Weed Identification

Australia > > Perennial Ragweed

Perennial Ragweed

Ambrosia psilostachya

Family: Asteraceae.

Form: Herb

Origin: Native of North America.

Flowers/Seedhead: Male flower spikes to 15 cm long. Male flowers cream to greenish-yellow, several per head. Female flowers 1 per head, inconspicuous. Flowers summer and early autumn.

Description: Perennial herb to 1.5 m high. Stems hairy. Leaves opposite below and alternate above, grey-green, both surfaces hairy; ovate to lanceolate in outline; lower leaves 5–12 cm long, 2–4 cm wide almost without a stalk. Seeds 3–4 mm long with an obvious beak.

Distinguishing features: Distinguished by perennial creeping roots sending up shoots to form large colonies; lower leaves pinnately divided with glandular hairs; male flowers in drooping cone-shaped clusters in spikes above female flowers in leaf axils; seeds hairy with a ring of 4–5 short blunt spines.

Dispersal: Spread by seed attached to animals or in mud, and by creeping roots.

Confused With: Other Ambrosia species, see taxonomic texts for distinguishing features. May also be confused with Chinese Wormwood, Artemisia verlotiorum, but Artemisia species have heads that contain clusters of flowers of both sexes.


Deeply divided leaves & flowerheads
Inset: seed

Notes: Forms dense colonies. A weed of crops and pasture. Plants not eaten by stock. Major cause of allergy with pollen causing flu-like symptoms; plant contact often causes skin allergies. Epiblema strenuana, a stem-galling moth introduced for control of Parthenium Weed, Parthenium hysterophorus, reduces Perennial Ragweed populations in warmer areas.

References:

    Noxious Weeds of Australia. W. Parsons and E. Cuthbertson, 1992, pages 250–253. Flora of NSW. G. Harden (ed), Vol. 3, 1992, page 268.

Web References: Search Australian web sites for further information on this weed.

Creeping roots
Stem-galling moth in
ragweed stem
Spikes of
male flowers
Erect stand at
Wellington, NSW
photo J.J.Dellow
Forms clumps,
Lithgow, NSW

This weed has been included in the WEEDeck field guide as card H44

More information about WEEDeck is available from Sainty & Associates Pty. Ltd.

 

Australia > > Perennial Ragweed

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