Report 2016
Chromolaena odorata, also known as Siam Weed, is an herbaceous to woody perennial invasive
plant species that is considered one of the world’s worst weeds. The plant has a bushy habit
which forms a very dense thicket about 2 m high. After the first year of growth, the plant
develops a strong, woody underground storage organ, which can reach a diameter of 20 cm
(http://www.cabi.org/isc/datasheet/23248)
The weed has effective short and long distance dispersal mechanism, jeopardizing pasture and
farmlands in the tropical region, including Tanzania (Crutwell McFadyen and Skarrat; 1996;
Kriticos et al., 2005; Raimundo et al., 2007). Most of the seeds produced by the plant enter the
soil and build up a seed bank which may survive up to 6 years (Waterhouse and Zeimer, 2002).
The seeds are generally wind-disseminated but they can also stick to fur, feather and clothes.
Siam weed is highly competitive. It has prolific reproduction, fast growth and branching habit,
which ensures rapid domination and suppression of other species. Under its very dense canopy
thicket, light is scarce and other fast-growing species cannot survive. Slow-growing, shade-
tolerant species are regularly bent to the ground by the continuous pressure of the growth of
new C. odorata twigs on the upper layer of the thicket (Gautier, 1992b). The plant has a very
efficient root system for nutrients absorption (Bennet and Rao, 1968), and allopathic effects
may also be involved in suppressing other vegetation (Ambika and Jayachandra, 1980b;
Nakamura and Nemoto, 1993).
Siam weed is considered as a weed in all perennial crops of the humid tropics, pasture and
forestry. Its aggressiveness is much more serious where it is an exotic plant, rather than where
it is native. The weed grows in areas with an annual rainfall below 1000 mm, provided the dry
season is not too long and it is limited to around 2000 m altitude. It grows on soils ranging from
sand dunes to heavy clays (Liggit, 1983), and it is heavily dependent on the availability of light.
The weed has a lot of negative impact in grassland and cropland. In low-growing annual and
perennial crops, C. odorata can completely overwhelm the crop, whereas in taller crops, as
soon as the canopy is closed the weed is no longer a problem. In shifting cultivation, the weedreplaces the natural secondary succession and becomes the dominant fallow species (Slaats,
1995). The weed out competes and causes severe problems in pastures growth (Audru et al.,
1988). It has high nitrate content in its leaves leading it to be poisonous to cattle (Sajise et al.,
1974). C. odorata can also transmit pathogenic fungi (Oritsejafor, 1986), and act as a host for
insect pests including Zonocerus variegatus (Chapman et al., 1986). In regions where there are
dry seasons C. odorata can be a fire hazard (Englberger, 2009).
The weed’s presence in Serengeti district was first documented in Rung’abure village less than
five years ago. Since then, the weed has prevailed and its distribution has been increasing fast
to areas which were previously not infested. Both croplands and pastureland are affected, and
the magnitude appears to increase rapidly with time. This study intended to establish the
extent of the spread of C odorata in the Serengeti District at the time of the study, and relate it
to some biophysical factors from existing database.