Parthenium hysterophorus, commonly called parthenium or congress weed, is an aggressive invader that is native to Mexico. The weed was first seen growing in Ethiopia in 1988 near food-aid distribution centres, so it is presumed that imported wheat grain was contaminated with its seeds. Once introduced, the weed was able to spread rapidly, as the seeds are readily dispersed in mud adhering to vehicles, machinery and animals, as well as by water and wind. The seeds can remain viable on the soil surface for up to two years, while buried seeds can stay dormant for as long as 20 years before germinating.
Parthenium is a herbaceous annual with an erect stem that becomes woody with age, allowing it to reach a height of two metres. It colonises disturbed land, including overgrazed and recently ploughed or cleared areas, and because it has an allelopathic effect – the chemical inhibition of growth and seed germination of other plants – it can quickly dominate pastures and crop fields.
Indeed, it has had such a devastating effect on crop production in Ethiopia that it has earned alocal name meaning ‘no crop’. The weed is unpalatable to livestock, so its invasion results in grazing shortages; if it is mixed in with fodder, it taints the meat and milk. Parthenium also poses a health problem for both humans and livestock, because contact with the plant or pollen can cause allergic reactions such as dermatitis, asthma and hay fever.
Elsewhere in Africa the weed has invaded the sub–tropical regions of South Africa – where it is especially problematic in sugarcane and banana plantations – as well as Swaziland, Mozambique, Zimbabwe and Madagascar. Although individual plants can be killed with foliar application of herbicides, rapid regeneration from seed soon follows.
The only successful chemical control method is to use residual soil-applied herbicides to kill pre-emergent plants, but these are non-selective and environmentally hazardous. The best method of control is to maximise competition against the weed by maintaining good grass
growth. This requires exclusion of grazing livestock until grass has become re-established, followed by a reduction in stocking rates to prevent reinvasion by the weed. Biocontrol agents have been released in some countries, but these have not yet achieved adequate control.
Reference: Matthews S. & Brandt K. Africa Invaded:The growing danger of invasive alien species Global Invasive Species Programme 2004