Tropical Topics A n i n t e r p r e t i v e n e w s l e t t e r f o r t h e t o u r i s m i n d u s t r y Weeds in the wet tropics No. 27 April 1995 Don’t give a weed a break Notes from the In the 1840s someone decided that a Central American flowering plant would look good in their garden in Calcutta in India. It grew well. It spread throughout Editor India, across Asia to Siam (Thailand) and on to China, the Pacific islands and A weed is a plant which grows where Papua New Guinea. Within a century it was also well established in Africa. it is not wanted. A plant introduced to improve pasture may be unwelcome in our back yards. A plant which we Siam weed, Chromolaena odorata, is boots or to machinery used overseas. admire in our gardens may well cause now considered one of the world’s The Department of Lands has been the destruction of native vegetation. worst weeds. It is fast-growing and spraying the infestation with This issue of Tropical Topics competitive. In open areas it forms herbicides; the third program is due to concentrates on environmental dense tangled bushes which can start at the end of this month. It is weeds, specifically those which cause intense bushfires in the dry hoped that the weed is confined to the endanger the natural environment in season. It can climb up to 20m on Tully area and can be controlled but trees, smothering them with its dense there are concerns it may have been the wet tropics. growth. Growing over a metre in just a carried further. couple of months it can overwhelm Each component in an ecosystem has sugar cane and other crops and it is Siam weed is not the only weed natural checks on its population toxic to stock. threatening the wet tropics. As can be growth; over time providers and seen on the following pages vines and consumers have struck a balance. Australia was thought to be one of the creepers, trees, shrubs and grasses all However, when one element of an few tropical areas free from this weed have a destructive potential. However, ecosystem is transplanted into — until July 1994 when a DPI botanist very few of these weeds are able to another it usually travels without its spotted it near Mission Beach. No one establish themselves in unbroken enemies, the diseases and predators knows how it got rainforest. If a siam weed seed is which would normally check its there but the dropped under the forest canopy it growth. Some plants fail to thrive in an feathery seeds can’t grow in the dim light. However, if alien situation, others fit in to some may easily have the same seed is deposited wherever degree and yet others achieve attached there is a break — along a river bank, outrageous success — at the expense themselves to in a clearing, at the roadside or of the locals. A successful weed is a visitor’s anywhere there has been a hardy, fast-growing, competitive, disturbance — it will spring to life and aggressive, reproduces quickly and quickly destroy surrounding abundantly and disperses with ease. vegetation. The more the forest is disturbed the more danger there is of It is difficult to predict which plants weeds attacking the remaining part. will become weeds. A species may, at The best way to defend the rainforest first, adapt to its new environment is not to give the weeds a break. without appearing to be a problem. This is known as the lag phase. Then it may suddenly launch into the expansionist phase — a rapid population increase which may Siam weed has a similar growth habit to eventually dominate neighbouring lantana, soft hairy roughly triangular leaves, Illustration courtesy Department of Lands vegetation. soft round stems, no prickles and masses of white or pale lilac flowers in winter. The Department of Lands is keen to hear of any sightings of the plant. If you see it call the Innisfail office on (070) 61 1446 or your local Department of Lands office. Weeds in the water Water weeds are a widespread problem in Australia. Many species have been introduced to decorate garden ponds or aquariums and have then found their way into natural waterways where they have become pests. Water weeds usually grow very fast under tropical Some biological control of four serious waterweeds, conditions, some infestations doubling in size each week. salvinia, water hyacinth, alligator weed and water lettuce They form dense mats of vegetation on the surface of the has been achieved by releasing weevils and beetles which water which: feed on these species. There is no known predator for • reduce light reaching submerged plant species cabomba. • use up oxygen in the water, depriving and killing fish and other aquatic creatures It is essential for aquarium and pond owners and those • restrict the movements of water-based dealing in water plants to be aware of the potential