parthenium weed

Parthenium hysterophorus
Herb
Alternate
Simple
White
Green
Discoloured

A short-lived herbaceous plant forming a basal rosette of leaves in the early stages of growth. Later producing an upright stems that are ribbed lengthwise, hairy, and much-branched at maturity. Its leaves are deeply divided and fern-like in appearance, but are smaller towards the top of the plant. Large numbers of small white flower-heads (4-5 mm across) are borne at the tips of its branches. These flower-heads produce five small winged 'seeds' (about 2 mm long).

Common names 
Also known as: parthenium weed, false ragweed, bitter weed, carrot grass, congress grass, parthenium, Santa Maria,
Family 
Asteraceae
Deciduous 
No
Flowering time 
Spring-Summer
Native/Exotic 
Exotic
Origin 
Native to Mexico, Central America, the Caribbean and South America.
Notifiable 
No
State declaration 
Class 2
Council declaration 
Class C – Containment and reduction
Known distribution 

This species is distributed mainly in the coastal and sub-coastal districts of central Queensland. It is also relatively common in parts of northern Queensland and present in south-eastern Queensland, inland New South Wales and the Northern Territory. Also widely naturalised in other parts of the world, including Africa (i.e. South Africa, Mozambique, Kenya and Ethiopia), Madagascar, the Indian sub-continent (i.e. India, Nepal, Bangladesh and Pakistan), south-eastern Asia (i.e. Vietnam, Taiwan and southern China), the Mascarenes (i.e. the Seychelles and Mauritius), eastern USA and Oceania (i.e. Vanuata, Tahiti, New Caledonia and Hawaii).

Habitat 

A weed of semi-arid, sub-tropical, tropical and warmer temperate regions. It is found along roadsides, railways and waterways and in pastures, floodplains, grasslands, open woodlands, waste areas, disturbed sites, lawns, gardens and crops. It is particularly aggressive in degraded or disturbed pastures in semi-arid environments.

Habit 

A much-branched, short-lived (i.e. annual), upright (i.e. erect) herbaceous plant that forms a basal rosette of leaves during the early stage of growth. It usually grows 0.5-1.5 m tall, but can occasionally reach up to 2 m or more in height.

Impact and control methods 

Parthenium weed (Parthenium hysterophorus) is one of the 20 Weeds of National Significance (WoNS). It is regarded as a significant evironmental weed in Queensland, and as a potential environmental weed in New South Wales, Victoria, the Northern Territory and Western Australia.

Stem and leaves 

The greenish-coloured stems are ribbed lengthwise (i.e. longitudinally striate), covered in small stiff hairs (i.e. hirsute), and become much branched at maturity. The alternately arranged leaves are simple with stalks (i.e. petioles) up to 2 cm long and form a basal rosette during the early stages of growth. The lower leaves are relatively large (3-30 cm long and 2-12 cm wide) and are deeply divided (i.e. bi-pinnatifid or bi-pinnatisect). Leaves on the upper branches decrease in size and are also less divided than the lower leaves. The undersides of the leaves, and to a lesser degree their upper surfaces, are covered with short, stiff hairs that lie close to the surface (i.e. they are appressed pubescent).

Flowers and fruits 

Numerous small flower-heads (i.e. capitula) are arranged in clusters at the tips of the branches (i.e. in terminal panicles). Each flower-head (i.e. capitulum) in borne on a stalk (i.e. pedicel) 1-8 mm long. These flower-heads (4-5 mm across) are white or cream in colour and have five tiny 'petals' (i.e. ray florets) 0.3-1 mm long. They also have numerous (12-60) tiny white flowers (i.e. tubular florets) in the centre and are surrounded by two rows of small green bracts (i.e. an involucre). Flowering can occur at any time of the year, but is most common during spring and summer, and least common during winter. Five small 'seeds' (i.e. achenes) are usually produced in each flower-head (i.e. capitulum). These achenes (1.5-2.5 mm long) consist of a black seed topped with two or three small scales (i.e. a pappus) about 0.5-1 mm long, two straw-coloured papery structures (actually dead tubular florets), and a flat bract.

Reproduction and dispersal 

Parthenium weed (Parthenium hysterophorus) reproduces by large numbers of seed.These seeds are dispersed by wind, water, animals, vehicles, machinery (particularly on harvesters and bulldozers) and in clothing. They are sometimes also spread in mud and contaminated agricultural produce (e.g. fodder and grain).

Similar species 

Parthenium weed (Parthenium hysterophorus) can be confused with annual ragweed (Ambrosia artemisiifolia), perennial ragweed (Ambrosia psilostachya), burr ragweed (Ambrosia confertiflora) and lacy ragweed (Ambrosia tenuifolia) when in the vegetative stage of growth. However, parthenium weed (Parthenium hysterophorus) can be distinguished from all these species by its ribbed stems, and also by white flower-heads (i.e. capitula) when it is in flower.The fleabanes (e.g.. Conyza bonariensis, Conyza canadensis, and Conyza sumatrensis) are also reasonably similar, but do not have highly dissected leaves or ribbed stems, and their seeds are topped with a ring (i.e. pappus) of whitish hairs.