Siam weed

Native to the Americas, Siam weed has become a major land management issue across tropical areas of Asia and Africa. In 1994 it was detected in the Tully area of Far North Queensland and until recently was confined to that region. An incursion of Siam weed has recently been detected on two properties in the greater Darwin region.

Siam weed has high nitrate levels and in some countries has led to cattle deaths and abortions when stock have been accidentally fed contaminated fodder. Its rapid rate of growth means that it has the potential to outcompete crops, pastures and native vegetation. In addition to this, Siam weed may cause skin complaints and asthma in allergy prone people.

These potential environmental, economic and health impacts, when combined with the costs associated with its control mean that it is considered one of the world’s worst weeds.

The Weed Management Branch (WMB) has notified the National Biosecurity Management Consultative Committee of this most recent incursion of Siam weed. WMB is working closely with the landholders and other government departments to contain and control the current known infestation and conducting surveys to determine how far it may have spread.

As with any weed management, the early reporting of a suspected Siam weed infestation is essential to successful control and eradication efforts.

Siam weed can look similar to other weed species but there are a few outstanding features that help in identification.

The pungent smelling leaves of this scrambling climber are soft, green, hairy and roughly triangular in shape. They also have a very distinctive three-vein ‘pitchfork’ pattern running the length of each leaf. Pale pink-mauve tubular flowers are held in clusters at the end of the branches. The flowers can appear white if seen from a distance but will usually turn darker lilac when mature.

Siam weed flowering is triggered in mid-year by shorter day lengths and they produce enormous numbers of seeds within 8-10 weeks after flowering. Each seed has a tuft of white or brown hairs that act as a little parachute, allowing it to be easily transported by wind or water.

Preventing weed spread through biosecurity measures is as important to Siam weed management as early detection. The hairs on the seed help it attach to vehicles, clothing, footwear and animal fur. It is very important that travel through known areas of infestation is controlled and that vehicles, machinery and any personal gear used in an infested area be cleaned thoroughly before leaving an infected area.

Let’s work together to stop the spread of Siam weed. If you have seen Siam weed, or think you may have it on your property please contact the Weed Management Branch on (08) 89 99 45 67 or at

Last updated: 25 September 2019

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