Siam Weed (Chromolaena odorata)

Siam weed is a tropical species of flowering shrub that belongs in the same family as the sunflower (Asteraceae) (Gunasekera, 2009). It is native to North America, Mexico and the Caribbean, it is a rapidly growing perennial herb that can reach a height of 2.5m in open areas and within shady areas it acts like a creeper allowing it to reach around 10 metres in height (Gunasekera, 2009).

Siam weed contains carcinogenic pyrrolizidine alkaloids making it extremely toxic to cattle and other organisms, while causing severe allergic reactions in humans (DAFF, 2014).

Close up of the flowers of Siam weed.

Source:http://www.daff.gov.au/plants/weeds-pest-animals-ants/weeds/a-z-listing-of-weeds/photo-guide-to-weeds/siam-weed. Accessed on the 27th of May 2014.

Siam weed is a hairy and glandular plant that gives off a pungent aromatic odour when the leaves are crushed. Its leaves can be between 4-10cm long with flowers of either white or pale pink, the are slightly hairy also and are spread through wind dispersal, dingy to fur, clothes or machinery (Gunasekera, 2009). Seed production within individual weeds can be anywhere between 80000 to 90000 per plant. The seeds need light in order to germinate and under ideal conditions can grow up to 3cm a day (Gunasekera, 2009).

An area of land Siam weed has invaded.

Source: http://www.lrm.nt.gov.au/weeds2/find/siamweed#.U4Q5F9wVfwI, accessed on the 27th of May 2014.

In Indonesia Siam weed has medicinal uses, when the young leaves are crushed the resulting liquid is used as a treatment for wounds on the skin (Gunasekera, 2009). In other countries Siam weed has been used as an ornamental plant, allowing for it to potentially cause problems if it escapes into the wild.

As an invasive species, siam weed is known for its invasion of field crops and the natural landscape of the areas it has been introduced into (Struhsaker, et al. 2005). Reports suggest that it has invaded the protected rainforests of Africa where it prevents the regeneration of important tree species, its flammability also affects forest edges causing damage to rain forests effected (Struhsaker, et al. 2005).

A biological control initiative to eradicate it from Sri Lanka and Guam was conducted in the early 1970's, this initiative used a defoliating artiid (Gunaserkera, 2009). This proved unsuccessful and was later scraped. Within Australia it was first discovered in 1994 and a harsh herbicide regime was implemented quickly, this proved much more successful than the 1970's eradication program (DAFF, 2014).

Current and potential distribution of Siam weed in Australia.

Source: http://www.weeds.org.au/cgibin/weedident.cgitpl=plant.tpl&state=qld&s=&region=nnc&card=S20, accessed on the 27th of May 2014.

References:

Gunasekera, L. (2009). Invasive Plants: A guide to the identification of the most invasive plants in Sri Lanka. Colombo. 116-117.

Siam weed (Chromolaena odorata). 2014. [Online] Available at: http://www.daff.gov.au/plants/weeds-pest-animals-ants/weeds/a-z-listing-of-weeds/photo-guide-to-weeds/siam-weed. Accessed on the 27th of May 2014.

Struhsaker, T., Struhsaker, P. & Siex, K. (2005). Conserving Africa's rainforests: problems in protected areas and possible solutions. Biological Conservation 123. 45-54.