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Guests and Pests - Australian Plants Overseas
Australians often look with dismay at exotic plants such as bitou bush, lantana and privet which invade and displace native plant communities. Few probably realise that there are Australian plant species which are having a similar effect on native ecosystems overseas. But there are also other Australian plants which have naturalised without causing a significant nuisance. This article looks at a few of these expatriate Australians.
It may come as something of a surprise to learn that a number of our acacias, eucalypts and hakeas are regarded as pest plants in some countries. In Europe, perhaps the climate has tended to restrict their spread and none has reached pest proportions. Nevertheless there are some 8 Australian acacias which are recorded as being naturalised (i.e. have been reported from particular sites for 25 years or more) and all were deliberately planted.
Probably the most widespread is A.dealbata, (Silver Wattle) a handsome tree to 25-30m which is widely naturalised in Southern Europe. While sometimes planted for timber, for soil stabilisation and as a source of tannin, it's main use is ornamental. Known as Mimosa in France, A.dealbata and its various cultivars provide considerable quantities of cut flowers to the European flower trade. In Australia, acacias usually have a short vase life of perhaps 2 or 3 days. In Europe, by judicious selection and breeding of cultivars and by the use of various preservative solutions such as Mimosa-Chrysal, this has been extended to 7 to 9 days. As seems to be so often the case these days, the initiative to use our native plants commercially lies with growers outside Australia.
Acacia dealbata is an ornamental and adaptable tree in many climates. It grows naturally in eastern Australia. Select the thumbnail image or plant name for a higher resolution image (22k).
Photo: National Botanic Gardens.
Several species were grown for tan-bark and have since become naturalised. These include A.mearnsii, A.pycnantha and A.dealbata while A.melanoxylon and A.dealbata are valuable sources of timber, especially in South West Europe. Because all the species are hardy and adaptable it is not surprising to find the same species being used for sand and dune stabilisation in Europe as were employed in South Africa, Libya, and a number of other countries. The plants used include A.saligna (still sometimes called A.cyanophylla), A.longifolia and especially A.cyclops. Because of their ornamental value, some of these same acacias are widely grown in Europe as garden specimens, including A.dealbata, A.mearnsii, A.longifolia and A.retinoides.
There are no real surprises in the list as the plants are all very hardy and several in Australia are often thought of as weeds. They have tended to acclimatise better in milder areas of Southern and South Western Europe notably Italy, Spain and Portugal, though some A.dealbata cultivars grow as far north as the Black Sea coast. As the 'Flora Europea' shows, apart from acacias and eucalypts, there are only relatively few Australian species naturalised in Europe - certainly much below the European plants naturalised here. It could be interesting to learn why.
Some Unusual Australian Potential Pest Plants
Dan Austin's article in the Decernber issue of "Australian Plants online" ("Three More Rogue Aussies") highlights something which is not often realised, namely that there are a very large number of Australian plants which, if given the right conditions, can become pest plants. Sollya heterophylla, Acacia baileyana and Hakea laurina are just three examples of popular gardens shrubs which have shown regrettable tendencies to escape into the bush in areas of Victoria. Geoff Carr in "Envionmental Weed Invasions in Victoria" has documented many others and has long campaigned for people to grow local indigenous plants and be very wary of "introduced" species from other areas and states.
The plant invasions of South Africa and the Florida Everglades by Australian plants has caused enormous concern among naturalists, botanists and conservationists in these countries. They are now much more careful about introducing 'foreign' plants and usually will conduct trial plantings, or extensive evaluations, to determine likely pest species.
Two species which I have seen evaluated as showing potential to escape are Grevillea pteridifolia and Banksia ericifolia. These are hardly anyone's idea of potential escapees!
Grevillea pteridifolia was tested at the Agricultural Research Center, Ft. Lauderdale, Florida from 1970 to 1976. At the time it was regarded as an attractive large shrub for a low maintenance landscape. Plants were raised from wild seed collected in the Atherton Tableland, North Queensland. They reached 2 m in two years and began flowering. Plants also set seed which gave around 80% germination in four weeks when tested. The staff at the Research Centre counted flowers on inflorescence and numbers of seeds set. From this they calculated that up to 300,000 seeds could have been shed from just 28 plants in a three year period!
- Grevillea pteridifolia (2)
Grevillea pteridifolia is a widespread species in northern Australia. It is a parent of a number of garden hybrids including G."Honey Gem". Select the thumbnail image or plant name for a higher resolution image (53k).
