Parthenium Weed

Parthenium hysterophorus L.

Family: - Asteraceae.


Parthenium is from the Greek partheniki the name of the Feverfew plant Tanacetum parthenium.

Hysterophorus from the Greek hystera meaning womb and phoros meaning carrying and refers to the large amount of fruit and seed produced.

Parthenium Weed because it is a weed from the Parthenium genus.

Other names:

Bitter weed

Carrot grass

Congress grass (India)

Escoba amarga (Caribbean)

Feverfew (Caribbean)

False Ragweed

Ragweed Parthenium (USA)

Whitetop (Jamaica)


An aromatic annual herb with light green much divided leave and daisy like flowers with white 'petals' and a yellow centre.





Form a rosette. Alternate.

Petiole - Short.

Blade - Pale green, 80-200 mm long by 40-50 mm wide, deep opposite lobes and the lobes themselves may be deeply and oppositely divided. Soft fine white hairs.

Stem leaves - Upper leaves are sessile, shorter, not or only slightly lobed with rounded tips. Soft fine white hairs.


Erect, lengthwise grooved, 300-1500 mm in height but may reach up to 2400 mm, and are much branched towards the top. Hairy. Becomes woody with age.

Flower head:

Daisy type, 4-10 mm in diameter with yellow disc florets in the centre and a ring of ray florets with creamy white ligules ('petals'). Borne at the ends of many branches that arise from the stem nodes and are held at a similar height.


2 types;

a) Many, yellow, male tubular flowers in the centre of the flower head.

b) 5, white 'petal', female flowers around the outside of the head.

Ovary -

'Petals' - Creamy white.

Stamens -

Anthers -


Black, flattened, 2 mm long, surrounded by 2 white, spoon shaped, membranous appendages attached to the top.


Enclosed in fruit.


Deep taproot with many finely branched feeder roots.

Key Characters:

White daisy type flowers with white 'petals' and a yellow centre.


Life cycle:

Annual. A flush of germination occurs after the first heavy rains between October and December with minor germinations at any time moisture is available. The seedling forms a rosette of leaves and sends up a flowering stem soon after so that flowering commences 4-8 weeks after germination and may continue for 6-8 months. The first flowers are formed in the top leaf axil and subsequent flowers form progressively in lower axils. The stem elongates in a manner that results in all flowers being held at about the same height. The five female flowers in each head usually only produce 4 seeds. The seeds normally remain on the plant until it dies. Up to 4 generations can be produced in a year.

The plant reproduces by seed in Queensland. In India vegetative reproduction by vegetative buds occurs.


Some of the hairs contain the allergenic substances.


By seed and vegetative buds.

Flowering times:

Seed Biology and Germination:

There is little dormancy and freshly produced seed normally germinates readily so that up to 4 generations can be produced in a single year.

Requires bare ground to germinate.

Seed viable in the soil for up to 2 years.

Tolerates a wide range of pH from 2.5-10 with an optimum of 5.5-7.1 at germination.

Tolerates a wide range of day/night temperatures from 15/4oC to 36/31oC.

White light retards germination and germination is enhanced when seed is transferred to dark conditions.

Germination is very sensitive to water stress ranging from 91% at field capacity to none at 0.9 MPa.

Seedling growth falls when temperatures fall below 5oC but they can stand a light frost at -2OC.

Vegetative Propagules:




Substances in the seeds, stems and leaves reduce the germination and growth of both Parthenium Weed, companion species and nitrogen fixing bacteria.

Pollen of Parthenium Weed reduces the fertilisation of other species flowering at the same time.

Population Dynamics and Dispersal:

Spread mainly by machinery, animals and in mud.

The spread in Queensland has closely followed the railway tracks and roads.

Spread by water during floods. Wind dispersal is mainly caused by whirlwinds.

There is some spread in hay and grain.

It colonises bared areas quickly and aggressively, leaving undisturbed and covered areas relatively clean.

Each plant can produce 15,000 seeds.

Plants require pollen from a different individual to set seed.

A dense mat of seedlings usually occur after a rain and the rosette form of growth prevents most other plant establishing so pure stand of Parthenium weed are common.

It spreads very quickly. Between 1951 and 1972 it spread over most of the western states of India. During the 1970's it spread over thousands of hectares in Queensland.

Origin and History:

North America. West Indies. Caribbean.

Probably first introduced to Australia with aircraft and machinery parts during the World War II and first recorded at Toogoolawah in Queensland in 1955.

Introduced again into Queensland in 1964 as a contaminant in grass seed from Texas, but not reported as a problem until 1973.

It spread rapidly through central and south east Queensland in the 1970's.

Found in NT and NSW in 1980.

Expected in Victoria soon!





Humid to sub humid tropics.


Most abundant on heavier fertile soils and alkaline clay loams.

In Queensland it prefers the grey and brown self mulching and cracking clay soils.

Plant Associations:

Brigalow and Gidgee country.



Concentrates potassium with around 4.8% KCl in the leaves and may be a source of potash.

Very high protein source with leaves containing over 54% protein. This has been developed into a stockfeed in India.

May have some potential as an insecticide as it has cholinesterase activity.

Formerly used in herbal medicine for fever and neuralgia but the side effects were too severe.


A significant weed of crops in many tropical and sub-tropical areas.

Weed of pastures, crops, sunflowers, grain sorghum, roadsides, railways, stockyards and disturbed areas.

Excludes nearly all other species when it infests an area.


It taints the meat of livestock forced to eat it too severely for human consumption. A months grazing on clean pastures is required to remove the taint.


The plant produces severe allergic reactions and asthma in persons sensitised to it. Men are more sensitive than women and cattle may also be affected and can pass it on in milk.

In India a number of deaths have been caused by the plant.


Contact dermatitis, itching eruptions on the face, neck, hands and back of the knees


Seek medical attention.

Wear protective clothing if there is a risk of exposure.


Noxious weed of NSW, NT, QLD, SA, TAS, VIC, WA.

Management and Control:


Eradication strategies:

Prevent stock or unclean machinery entering clean areas.

Maintain good pasture cover on threatened fields and if possible introduce perennial species.

Spray out small infested areas with a mixture of 2,4-D and atrazine.

Avoid hand pulling because of allergic reactions. If it is necessary, use gloves and protective clothing and thoroughly wash them regularly.

On large areas allow pasture to grow, burn it, then spray with 2,4-D plus atrazine or Tordon 75-D or hexazinone.

In cropping areas, summer fallow followed by winter crops give good control. If summer crops have to be planted then grain sorghum is preferred. Spot spray Parthenium weed appearing in fallows.

Isolated mature plants can be sprayed with diesel to kill the plant and sterilise the seed.

A number of selective herbicides are available for particular crops and situations.

Herbicide resistance:

Biological Control:

A number of biocontrol agents have been released and research is continuing.

Related plants:


Plants of similar appearance:


Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P109. Photo.

Hyde-Wyatt, B.H. and Morris, D.I. (1980) The Noxious and Secondary Weeds of Tasmania. (Tasmanian Department of Agriculture, Hobart, Tasmania). P36.

Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #936.1.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P292-296. Photos.


Collated by HerbiGuide. Phone 08 98444064 or for more information.