perennial ragweed

Ambrosia psilostachya
large colony in a pasture near Blackbutt (Photo: Sheldon Navie)
dense infestation (Photo: Sheldon Navie)
habit in flower (Photo: Sheldon Navie)
close-up of rhizome showing young shoots beginning to develop (Photo: Sheldon Navie)
stems and deeply-lobed lower leaves (Photo: Sheldon Navie)
close-up of hairy younger stem (Photo: Sheldon Navie)
elongated clusters of male flower-heads (Photo: Sheldon Navie)
close-up of male flower-heads from below (Photo: Sheldon Navie)
close-up of older male flower-heads (Photo: Sheldon Navie)
female flower-heads in the forks of the smaller upper leaves, directly below the male flower clusters (Photo: Sheldon Navie)
close-up of immature fruit (Photo: Sheldon Navie)
young plant (Photo: Sheldon Navie)
Herb
Opposite
Simple
Cream
Green

A long-lived herbaceous plant growing 30-150 cm tall. It produces numerous creeping underground stems that send up short-lived upright shoots. Its paired or alternately arranged leaves are deeply lobed. Separate male and female flower-heads are produced on different parts of the same plant. The drooping greenish-yellow male flower-heads (2-4 mm across) are borne in upright elongated clusters. The stalkless female flower-heads are borne in the forks of the uppermost leaves.

Common names 
Also known as: perennial ragweed, ragweed, Roman wormwood, dunbible weed, cuman ragweed, western ragweed, naked-spike ambrosia,
Family 
Asteraceae
Deciduous 
No
Flowering time 
Autumn - early Winter
Native/Exotic 
Exotic
Origin 
Native to western North America.
Notifiable 
No
Council declaration 
Class C – Containment and reduction
Known distribution 

This species is occasionally naturalised in south-eastern Queensland. It has been recorded from the Moreton and Burnett districts. It is also naturalised in southern and eastern New South Wales, northern Victoria, south-eastern South Australia and south-western Western Australia.

Habitat 

A weed of roadsides, waste areas, creek banks, pastures and uncultivated fields.

Habit 

An upright (i.e. erect) and long-lived (i.e. perennial) herbaceous plant usually growing from 30-100 cm tall, but sometimes reaching up to 2 m in height. It quickly forms dense clumps and eventually develops into extensive colonies.

Impact and control methods 
Stem and leaves 

Young plants form a basal rosette of leaves during the early stages of growth. As they mature, they produce creeping underground stems (i.e. rhizomes) that send up aboveground (i.e. aerial) shoots at various intervals. The upright (i.e. erect) stems are hairy (i.e. pubescent) and finely ridged (i.e. striate). They are much-branched in the upper parts and woody towards the base. The leaves are oppositely arranged towards the base of the plant and become alternately arranged further up the stem. These leaves (5-14 cm long and 1-6 cm wide) are divided into several pointed lobes (i.e. they are pinnatifid) and are borne on short stalks (i.e. petioles) or are almost stalkless (i.e. sub-sessile). They are hairy (i.e. pubescent) and often somewhat rough to the touch (i.e. scabrous).

Flowers and fruits 

The flower-heads (i.e. capitula) are small and either cream or pale greenish-yellow in colour. Separate male and female flower-heads are produced different parts of the same plant. The cup-shaped (i.e. hemispherical) male flower-heads (2-4 mm across) are borne in branching spike-like clusters (up to 15 cm long) at the top of the stems. They are borne on short stalks (i.e. peduncles) 0.5-2 mm long and are held in a drooping position (i.e. they are pendulous). The female flower-heads consist of a single tiny flower (i.e. floret) and are stalkless (i.e. sessile). They are borne in the forks (i.e. axils) of the uppermost leaves, just below the male flower-heads. Flowering occurs from autumn through to early winter, but is most apparent in late summer. The 'seed' (i.e. achene or cypsela) is egg-shaped (i.e. ovoid) and turns from green to brown or greyish-brown in colour as it matures. These woody 'seeds' (2.5-4 mm long) are faintly haity and are topped with a small pointed 'beak' (about 1 mm long) which is also surrounded by four or five smaller blunt projections.

Reproduction and dispersal 

This plant reproduces by seeds and vegetatively via shoots which are produced along the creeping underground stems (i.e. rhizomes).Infestations spread laterally over time through the creeping underground stems (i.e. rhizomes) and eventually develop into large colonies. Seeds are dispersed by animals, vehicles, water and during the movement of soil, while rhizome fragments may be spread by cultivation machinery.

Similar species 

Perennial ragweed (Ambrosia psilostachya) is very similar to annual ragweed (Ambrosia artemisiifolia) and is also relatively similar to parthenium weed (Parthenium hysterophorus) when in the vegetative stage of growth. These species can be distinguished from each other by the following differences: 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 clusters at the tips of the branches. Its small hairy fruit (i.e. pubescent achenes) are borne in small clusters in the upper leaf forks. These fruit (about 2 mm long) and have a pointed beak surrounded by about five small blunt spines. It also produces a large network of creeping underground stems.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 clusters at the tips of the branches. Its hairless fruit (i.e. glabrous achenes) are borne in small clusters in the upper leaf forks. These fruit (2-5 mm long) have a have a pointed beak surrounded by a row of 4-8 small blunt spines.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' (i.e. achenes). Perennial ragweed (Ambrosia psilostachya) may also be confused with some of the wormwoods (Artemisia spp.). However, these species have bisexual flower-heads that are produced in branched clusters at the tips of the branches.