( Echium plantagineum ) die off in summer exposing bare soils to erosion by wind and rain. • Impacts on geomorphic processes Coastal native plant species encourage the formation of low sand dunes. Introduced Marram grass (Ammophila arenaria ) causes steep unstable sand dunes to form which can easily collapse during storms. By holding huge amount of sand with its roots, Marram grass also affects natural sand drift patterns along the coast. • Changes in biochemical cycling A weed of national significance, Lantana ( Lantana camara ) releases chemicals, which affect soil chemistry stopping seeds from other plants germinating near them. This process is known as allelopathy. Radiata Pine ( Pinus radiata ) affects soil nutrient cycles. Fallen pine needles take a long time to break down. This slowed recycling of nutrients back into soil creates a thick mulch of dead pine needles which stop germination of native seeds. • Changes to hydrological cycles Exotic willow trees ( Salix spp.) along waterways alter stream flows and flooding patterns. This can result in bank erosion and rivers changing course. The aquatic Alligator weed ( Alternanthera philoxeroides ) can choke waterways, reducing light and oxygen to other aquatic plants and animals. Both of these invaders are Weeds of National Significance. • Changed fire regimes Weeds such as English Broom ( Cytisus scoparius subsp. scoparius ) and Pampas grass ( Cortaderia spp.) grow quickly into dense, highly flammable thickets. These weeds present dangerous fire risks during dry seasons. • Changes to genetics Introduced eucalyptus timber plantations may exchange pollen with surrounding native forest eucalyptus species, causing hybridization between species and genetic pollution. • Affect cultural heritage Weeds can impact upon areas important to indigenous Australians by affecting availability of bush tucker and medicine, and reducing the health of traditional fishing and hunting sites. They also change the overall appearance and feel of the natural Australian bushland, which is important to many people for a sense of peace, place and belonging. • Reduced crop and pasture yields Weeds compete with crops and pastures for moisture, nutrients and light. They also provide harbour for pests and diseases, can poison livestock, and contaminate produce. • Impacts on recreation and tourism Weeds degrade the beauty and interest of natural areas that people come to visit and enjoy. Case Study: the opportunistic Giant Rush The Barmah Forest 's Giant Rush ( Juncus ingens ) is a great example of a native plant ‘out of balance' in its own habitat. A semi-aquatic plant, the Giant Rush grows in heavy soggy soil along the Murray River and its billabongs. The tallest rush in the world, its 5 metre high stems provide nesting sites for birds such as ibis. It can tolerate shallow flooding for six to eleven months. However Giant Rush seeds can only germinate in wet mud - not under water. Normally Giant Rush does not grow in native Moira grass ( Pseudoraphis spinescens ) wetlands. The Moira grasslands thrive on prolonged deep flooding and provide important habitat for fish and birds. However the Murray River 's natural water flow patterns have been disturbed to allow agricultural irrigation. Reducing the Murray River's flooding frequency has allowed the Giant Rush to invade the Moira grasslands threatening biodiversity in the Barmah Forest . Although native to the area, the Giant Rush is now being managed as a weed. Weed ControlWeed control is an important part of caring for nature. Land managers spend huge amounts of time and money controlling weeds both on land and in waterways. In order to control a weed many questions need answering. How does the weed spread? What is its life cycle? What restricts this weed's growth in its natural habitat? What are the impacts of this weed? A combination of methods is used to control weed species, including herbicide, fire, steam, manual removal, mulching and biological control. Case Study – biological control of Boneseed Without natural pests and diseases to keep numbers in check, exotics plants such as Boneseed (Chrysanthemoises monilifera ) have become major weeds of national parks. Biological control uses a living organism, such as a parasite, insect or disease, to control weeds. By researching what naturally controls Boneseed in its South African habitat, a biological control agent (the Leaf roller moth) was discovered. Careful research is needed to make sure that the introduced control organism does not become a pest itself. Developing biological control can take up to ten years so it is a costly and time consuming operation. What can you do?• Research your local environment or contact your council for weed information and a list of native plants to use in your garden. • Dispose of garden waste by composting or taking to the council tip. • When visiting a national park, be sure that whatever you take in you take out again - including food scraps. Did you know that discarded apple cores and pips from stone fruit can sprout and become weeds? • Before visiting a park ensure that your shoes, socks, picnic blankets, pets and vehicle tyres are clean of hitchhiking weeds. • Always keep to the tracks when walking, driving or riding in parks to avoid spreading weed seed. • Join your local Park Friends or Landcare group to help control weeds, replant native plant species and make new friends.
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