CADAGHI TO BE DECLARED A NOXIOUS WEED?

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David McMinn

The weed status of Cadaghi is currently being reviewed and it may soon be declared noxious. This will mean that land owners will have to take measures to control the spread of this tree on their properties. It already is regarded as an environmental weed under the Bushland Friendly Nursery Scheme. Despite this adverse classification, Cadaghi could still be purchased at The Channon market until recently. Alas, a few nursery people are willing to sell environmentally damaging weeds so long as they make some money. This tree was widely used during the 1980's for garden plantings even though black sooty mould fungus and attack by monolepta beetles and scale insects made it unsuitable for this purpose. It was also extensively promoted for wind breaks and other farm plantings throughout the North Coast. By the 1990's, it was increasingly apparent that this species was becoming a major weed threat.

Description. Cadaghi was originally classified in the genus Eucalyptus, but in 1995 this was split into two genera and the scientific name for this species changed to Corymbia torelliana. This species is often a large emergent tree up to 30m high in the tropical rainforest and wet sclerophyll forests of north east Queensland. The light green trunk and branches are its most striking feature. The leaves are also light green, often with pinkish new growth. It flowers profusely in summer with masses of cream flowers, which are heavily honey scented. The fruit is a round woody capsule and the tiny seeds are spread by wind and gravity. It is one of the few species of the eucalyptus group that grows in rainforests.

Adverse Impacts

Kills Native Bees. It is not clear how Cadaghi actually kills these insects. Apparently, native bees collect Cadaghi seeds for the waxy appendage, but this resin melts easily, clogging up hives and resulting in bee deaths. Native bees are key species for the pollination of many native plants. Reduced numbers or extinctions of native bees would have a severe impact upon many local plant populations. Commercial European bees are removed when Cadaghi is in flower and thus it is not a problem for  honey production. 

Hybridisation. Cadaghi and the Large Leaved Spotted Gun (Corymbia henryi) have been recorded as hybridising in the North Coast - Moreton Bay regions. If this hybrid produces viable seed, the gene pool of C henryi will become contaminated and interbreeding could result in eventual extinction of this species. Other species of Corymbia may hybridise with Cadaghi presenting even more environmental concerns. Fortunately, Cadaghi and the Spotted Gum (Corymbia maculata) do not flower at the same time and can never cross pollinate naturally. 

Rampant Weed. The absence of controlling agents such as insects, parasites and pathogens means that many plant species can spread out of control in a new environment. Fast growing weeds out compete native species thereby reducing their populations and biodiversity. Curiously on our farm at Blue Knob, Cadaghi has not presented a problem. A few seedlings have come up (easily eliminated) but that is all. However, 6 kilometres on the other side of the valley near Djanbung Gardens, it has become a serious pest invading the nearby sclerophyll forest and, remarkably, even growing well under large, shady Camphor Laurels. Why two nearby locations should produce such contrasting outcomes is debatable. Different soils and microclimates are possibilities. We are in the process of eradicating Cadaghi trees at the back of our property and will keep a close watch on the trees near the house to make sure they do not get out of control. Like many people, we used Cadaghis in windbreak plantings because they keep their lower branches and were very suitable for this purpose. We want to retain these trees if possible, as removal would leave large gaps in the windbreaks around our house.

Uses. If widespread elimination of trees becomes necessary, profitable uses need to be found for Cadaghi timber. Unfortunately, there is not much information available on this hard wood. According to K R Bootle (Wood In Australia), its heart wood was 'durable', but no information was given to allow comparisons to be made with other timbers. Nan and Hugh Nicholson (Rainforest Plants V) mentioned that it made excellent planking for boats, although finding outlets for this purpose may present problems. A local furniture maker also uses Cadaghi branches to produce bush furniture because it is both light and strong. How well it would burn as fire wood remains unknown.

Native & Semantics  

Because a plant is 'native', people seem to have the mistaken belief that it is environmentally friendly. This is not necessarily the case and the term native may be interpreted in various ways. 

*          It is a species native to Australia - the meaning most commonly used. Australian natives are considered to be OK to use in gardening/farming and it is assumed that they will not become weeds or cause ecological problems. This assumption can prove to be completely incorrect, with Cadaghi being only one of many examples. Silky Oak, Native Daphne, Cootamundra Wattle and Umbrella Tree are just some of many 'native' species that have gone feral when planted outside their natural habitat. Then there is the major problem of hybridisation. The Alexander Palm (Archontopheonix alexandrea) from north east Queensland hybridises with the local Bangalow Palm (Archontopheonix cunninghamiana) potentially degrading the integrity of the gene pool of that species in the wild. For this reason, plant only Bangalow Palms in our region, especially as they have the added advantages of faster growth, better cold hardiness and, arguably, are more attractive. An extreme example of hybridisation is Eucalyptus species in southern France. Various species of this genus were introduced into that region in the 19th century. They have interbred so completely that botanists have trouble determining the taxonomy of a particular tree without resorting to DNA testing. To prevent unwanted hybridisation, only plant species of Eucalyptus/Corymbia native to our region. There are many North Coast species to chose from, which are suitable for all sites: dry/wet, boggy/well drained, steep/flat, deep/stony soils and so forth.

*          Other people use 'native' in relation to species in the Northern Rivers. Even so, people have introduced new species into areas where they were not part of the natural ecology.  Pink Euodia (Meliocope elleryana) is native to the Northern Rivers but was originally only found along the coast. People have now widely planted this attractive species in inland frost free sites. Similarly, Dunn's White Gum (Eucalyptus dunnii) has only a limited natural distribution, but is now widely utilized in forestry, farming and in large gardens throughout the region. 

*          Environmental purists consider 'native' applies to those plant species found in a local area prior to white settlement. This is ignored by virtually everyone. How many locals only plant species native to the Nimbin Valley? In the long term, this concept of native is really only academic. Another 100 years of global warming and the ecology of our valley could change from subtropical rainforest/wet sclerophyll to tropical monsoon forest with completely new species. Plant communities are naturally in a state of flux over thousands of years due to alterations in climate. Human induced changes are another matter and global warming will cause sever strains on forest ecology throughout Australia over the next century.

People will not change and they will continue to plant inappropriate plant species with a severe detrimental effect on the local ecology. What seems like a good plant option, can easily turn into a weed nightmare. Cadaghi is yet another example of the need to plant only suitable tree species in our region. This includes only natives to the North Coast and those native/exotic species known to have no adverse environmental impact. Over the long term, I remain pessimistic about maintaining the ecological integrity of our beautiful valley. Weeds, exotic diseases, global warming and human stupidity do not bode well for the future.

Copyright.   2004. David McMinn. All rights reserved.