Not all environmental weeds are exotic species; some native plants can behave like weeds when planted beyond their native range.
The beautiful Corymbia torelliana, commonly known as Cadaghi, is one such plant. Cadaghi was originally classified in the genus Eucalyptus but reclassified to genus Corymbia in 1995.
The Cadaghi is native to coastal ranges of northern Queensland from Ingham to Cooktown and once only grew in this area. However, it was only a matter of time before propagators would realise its marketing potential for the nursery industry. There were some concerns raised at the time but the fateful decision was made to release plants for sale. In hindsight, many curse that more consideration should have been given to the effects on ecosystems and wildlife including native bees and koalas – Cadaghi is not palatable to koalas.
The Cadaghi has proven to be somewhat of an unwelcome guest further south where it was initially planted widely as an ornamental and on farms as a windbreak. Since then, it has gradually naturalised itself across Queensland and into districts of northern New South Wales, in some areas even hybridising with local gums.
Growing to a height of 30 metres, and with a very dense canopy of large leaves, the Cadaghi creates a heavy shade over native understorey plants preventing them from growing. It has a significant potential to modify the diversity and structure of the native forests in sub-tropical Australia and is now regarded as an environmental weed in these areas.
Reproduction and Dispersal
The Cadaghi is able to produce large amounts of wind borne seed; a single specimen may be responsible for widely dispersed and numerous offspring.
Seedlings can easily be identified in bushland areas by the dense cover of reddish hairs on their stem and leaf stalks, which makes them rough to touch.
Seeds are also spread by native bees as they actively forage on the flowers during October and November. The Cadaghi is one of the few Corymbias where the opening of the gumnut is large enough to allow the entry of a stingless bee.
The sticky resin inside is used for nest structure but often seeds stick to the bees in the process. They struggle to carry them as the seeds are about half the weight of the bee itself.
Native bee expert Bob Luttrell, known as Bob the Beeman, said the bees do not intentionally collect these seeds and will go to some trouble to remove them. “This is why you’ll find groups of young seedlings at the base of a nest – they can come up in their hundreds. Sometimes seeds coated in sticky resin can clog or even seal the entrance of the nest. It appears to be more of a problem with poorly designed boxes that have tiny entrances rather than in natural hollows.”
Bob said bee keepers began noticing a new behavioural anomaly with their bees when the trees began flowering in southern regions.
“There is a difference in their pattern here. I believe that in the north there are greater sources of resin available over the season, and the bees do not build up a ‘deficit’ demand for resin. There is no need for them to rush to collect it when it does become available.
“I have observed my bees to fly just over a kilometre to collect this resin and bring seed all the way back. They would also have dropped seed on the flight path and that is a very extensive spread of seed. The bees have changed the character of the landscape by spreading these trees,” Bob said.
In the 1970s Cadaghi, with its easily recognizable light green trunk and flaky base, was extensively planted as a garden and street tree. Its fast growth rate, strikingly smooth branches and attractive red growth tips made it popular with landscapers.
However, as the trees reached maturity, gardeners began noticing some negatives. Its large horizontal limbs are prone to snapping but a more common complaint is the black sooty fungus, which quickly coats whatever is under the canopy including pavers, outdoor furniture and vehicles. Cadaghis are highly attractive to Red-shouldered leaf beetles (Monolepta australis );yellow sticky traps provide an early indication of beetle presence.
Brisbane City Council has listed Cadaghi as a “Class R” (reduce populations) plant and other Councils have done similar as the best way to stop the spread of invasive plants is stop their initial incursion. The Nursery & Garden Industry Australia (NGIA) acknowledges the large number of ornamental plantings has created a large ‘seed bank’ that enables Cadaghi to invade natural bushland. Its ‘Grow Me Instead’ website encourages growers not to grow and sell plants which could be invasive. The advice to consumers considering planting Cadaghi trees is to seek out and plant local native species instead.