annual ragweed

Ambrosia artemisiifolia
habit prior to flowering (Photo: Sheldon Navie)
habit in flower (Photo: Sheldon Navie)
hairy rounded stem (Photo: Sheldon Navie)
highly-divided and fern-like lower leaf (Photo: Sheldon Navie)
smaller, alternately arranged, and less-divided upper leaves (Photo: Sheldon Navie)
young flower clusters (Photo: Sheldon Navie)
male flower-heads in elongated clusters (Photo: Sheldon Navie)
female flower-heads and fruit, left, and old male flower-heads, right (Photo: Sheldon Navie)
seedling (Photo: Sheldon Navie)
young plant forming a rosette of oppositely arranged leaves (Photo: Sheldon Navie)
adult Zygogramma bicolorata beetles on annual ragweed leaves (Photo: Sheldon Navie)
Herb
Opposite
Simple
Cream
Green

An upright herbaceous plant (growing up to 2 m tall) that forms a basal rosette of leaves during the early stages of growth. Its rounded stems bear deeply divided leaves that are fern-like in appearanceseparate male and female flower-heads are formed on the same plant. The drooping male flower-heads are borne in elongated spike-like clusters (up to 20 cm long) at the tips of the branches. The inconspicuous female flower-heads are borne in the upper leaf forks.

Common names 
Also known as: annual ragweed, ambrosia, American wormwood, carrot weed, hay-fever weed, hog weed, common ragweed,
Family 
Asteraceae
Deciduous 
No
Flowering time 
Summer - early Winter
Native/Exotic 
Exotic
Origin 
Native to large parts of North America (i.e. in southern Canada and throughout most of the USA).
Notifiable 
No
State declaration 
Class 2
Council declaration 
Class R – Reduce populations
Known distribution 

Widely naturalised in eastern Australia and also present in southern Australia. It is most common and widespread in eastern Queensland and the coastal and sub-coastal districts of central and northern New South Wales (particularly in south-eastern Queensland and north-eastern New South Wales). Also occasionally naturalised in some parts of Victoria. Naturalised in many other parts of the world, including China and Hawaii.

Habitat 

This species is mostly found in warmer temperate and sub-tropical environments. It is a common weed of pastures, open woodlands, roadsides, disturbed sites, waste areas, creek banks and riparian vegetation, and is occasionally also found growing in cultivation.

Habit 

A short-lived (i.e. annual) herbaceous plant with an upright (i.e. erect) habit, growing up to 2 m tall.

Impact and control methods 

Annual ragweed (Ambrosia artemisiifolia) is mainly seen as a weed of disturbed sites and pastures in eastern Australia. However, it is also regarded as an environmental weed in some parts of New South Wales and Queensland and is listed as a priority environmental weed in at least one Natural Resource Management region. This species can become very abundant in overgrazed natural pastures, as well as in riparian areas (i.e. along creek banks, on floodplains, and on sandy creek beds), where it replaces native species.In New South Wales, annual ragweed (Ambrosia artemisiifolia) is mainly a problem in coastal districts north of Sydney and it appears on several environmental weed lists in this region (i.e. it is an alert weed in the Sydney North region, is on the NSW North Coast environmental weed survey list, and is listed as an environmental weed in Byron Shire). It has also been recorded in conservation areas in north-eastern New South Wales (i.e. Billinudgel Nature Reserve) and south-eastern Queensland (i.e. Tugun Hill Conservation Area). Dense stands of this and other weed species are also seen as a threat to the integrity of remnant littoral rainforests in the coastal regions of New South Wales, and these rainforests are regarded as an endangered ecological community in this state.Annual ragweed (Ambrosia artemisiifolia) is also considered to be an invasive species in Europe (e.g. France and Switzerland) and parts of Asia, although it is not an extremely aggressive species. It also displaces native vegetation in these countries, especially after disturbance events which put competitive pressures on the native flora.A stem-galling moth (i.e. Epiblema strenuana) and a leaf-feeding beetle (i.e. Zygogramma bicolorata), which were introduced as biological control agents of parthenium weed (Parthenium hysterophorus), also attack annual ragweed (Ambrosia artemisiifolia) and may reduce its invasiveness in sub-tropical regions.

