Be aware: Siam weed detection

An incursion of Siam weed (Chromolaena odorata) has recently been detected on two properties in the greater Darwin region. Primary producers should be aware of this and check any new plants they find on their properties. As with any weed management, early reporting of a suspected Siam weed infestation is essential to successful control and eradication efforts.

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.

The NT Weed Management Branch notified the National Biosecurity Management Consultative Committee of this most recent incursion of Siam weed. We are working closely with landholders and other government departments to contain and control the infestation and are conducting field surveys to determine how far it may have spread.

Why is it a problem?

Siam weed has high nitrate levels and, in some countries, has led to cattle deaths and abortions when stock have been accidently fed contaminated fodder. Its rapid growth rate means that it has the potential to out‑compete crops, pastures and native vegetation. In addition, 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 control, mean that it is considered one of the world’s worst weeds.

How to identify it

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.

Flowering is triggered mid-year by shorter day lengths. The flowers produce enormous numbers of seeds within 8-10 weeks of 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.

Siam weed  in flower in the NT

Come clean, go clean

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.

Report it

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

Last updated: 28 September 2020

Share this page:

Was this page useful?

Describe your experience

More feedback options

To provide comments or suggestions about the NT.GOV.AU website, complete our feedback form.

For all other feedback or enquiries, you must contact the relevant government agency.