Annual Ragweed

Ambrosia artemisiifolia L.

Synonyms - Ambrosia diversifolia, Ambrosia elatior, Ambrosia media, Ambrosia monophylla.

Family: Asteraceae.

Names:

Annual Ragweed refers to its annual growth cycle and ragged appearance of the leaves. Ambrosia is derived from the Greek meaning 'food of the gods' because this plant was supposed to give immortality. Artemisiifolia is from the Greek virgin goddess of the hunt, 'Artemisia', and 'folia' meaning leaf and refers to the deeply divided leaf.

Other names:

Bitterweed
Common Ragweed
Dunbible weed
Hay-fever weed
Hog weed.

Summary:

An erect, branching, shallow rooted, annual herb to 3.5 m tall (usually 0.5-2 m) with resinous and aromatic glands. It has mainly opposite, deeply lobed, greyish leaves and yellow-green flowers with no 'petals' around autumn. The stems are angled and often reddish.

Description:

Cotyledons:

Two.

Leaves:

Mainly opposite. Green. Upper ones tend to be alternate.
Stipules -
Petiole - To 30 mm long.
Blade - Egg-shaped. 20-100 x 10-70 mm. Lobed about half way to the midrib or double lobed. The lobe is oval with a pointed tip directed forward. Wings up to 3 mm wide on midrib of lobe. Upper leaves may have no lobes.
Stem leaves - Very short petioles or none.

Stems:

Erect. Branching. 500-3500 mm tall, brown/green. Hairless to roughly hairy.

Flower head:

Each head has only one sex of flower. Male heads are higher on the plant than female. Male heads have 10-100 flowers in drooping hemispherical clusters with a set of cup-like bracts underneath. Many of these clusters are attached to a bare stalk, up to 200 mm long, at the ends of stems or branches. Females in single saucer shape, wavy edged flowered heads, 2.5-5 mm long, and borne singly or in clusters in the upper leaf axils or on short branches. Often very branched. 5-7 fine and tapering spines surround the head near the top.

Flowers:

Cream or pale green. Separate male and female flowers.
Males, 2.5-4 mm, tubular.
Females have no petals.
Bracts - Males are fused, toothed and hairy. Females are egg-shaped, 3 mm long, hairy or smooth with a net pattern.
Ovary - Style has 2 long slender branches.
Perianth -
Stamens - 5
Anthers - Free. Shed large quantities of allergenic, fine, yellow pollen. (Estimated at 1 million grains per plant per day)

Fruit:

Head with 4-7 spines or teeth about 1 mm long near the top.

Seeds:

Light brown. 3-5 mm long, 2-3 mm diameter. Woody. Top shaped with an awl shaped, 1-2 mm long, beak and surrounded by 4-7, 1 mm long, spines.

Roots:

Strong taproot to 500 mm deep. Many branching fibrous roots.

Key Characters:

Biology:

Life cycle:

Annual. Seeds germinate in August to October after rain and grow quickly. Flowering starts in late summer and continues to May. Plants die in winter.

Physiology:

Reproduction:

By seed.

Flowering times:

Late summer to May.
There are several ecotypes that vary in their time of flowering.

Seed Biology and Germination:

Produces an average 3000 seeds per year with some producing up to 62,000. Seeds won't germinate for some time after maturity. Exposure to light and alternating temperatures improves germination. Seeds become dormant with high temperatures and may survive for over 40 years.

Vegetative Propagules:

None.

Hybrids:

Several ecotypes.

Allelopathy:

It produces toxins that affect the growth of neighbouring plants, especially ryegrass, oats, onions and Amaranthus.

Population Dynamics and Dispersal:

The main spread is by the spiny seed attaching to animals, clothing or machinery. It is also spread by water and top soil used for roads and gardens.

Origin and History:

North America.
Has become a serious weed of the Australia, Soviet Union, Japan and New Zealand.
Introduced to NSW in 1908. It spread slowly until 1940 then spread rapidly.
Occurs in isolated patches in the south west of WA.

Distribution:

NSW, QLD.

Courtesy Australia's Virtual Herbarium.
Not in WA - name misapplied to Ambrosia polystachya.

Habitats:

Climate:

Sub-humid temperate to sub-tropical areas.

Soil:

Prefers wet areas.

Plant Associations:

Open areas.

Significance:

Beneficial:

Used to stop bleeding or as a cure for dyspepsia.
Good forage value for quail and cattle but unpalatable to horses.
The seeds produce an oil with good drying qualities that is potentially useful for paints and varnishes.

Detrimental:

Weed of pastures, stubbles, fallows, arable and disturbed land, water courses, roadsides.
Causes severe hay fever and dermatitis. Cattle eat young plants, but horses avoid it so that it often over-runs horse paddocks. It is a serious weed of many crops in other countries. It produces toxins that affect the growth of neighbouring plants. An aggressive coloniser of waste ground.
Hosts sclerotinia rot of cabbages.

Toxicity:

Toxic. The unpleasant odour usually makes it distasteful to stock.

Symptoms:

It may cause a sore mouth in stock that graze it.
Hay fever and skin irritations in people.

Treatment:

Exclude stock from infested areas.

Legislation:

Noxious weed of NT, QLD, SA.

Management and Control:

Cultivation, mowing and grazing are generally not effective control techniques because of the high levels of dormant seed. Herbicides provide good short term control for crops. Establishment of perennial pasture provides good long term control of the weed. It is not common as a contaminant of agricultural seed or produce.

Thresholds:

It is an aggressive coloniser of waste ground.
It is a serious competitor in crops overseas.
1-5 plants/m2 reduce yields in row crops.

Eradication strategies:

It is susceptible to many common herbicides, but the seed dormancy and longevity make eradication difficult.

Herbicide resistance:

Resistant forms of ragweed have developed where triazine herbicides have been regularly used.

Biological Control:

A number of bio control agents have been selected and two have been released in Qld and NSW.

Related plants:

Burr Ragweed (Ambrosia confertiflora)
Lacy Ragweed (Ambrosia tenuifolia)
Perennial Ragweed (Ambrosia psilostachya).

Plants of similar appearance:

References:

Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P83. Photos.

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

Gardner, G.A. and Bennetts, H.W. (1956). The toxic plants of Western Australia. (West Australian Newspapers Ltd, Perth). P194.

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). #70.1.

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). p245-248. Photos.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.