problems animals including waterbirds not just of proven pest plants but also of those not yet • choke drains and restrict the flow of released on the environment. Unwanted aquatic plants water should be burned or composted and NEVER NEVER • clog up pumps, seriously dumped in waterways. Pond owners should also be careful affecting irrigation and to prevent water plants being carried away when there is aquaculture equipment heavy rain and flooding. (Likewise, unwanted fish should • foul boat engines and never be released into waterways.) restrict navigation, watersports, swimming and The best native plants to use are fishing probably Blyxa species and stoneworts • cause water loss, at up to four times the normal — Chara and Nitella species. Others evaporation rate, through rapid transpiration from the leaves are Pacific and ferny azolla (Azolla • during floods, together with trapped debris, can filiculoides, A. pinnata), hydrilla cause the collapse of bridges and fences. (Hydrilla verticillata), clasped pondweed • restrict livestock access to water sources (Potamogeton perfoliatus) and ribbonweed • endanger stock and children who may (Vallisneria gigantea). (See Waterplants become entangled and drown book in Bookshelf) Please remember that • provide breeding places for disease- even native plants can cause problems if carrying mosquitoes. introduced to the wrong area so please do not dump them in watercourses. Also, please do not collect from the wild. Some natives are rare or threatened. Water hyacinth (Eichhornia crassipes) (left) has beautiful purple flowers and despite its Cabomba, or fanwort, (Cabomba remarkable abilities to clean caroliniana) (right) is a popular aquarium up polluted water, has become plant from America which is causing an invasive and unwanted problems in ponds, lakes, dams and quiet species all over rivers in Queensland. In some areas it the tropical has been declared a category P2 world outside its plant. Sale of cabomba is illegal in native Brazil. In Queensland it is a category P2/P3 weed. Queensland. What is a declared plant? In a word A declared plant (formerly termed ‘noxious plant’) is a plant considered a Exotic is a word which conveys a positive serious enough pest to warrant its control being enforced under legislation (the image to most people. Exotic holidays, Rural Lands Protection Act 1985-1988). exotic food, exotic fruits — the word has come to mean things which are strange or Declared plants are placed in various unusually colourful or beautiful. In fact, categories, though some plants may the word means foreign, from outside be in more than one category. that country. While we can safely admire exotic animals and plants when we travel, P1 — plants which do not occur in if transported elsewhere they can become Queensland and whose introduction pests. Pigs, foxes, cats, rabbits and so on into the State is PROHIBITED. are all exotic animals — but unwanted in P2 — plants which are to be natural areas of Australia. Similarly many completely DESTROYED throughout exotic plants can become exotic weeds. an area. P3 — plants whose numbers and distribution should be REDUCED in an Rubber vine (Cryptostegia Weeding in the Wet area. grandiflora) (above) is a declared It is tempting, when the ground is wet, to P4 — plants which should be category P3 weed. A garden escapee, pull up all those nasty weeds by the PREVENTED FROM SPREADING originally from Madagascar, it roots. However, this may leave the soil beyond places in which they occur. threatens mainly dry rainforest and vulnerable to soil erosion and simply P5 — plants which should be riverside forest. It invades prepare the ground for the next generation CONTROLLED only on land under the agricultural land and is poisonous to of germinating seeds. A more effective control of a Government Department stock, costing the grazing industry method is to snip the offending plants or Local Authority. about $8 million each year. and cover the ground with a thick layer of newspaper and mulch. Few weeds will For more information contact your local Department of Lands office. survive this treatment. 2 Cartoon illustration courtesy ACT Parks and Conservation Service Plant illustrations courtesy Department of Lands Nature notes A diary of natural events creates a pleasing journal which grows richer with the passage of time. Watching for the recurrence of an event after noting it in a previous year, and trying to understand what could have caused changes in timing, is intriguing. These notes are from the author's own notebook, or were offered by researchers and fellow naturalists. Readers will, inevitably, note variations between their observations and those appearing here. If you do not keep a nature diary perhaps this will inspire you to begin one.