Common names: Carrot weed, White top, Star weed, Congress weed (India) (English), Parthenium (Sinhala)
Synonyms: Argyrochaeta bipinnatifida
Taxonomic notes: N/A
This species is an ephemeral, herbaceous, annual weed which is spread by seeds. Plants first form a basal rosette up to around 30 cm (a foot) in diameter, of finely lobed leaves. Plants then form a paniculately branched pubescent stem with lengthwise grooves. The leaves on the stem are alternate, and the upper leaves are entire to slightly lobed. All leaves are light green in colour and pubescent on both sides. Flowers, are borne on the tip of the stems and are small (3.2-5.1 mm wide) and whitish with tiny ray florets at each of 5 distinct corners. A small seed is produced in each of the five corners.
Total height: It grows to heights of 2 m.
History and introduction:
In India this species was first described in 1810 but was not considered a serious problem until after it was introduced in 1955 in contaminated cereal grain (Rao, 1956). It has ever since spread to most of the sub-continent (Nath, 1988). The weed is thought to have entered Nepal from India and is currently found throughout most of the lowland Tarai region that borders India and within most of the cities.
It has been stated that this species was introduced to Sri Lanka through goats imported from India by the Indian peace keeping force in 1987. This plant was first identified from the Vavuniya district. Seeds of P. hysterophorus are believed to have entered the island along with seeds of onion and chillies imported from India as a contaminant (Wijesundara, 2010).
This species is abundantly found in northern dry and arid zones of Sri Lanka. The affected habitats/ecosystems are listed as wastelands and irrigation canals.
Dispersal and reproduction:
The seeds are mainly dispersed through water currents, animals and the movement of vehicles, machinery, livestock, grain, stock feed and other products. Germination of these seeds can occur between 8-30 ⁰C, the optimum temperature for germination being 22 to 25 ⁰C. It can also be spread by wind because its seeds are small (1-2 mm diameter) and light (50 µg) and therefore are able to travel long distances (Navie et al., 1996; Taye, 2002). The transportation of soil, sand and gravel from Parthenium-infested areas to non-infested areas for construction purposes may be the reason for high infestation levels along roadsides and around buildings (Taye, 2002).
Impact on native species and habitats:
Parthenium weed, due to its allelopathic potential, replaces dominant flora and suppresses natural vegetation in a wide range of habitats. Thus it is a serious threat to biodiversity. Wherever it invades, it forms a territory of its own, replacing indigenous grasses and weeds that are supposedly useful for the grazing animals (De & Mukhopadhyay, 1983). Parthenium weed has an adverse effect on a variety of natural herbs that are the basis of traditional medicines systems for the treatment of several diseases in various parts of the world (Mahadevappa et al., 2001; Shabbir & Bajwa, 2006). It is also a poisonous or lethal weed for agricultural labourers and city-dwellers who are sensitive to it (Mukhopadhyay, 1987). It can cause asthma, bronchitis, contact dermatitis, eye irritation, rhinitis (Gupta et al., 1996), and sinusitis (hay fever) (Agarwal & D'Souza, 2009; Towers & Mitchell, 1983).
Direct exploitation/ destruction of native species:
It is an environmental weed which can cause irreversible habitat changes in native grasslands, woodlands, river banks and floodplains (Jayachandra, 1971). The presence of Parthenium pollen grains inhibits fruits of tomato, brinjal, beans, etc., and is also responsible for the bitter milk disease in livestock fed on grass mixed with its leaves (GRIN 2008).
This species is not utilized for any purpose in Sri Lanka. However, it is reported to have insecticidal, nematicidal and herbicidal properties. It is also used for composting (FAO 2015).
Natural Threats (Pests): Not known.
Prevention and Control:
Prevention: Several Parthenium eradication campaigns have taken place in Sri Lanka to control this serious invasive species, especially the campaigns undertaken by Sri Lankan troops, department of Agriculture and Forest department are substantial efforts. Although this species has been controlled in some areas, it is still spreading in Jaffna peninsula. Since the seeds can be spread via wind or flowing water, seed production should be prevented by destroying the plants before flowering or seed setting. Also continuous removal of the weed is required. The spread of seeds through the trading and transport of goods, animals grazing on infested fields, and the transportation of sand, soil and compost from infested areas to uninfested areas also must be regulated. Mechanical control: Grading, mowing, slashing and ploughing are considered inappropriate as they may promote seed dispersal as well as rapid regeneration from lateral shoots close to the ground (Navie et al., 1996). Fire has been used to control the first flush of emergent weeds at the beginning of the rains and is only considered to be a short-term control measure. Chemical and biological control: A beetle native to Mexico, Zygogramma bicolorata, was first introduced to India in 1984 and has since become widespread and well-established, and is capable of defoliating and killing this weed. P. hysterophorus has been found to be resistant to glyphosate, a popular systemic herbicide, prohibiting its control in this manner. Paraquat (Gramxone) solution is sometimes applied to plants, when the weeds are young. If the population in a cultivated field is light, it can be removed manually.