Species:
Parthenium hysterophorus
Family: Asteraceae (Daisy family).
Common names: Parthenium weed, Bitterweed, Carrot grass, Congress grass, Escoba amarga, False ragweed, Feverfew, Parthenium, Ragweed, Ragweed parthenium, Whitetop.
Description: Seedlings – cotyledons are a rounded paddle shape, 3 - 4 mm long, borne on short stalks 1 – 2 mm long. The first true leaves are egg-shaped and covered in fine, white hairs. Older leaves become increasingly lobed and deeply divided.
Young plants – develop into a rosette, with leaves to 80 - 200 mm long and 40 - 50 mm wide.
Older plants – develop an erect, highly branched stem 30 – 150 cm high, and a deep taproot. Deeply divided leaves develop along the stem, which is deeply grooved. Stems and leaves are covered in short, white hairs. Plants develop a bluish or greyish appearance.
Flower heads – occur in clusters at the top of the plant, borne on short stalks, arising from the leaf forks. Flower heads are white, 4 - 10 mm across and form an unusual, 5-sided shape, with longer white flowerlets (ray flowers) in the corners. Flower heads become hard and brown as they mature.
Seeds – are striped grey to black and a narrow diamond shape, 2 mm long and flattened. They have a brown tuft on the end formed from 2 broad scales 0.5 mm long. Seeds are tightly grasped in a brown outer coat, which gives them more of a tufted triangle appearance. Generally only 4 seeds develop in each head.
Lifecycle/Biology: Parthenium weed can germinate at any time of the year, with the main germination in spring and early summer. Plants can flower 4 – 8 weeks after germination, and flowering may continue for 6 to 8 months. Seed has no dormancy. Mature plants have some frost tolerance.
Ecology: Parthenium weed is a perennial plant which is well adapted to invade much of the Australian farmland area.
The problem: Parthenium weed is an aggressive invader of fallows, wastelands, roadsides, overgrazed pastures and less competitive crops. It produces a range of toxins which affect other plants and animals. It is not commonly eaten by livestock but can taint meat and toxins will pass into milk. Regular contact with parthenium causes dermatitis in animals and humans and can cause respiratory problems including asthma.
Distribution: Parthenium weed has become established in central and northern Queensland, the western Downs and the Northern Territory.
New South Wales is a parthenium weed exclusion zone. Any parthenium plants observed in New South Wales must be immediately reported to the NSW Dept. Primary Industries or a Local Council Weeds Inspector who will organise for their eradication.
Farmers should pay particular attention to sites of header breakdowns, as these have been a common source of infestation due to the release of parthenium seeds following the removal of panels during repairs.
Origin: An introduced weed from North and South America.
Reference: Crop Weeds of Northern Australia, p. 63 - 64.
Noxious Weeds of Australia, p. 292 - 296.
Compiled by: Graham Charles
