Perennial Ragweed

Ambrosia psilostachya DC.

Synonyms - Ambrosia artemisiifolia, Ambrosia coronopifolia, Ambrosia californica.

Family: - Asteraceae.


Perennial Ragweed refers to the ragged appearance of the plant and it perennial life form. Ambrosia is from the Greek word for 'food for the gods' because this plant was supposed to give immortality. Psilostachya is from the Greek words meaning 'bare ear' because this plant has a leafless spike or ear of male flowers.

Other names:

Herbe solferino (Mauritius)

Western Ragweed (USA)


Erect, stout, branching, hairy, rhizomatous perennial herb but often with annual tops, 30 to 200 cm tall that form dense clumps. The leaves are lobed and rough to touch.





Mainly alternate with lower ones opposite. Grey green.

Stipules -

Petiole - Short.

Blade - Oval in outline, 30-120 x 15-75 mm. Lobed halfway to midrib or double lobed. Lobes are oval with a pointed tip, edges may have a few teeth that are 2-4 mm wide. Rough to touch. Glandular hairs with blistery bases. Aromatic.


Erect, branching, green to grey green or whitish. Hairy. Up to 2000mm. Striped. Woody at the base.

Flower stem - Branched panicle.

Flower head:

Separate heads for male and female flowers. Male heads higher than the female heads. Often very branched. 35- 100mm long. Male heads in raceme like clusters and tend to be nodding, hemi spherical clusters about 3 mm diameter. Female heads in a few to several headed clusters on short branches, or in single flowered heads in the axils of the upper leaves underneath the male spikes.


Male and female on the same plant.

Male - 2.5mm diameter, bracts toothed and hairy with blistery bases. Cream or pale yellow green. Tubular.

Female - Few, single, 2.5mm long, hairy. 4-6 short spines that may be pointed or blunt surround head near the top. No corolla.

Bracts - Involucre of fused bracts. Roughened or hairy.

Ovary -

Perianth -

Stamens -

Anthers -


Woody hull with network pattern on the surface. 3-4 mm long with a short pointed beak and 4-5 wart like protuberances near the top.


Grey brown to black. Egg shaped to spherical.


Short thick rootstock, underground creeping rhizomes that produce large numbers of shoots and feeder roots. Sometimes has above ground creeping stems or stolons.

Key Characters:


Life cycle:

Perennial. Flowers February to April. Seeds germinate in autumn, grow quickly over winter and spring forming a network of perennial rhizomes in the top 300 mm of soil with occasional roots to 1000 mm deep. Flowering stems emerge in late spring and flowering usually starts in November and continues to April under favourable conditions. During periods of drought the top growth dies in summer. Plants quickly re-establish in autumn from buds on the roots.



Flowering times:

February to April.

Seed Biology and Germination:

Germination is a complex function of temperature and light. Dormancy is also temperature dependent. Seed longevity in the soil is many years.

Vegetative Propagules:

Rhizomatous roots, stoloniferous shoots and a strong central rootstock.



Produces toxins that reduce the germination or growth of crop and pasture plants.

Population Dynamics and Dispersal:

The fruit tangles in wool, fur and clothing. The seed is transported in mud clinging to vehicles and stock. Contaminated soil used for road making or garden dressing also contributes to spread. Clumps increase in size by extension of the underground root system that produces new shoots. Much of the spread around the world and in NSW was probably associated with the movement of Army vehicles during World War 2.

Origin and History:

North America.

Prevalent through the western half of the US and central Canada.

Serious weed of eastern Europe and western Asia.

First recorded in NSW in 1922.

Spreads rapidly.

Small areas occur west of Bunbury in WA.

Small areas found in Victorian Mallee in the 1960's have been successfully eradicated.





Semi arid.


Sandy soils.

Plant Associations:

Grasslands and irrigation areas.




Weed of roadsides, pastures, arable and disturbed areas, and creek and channel banks. It is not eaten by stock at any stage. Causes severe hay fever and contact dermatitis. Strongly competitive. Produces toxins that reduce the germination or growth of crop and pasture plants.

Strongly competitive.


Not recorded as toxic.

May cause hay fever and contact dermatitis in people.


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

Management and Control:

Has a complex seed dormancy mechanism. Some seed remains dormant in the soil for many years. Cultivation is ineffective and often results in worse infestations because the roots are distributed and re-shoot. Hormone herbicides such as 2,4-D ester, dicamba, triclopyr and picloram provide good control when applied at the budding stage, but need to be repeated annually for a number of years to exhaust dormant seed banks. Biological control with a stem galling moth is expected to be successful in tropical areas but not in the cooler southern areas. All stock should be kept out of infestations when seed is present because of their ability to spread it.


Eradication strategies:

Small areas found in Victorian Mallee in the 1960's have been successfully eradicated.

Herbicide resistance:

Biological Control:

Biological control has been reasonably successful in warmer regions north of 32 degrees south latitude but less promising in cooler areas.

Related plants:

Annual Ragweed (Ambrosia artemisiifolia)

Burr Ragweed (Ambrosia confertiflora)

Lacy Ragweed (Ambrosia tenuifolia)

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). P83.

Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P873.

Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P728.

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). #70.3.

Marchant, N.G., Wheeler, J.R., Rye, B.L., Bennett, E.M., Lander, N.S. and Macfarlane, T.D. (1987). Flora of the Perth Region. (Western Australian Herbarium, Department of Agriculture, Western Australia). p655.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). p250-253. Photos.


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