Many grevillea seeds have a wing structure which allows for wind dispersal once the seed capsules have opened. Within a year of the first plants flowering, seedlings were
discovered in an adjacent field and a windbreak row. Subsequent counts after twelve
months showed that some 328 seedlings had established without benefit of watering,
fertiliser addition or weed control. At this stage, the authorities, believing that Grevillea
pteridifolia posed a serious threat to the Florida natural environment, ordered all plants
destroyed. I am not aware of G.pteridifolia exhibiting similar pest plant potential in
Australia but this is a good demonstration of how an apparently harmless plant can cause
havoc given the right circumstances.
A number of banksias, mainly from Western Australia, are currently being grown on wildflower farms or as horticultural specimens in South Africa, Given the demonstrated ability of other Australian plants, notable Acacia, Hakea and Leptospermum, to invade the natural bush (or fynbos), experimental work was carried out on Banksia ericifolia to compare its invasive potential with Leucodendron laureolum, an indigenous proteaceous species.
A characteristic of many Australian and South African proteaceae is 'serotiny' or ability to store seeds in protective woody capsules or cones in the plant canopy until they are released by a fire or death of a parent plant. The authors of this study identified a number of characteristics of successful invasive species including:
As Table 1 shows, on most counts, B.ericifolia proved superior to Leucodendron laureolum and was considered by the authors to be likely to be highly invasive in fynbos.
- large numbers of seeds produced in a given time period,
- high seed viability,
- slow release of seeds from cones,
- ability to be dispersed by wind over considerable distances
- rapid seedling growth, and
- rapid establishment of plants.
Table 1: Comparison of invasive potential of Banksia ericifolia with Leucodendron laureolum
|Seed production ||16,500 seeds/plant**||570 seeds/plant|
|Seed release ||Up to 12 weeks||Most released in 1 week|
|Seed dispersal ||Up to 7.6 m||Up to 3.0 m|
|Seedling growth||Both were similar but B.ericifolia produced a stronger root system|
|Rate of growth in the field|
(4 yr old plants)
Stem diam. 26mm
|Less than 0.5m|
Stem diam. 7.6mm
|*||Approximately the same age.|
|**||Note that this is considerably higher than in Australia. When measured in terms of seeds per sq.m of projected canopy cover, the South African plants produced around 1,100/sq.m whereas a population near Sydney gave 200-330/sq.m.|
It remains to be seen whether banksias will prove to be aggressively invasive in South Africa but the high viability of seed, the large seed production and the virtual absence of seed predators ensures that given the right circumstances, they certainly have the ability to out-compete the local species.
Unwelcome invaders...What are the Culprits?
While almost any plant can become a weed or pest plant, some are more successful than others and Australian plants have an unenviable reputation in South Africa and Florida (USA) for invading and suppressing the local vegetation.
Beyond knowing that some acacias and hakeas in South Africa and a melaleuca in Florida are the cause of the problem, most of us probably have little idea of which species are responsible. The following lists give this information. I have also included several references for those who wish to learn more about the effects the plants are having. It is worth noting that the only work on biological control of these species is being done in South Africa.
Table 2: List of Pest Plants and Potential Invaders - Florida
(now M.viridiflora var rubriflora)
|Cajeput - tree||Broad leaf paper bark|
|Casuarina equisetifolia||Australian pine||Horsetail she-oak|
|Potential Pest Plants|
|Acacia auriculaeformis||Ear-leaf acacia||Ear-pod wattle|
|Cupaniopsis anacardioides||Australian carrot wood|| Tuckeroo|
|Eucalyptus camaldulensis||Red gum||River red gum|
|Schefflera actinophylla||Umbrella tree||Umbrella tree|
Table 3: List of Pest Plants and Potential Invaders - South Africa
|Acacia cyclops||Rooikrans||Western coastal wattle|
|Acacia longifolia||Long leaf wattle||Sydney golden wattle|
|Acacia mearnsii||Black wattle||(late) black wattle|
|Acacia pycnantha||Golden wattle||Golden wattle|
|Acacia saligna||Port Jackson wattle||Golden wreath wattle|
|Hakea gibbosa||Rock hakea||Peeling hakea|
|Hakea sericea||Silky hakea||Silky hakea|
|Acacia saligna||Port Jackson wattle||Golden wreath wattle|
|Sweet hakea||Sweet scented hakea|
|Leptospermum laevigatum||Australian myrtle||Coast Ti tree|
|Paraserianthus lopantha*||Stinkbean||Cape Leuwin wattle|
|Potential Pest Plants|
|Acacia baileyana||Cootamundra wattle||Cootamundra wattle|
|Acacia dealbata||Silver wattle||Silver wattle|
|Acacia decurrens||Green wattle||Sydney green wattle|
|Acacia elata (terminalis)||Cedar wattle||New Year (sunshine) wattle|
|Eucalyptus gomphocephala||Sand gum or tuart||Tuart|
|Eucalyptus lehmanniana||Spider gum||Bushy yate|
|Omalanthus populifolius||Queensland poplar||Poplar leaved omalanthus|
|Pittosporum undulatum||Sweet pittosporum||Sweet pittosporum|
|* Formerly known as Albizzia lopantha. Some authorities regard this species as being an introduced species rather than an Australian native.|
A. Grevillea pteridifolia and Banksia ericifolia
- Honig, M.A. Cowling, R.M. and Richardson, D.M. (1992). "The invasive potential of Australian banksias in South African fynbos : A comparison of the reproductive potential of Banksia ericifoIia and Leucodendron laureolum", Australian Journal of Ecology, 17 : 305-314.
