Changes

At the end of the dry season a Siam weed fallow contains per 100 g dry matter: *leaves: N 2.1-3.0 g, P 0.17-0.21 g, K 1.4-1.6 g, Ca 1.6-1.9 g, Mg 0.4-0.5 g;

Leaves and petioles have glandular dots emitting a strong pungent smell when crushed. Phenols and alkaloids in the plant, in particular in the leaves, have an allelopathic effect, inhibiting the germination of its own seeds and seedling development of other plants. Siam weed contains essential oils having an anti-bacterial activity on ''Staphylococcus aureus'' and ''Escherichia coli'' . Applied as a green manure in rice paddies, it may kill fish. It contains 4',5,6,7-tetramethoxyflavone, which enhances blood coagulation.

''C. odorata'' has been excluded from the genus ''Eupatorium'' L.; it can be distinguished by its rather consistent pattern of many rows of bracts, giving a cylindrical appearance to the head, by the 3 prominent veins of the leaves, and by the pungent smell of crushed leaves. In Hawaii, the name ''Eupatorium odoratum'' is often used for ''Ageratina adenophora'' (Spreng.) R.M. King & H. Robertson. The name ''Eupatorium conyzoides'' Miller is a synonym of ''Vernonia arborescens'' Swartz, but is sometimes used erroneously for ''C. odorata'' .

Due to its fast growth and nutrient accumulation and its copious litter production, Siam weed may play an important role as a fallow crop for restoration of soil fertility in cropping systems where shortening of the fallow period is inevitable. Used as a mulch it may contribute to the increase or maintenance of soil fertility and the improvement of the physical condition of the soil. Due to its rapid lateral spreading and superficial rooting, Siam weed also has good prospects for the control of soil erosion and ''Imperata cylindrica'' . However, more research is needed to incorporate Siam weed into semi-permanent cropping systems and to develop adequate measures to control its weediness in extensively managed crop production and rangeland systems.