Siam weed (Eupatorium odoratum), native to the neotropics from the eastern USA to northern
Argentina[1], has become a major weed of crops, plantations, savannas and natural forests in many parts
of the world[2]. It is a perennial shrub forming dense tangled bushes 1.5-2.0 m in height, occasionally
reaching 6 m as a scrambler up trees. Seed production is prolific, the achenes bears a small stiff pappus
and are wind-dispersed[3], so it has great potential to increase in distribution and density.
Siam weed was first recorded in the southern parts of Yunnan and Hainan provinces of China in
1934 , it has spread extremely rapidly, now it prevails in Yunnan and Hainan provinces, and has expanded
its distribution to the southwestern part of Guangxi province, the coast south of Guangdong province. It
also tends to be a sporadic problem in Hongkong city, Guizhou and Taiwan provinces[4]. The weed
decreases the carrying capacity and species diversity in both grassland and forest[5]. It is one of the top 9
alien invader plants in China[6], and attract significant attention because of threats they pose to local
diversity and economics.