- Neel, P.L. and Will, A.A. (1978)."Grevillea chrysodendron R Br: potential weed in south Florida". Hort Science, 13 (1): 18-21.
(Note: G.chrysodendron is an old name for G.pteridifolia)
The major accessible references for Florida are:
C. South Africa
- Austin, Daniel F. (1978). "Exotic plants and their effects is Southeastern Florida". Environmental Conservation, 5 (1) : 25 - 34.
- Austin, Dan (1996). "Three (more) rogue Aussies". Australian Plants online, December (n.p.)
- Lahart, David (1977). "Invaders of the Everglades", Florida Wildlife, March-April : 33 - 36.
- Morton, Julie F.(1969). "The cajeput tree - a boon and an affliction". Economic Botany 20 : 31-39.
The most comprehensive reference is:
A selection of other papers includes:
- Stirton, C. H. (ed) (1978). Plant Invaders : Beautiful but Dangerous, The Department of Nature and Environmental Conservation of the Cape Provincial Administration, Cape Town.
(This describes 26 of the "army of plant invaders that today threaten the vegetation of the great landscapes of our beautiful Cape Province". It also includes chapters on control of pest plants, lists of indigenous plants as alternatives and an extensive bibliography.)
- Hall, A.V. (1961). "Distribution studies of introduced trees and shrubs in the Cape Peninsula". Journal of South African Botany, 27 : 101 - 110.
- Hall, A.V. and Boucher, C. (1977) "The threat posed by alien weeds to the Cape flora" in Proceedings of the Second National Weeds Conference of South Africa Stellenbosch, 2-4 February, pp. 35 - 45. A. A. Balkema, Cape Town.
- Macdonald, I.A.W. (1984). "Is the fynbos biome especially susceptible to invasion by alien plants? A re-analysis of available data." South African Journal of Science, 80 : 369-377.
- Macdonald, I.A.W., Kruger, F.J. and Ferrar, A.A. (eds.) (1986). The ecology and management of biological invasions in Southern Africa, especially chapters by G. Shaughnessy, "A case study of some woody plant introductions to the Cape Town area", pp.37 - 43 ; Macdonald, I.A.W. and Richardson, D.M., "Alien species in terrestrial ecosystems of the fynbos biome", pp. 77 - 91 : and Macdonald, I.A.W., Powrie F.J. and Siegfried, W.R., "The differential invasion of southern Africa’s biomes and ecosystems by alien plant and animals," pp. 209 - 225. Oxford University Press, Cape Town.
- Macdonald, I.A.W. Clark, D.L. and Taylor, H.C. (1989). "The history and effects of alien plant control in the Cape of Good Hope Nature Reserve, 1941 - 1978", South African Journal of Botany, 55 : 56 - 75.
- Milton, S.J. and Hall, A.V. (1981). "Reproductive biology of Australian acacias in the south-western Cape Province, South Africa". Transactions of the Royal Society of South Africa, 44 (3) : 465 - 485.
- Richardson D.M. et al (1992). "Plant and animal invasions", in R.M. Cowling (ed) "The ecology of fynbos, pp. 271 - 308. Oxford University Press, Cape Town. (This paper has an extensive bibliography up to 1992).
- Roux, E.R. (1961). "History of the introduction of Australian acacias on the Cape Flats". South African Journal of Science, 57 (4) : 99 - 102.
- Taylor, H.C. (1975) "Weeds in the South Western Cape vegetation". South African Forestry Journal, (93) : 32 - 36.
Tony Cavanagh has been growing Australian plants since the early 1970s and has had a long time interest in members of the Proteaceae, notably banksias and dryandras. He formed the Dryandra Study Group and is currently the Group's newsletter editor. In recent years he has been able to follow up information on the growing of Australian plants in England and Europe and has published several articles and lists of plants which were cultivated in the eighteenth and nineteenth centuries. He has other interests which include documenting the work done in other countries on Australian pest plants and producing a regular bibliography of articles on studies of banksias. Tony is currently Document Delivery Librarian at Deakin University-- useful in researching these topics.
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Australian Plants online - September 1997
The Society for Growing Australian Plants