Stem and leaves 

This plant forms a basal rosette of leaves during the early stages of growth. The much-branched, upright (i.e. erect), stems are rounded in cross-section(i.e. cylindrical) and reddish or brownish-green in colour. These stems vary from being almost hairless (i.e. sub-glabrous) to roughly hairy (i.e. hirsute). The leaves are oppositely arranged at the base of the plant, but are alternately arranged further up the stems. The leaf blades (1-16 cm long and 1-7 cm wide) are deeply divided (i.e. pinnatifid to bi-pinnatifid) and fern-like in appearance. They are borne on leaf stalks (i.e. petioles) usually about 1-3 cm long (occasionally up to 10 cm long). The uppermost leaves are usually much reduced in size and less divided than the lower leaves. All leaves are usually covered in hairs (i.e. pubescent), particularly on their undersides, and these hairs may be long and spreading or short and soft.

Flowers and fruits 

Separate male and female (i.e. unisexual) flower-heads are formed on different parts of the same plant (i.e. this species is monoecious). The male (i.e. staminate) flower-heads outnumber the female (i.e. pistillate) flower-heads and droop from branching spike-like flower clusters (up to 20 cm long) that are borne at the tips of the stems. These male flower-heads are small, hemispherical in shape, and either cream, yellowish or pale green in colour. The female flower-heads are less conspicuous and consist of a single tiny flower (i.e. floret). The base of these flower-heads (i.e. the involucre) is saucer shaped with 5-7 small bristle-like spines (each 3-5 mm long). These female flower-heads are held upright and borne singly in the forks (i.e. axils) of the uppermost leaves (i.e. below the male flower-heads). Flowering occurs mostly during summer, autumn and early winter. The fruit is a small brown or blackish achene (2-5 mm long) that is top-shaped (i.e. turbinate) and contains a single seed. These fruit become woody as they mature and have a pointed beak (1-2 mm long) and a ring of four to eight small blunt spines (each less than 1 mm long).

Reproduction and dispersal 

This plant reproduces mainly by seeds. The seeds are spread by animals, water, the movement of soil, and in contaminated agricultural produce (e.g. fodder and pasture seed).

Similar species 

Annual ragweed (Ambrosia artemisiifolia) is very similar to the other ragweeds (Ambrosia spp.) present in Australia, including burr ragweed (Ambrosia confertiflora), perennial ragweed (Ambrosia psilostachya) and lacy ragweed (Ambrosia tenuifolia). It is also very similar to parthenium weed (Parthenium hysterophorus) when in the vegetative stage of growth. These species can be distinguished by the following differences: annual ragweed (Ambrosia artemisiifolia) is a large short-lived (i.e. annual) herbaceous plant (growing up to 2 m tall) with rounded stems and leaves that are usually twice-divided (i.e. bipinnatifid). The single-sex (i.e. unisexual) greenish or yellowish male flower-heads are borne in elongated spikes. Its hairless (i.e. glabrous) fruit (2-5 mm long) are borne in small clusters and have a single row of 4-8 short blunt spines.burr ragweed (Ambrosia confertiflora) is a large long-lived (i.e. perennial) herbaceous plant (growing up to 2 m tall) with rounded stems and leaves that are usually twice-divided (i.e. bipinnatifid). The single-sex (i.e. unisexual) greenish or yellowish male flower-heads are borne in elongated spikes. Its abundant fruit (about 4 mm long) are borne in large clusters and are covered with short hooked spines.perennial ragweed (Ambrosia psilostachya) is a relatively large long-lived (i.e. perennial) herbaceous plant (growing up to 2 m tall) with rounded stems and leaves that are only once-divided (i.e. pinnatifid). The single-sex (i.e. unisexual) greenish or yellowish male flower-heads are borne in elongated spikes. Its small hairy (i.e. pubescent) fruit (about 2 mm long) are borne in small clusters and have five short blunt spines. It also produces a large network of creeping underground stems.lacy ragweed (Ambrosia tenuifolia) is a relatively small long-lived (i.e. perennial) herbaceous plant (growing up to 75 cm tall) with rounded stems and leaves that are usually twice-divided (i.e. bipinnatifid). These leaves are covered in long whitish hairs and are very finely divided, thereby giving them a greyish and lacy appearance. The single-sex (i.e. unisexual) greenish or yellowish male flower-heads are borne in elongated spikes. Its small fruit (about 2 mm long) are borne singly or in small clusters and have a few very short teeth.parthenium weed (Parthenium hysterophorus) is a large short-lived (i.e. annual) herbaceous plant (growing up to 2 m tall) with ribbed stems and leaves that are usually twice-divided (i.e. bipinnatifid). Masses of small, white, flower-heads are borne at the tips of the branches and each of these flower-heads usually gives rise to five small 'seeds'. Annual ragweed (Ambrosia artemisiifolia) may also be confused with some of the wormwoods (Artemisia spp.). However, these species have bisexual flowers in rounded flower-heads and they do not produce burr-like fruit.