Dealing with the devil

A long-term control program on Yourka Reserve is saving native animals and plants in Queensland from one of the world’s worst invasive weeds.

Moving through Yourka Reserve in tropical far-north Queensland is no easy feat. The vast landscape is a melange of rocky outcrops and eucalypt forests cut through by creeks and billabongs. When those waterways swell during the wet season, much of the 43,500 hectare reserve becomes inaccessible by land.

Paul and Leanne Hales first arrived at Yourka shortly after its purchase in 2007 as Bush Heritage’s first Yourka reserve managers. It quickly became apparent to them that the relative inaccessibility of their new home would make one of their highest management concerns – reducing the spread of Siam Weed, also known as Devil Weed – very difficult.

But a decade on, Bush Heritage’s long-term and creative approach to Siam control has seen dramatic results. Of the original 14,000 individual Siam weeds recorded in the priority management zone, 90% have now been destroyed. Success stories such as this are rare when it comes to dealing with the ‘devil’.

“After years of blood, sweat and tears it's very satisfying to see the success that Paul, Leanne and a host of volunteers have achieved in controlling this weed,” says Bush Heritage ecologist Terry Mahney.

A national threat

Native to parts of southern and central America, Siam spread to tropical and subtropical regions around the world thanks to its ability to invade quickly and aggressively.

A single plant can produce over 80,000 seeds a year, each of which can remain dormant for up to eight years. The plant grows up to 5 metres a year and will climb trees to heights of 20 metres. This allows it to smother native habitats.

It was discovered in Australia in 1994 and the Australian government has since declared it a priority because of its capacity to cause widespread environmental and economic (its seeds are toxic to cattle) harm.

Siam’s spread in Australia has remained relatively contained to a few infestations in far north Queensland, but it could establish along large parts of the east coast and tropical north. Preventing that from happening is at the core of Australia’s Siam strategy, which is where the work being done on Yourka comes in.

An integrated approach

Volunteers have been critical in the success of Bush Heritage’s Siam control on Yourka. They've put in hundreds of hours of groundwork, helping to record and map Siam’s distribution across the reserve as well as helping with the arduous task of physically removing it.

“We have groups of four to six volunteers walking the creek lines at a time. If we had to do that work ourselves we’d have been doing it six times, so our volunteers are a massive asset,” says Paul.

Bush Heritage’s approach to Siam control has been multi-pronged, involving mulching, burning, hand weeding, herbicides, and even horseback patrols to find infestation sites. Generous donations and grants have also allowed for the employment of spray contractors and bulk herbicides, freeing up Paul to get on with other tasks.

Now, Paul says it’s possible that some patches of Siam will “completely drop-off” with a few more years of management, as dormant seeds come to the end of their life spans.

“We’re working really effectively now,” Paul says. “We know the best herbicide to use and we’ve learnt some other tricks, too, so we aren’t having to go back and do as much follow up,” says Paul.

The flow-on effect

Bush Heritage’s success with Siam will benefit the more than 500 native plant and animal species that call Yourka home, but it will also be felt on a much wider scale. Yourka sits adjacent to Queensland’s Wet Tropics World Heritage Area and several national parks, as well as being upstream of agricultural areas where Siam could thrive if given the chance.

“By controlling Siam Weed in the catchment area, Bush Heritage is significantly reducing the spread of Siam Weed into these valuable ecological and economic areas,” says Terry. “Knowing that the control program has made a significant contribution to the protection of Yourka Reserve and the neighbouring World Heritage Wet Tropics makes it all worthwhile.”

Siam control work on Yourka Reserve was made possible through funding from the Australian Government.