Ambrosia tenuifolia Spreng.

Global description

Ambrosia tenuifolia is a vivacious, non-aromatic, herbaceous plant, with rhizomes and suckers growing in dense colony. It measures up to 1 m high. The stems are erect or ascending, sparsely branched, ribbed, hairy and leafy to the top. The leaves are simple, opposite at the base and becoming alternate in the upper part of the plant. The lamina, generally triangular in shape, is deeply cut into linear narrow lobes of 0.5 to 2 mm wide that are in turn cut. Both sides are green and pubescent. The inflorescence is a terminal spike consisting of many male capitulum; the female flower capitulum are usually solitary at the base of the upper leaves. The capitulums are whitish in color.
 
First leaves

The first leaves are simple, opposite, short-stalked, with a generally triangular lamina but immediately deeply divided pinnatifid. Both sides are shortly pubescent. The main veins are well marked.
 
General habit

Vivacious plant growing in colony from suckers. It can reach up to 1 m high. It has a pleasant odor but not aromatic.
 
Underground system

The underground system consists of branched rhizomes from which suckers are formed. Each node develop branched taproots.
 
Stem

The stem is cylindrical, solid, erect or ascending, sparsely branched, ridged, hairy with dense white hairs.
 
Leaf

The leaves are simple, opposite at the bottom of the plant and alternate in the upper part. They are held by a petiole, 1 to 4 cm long and hairy. The lamina is generally triangular in shape, 4.5 to 8 cm long and 3.5 to 6 cm wide. It is deeply bi-pinnatifid with linear segments, acute or rounded at the top, 0.5 to 2 mm wide, narrowed at their base to the rib. The margin is entire, both sides are green and pubescent.
 
Inflorescence

The inflorescence is composed of a terminal spike, 5 to 15 cm long with only numerous, sessile male capitulums; females capitulums are solitary flower at the base of the spikellets and at the upper leaf axils.
 
Flower

The capitulums are whitish. The male capitulums, 1 to 5 mm high and 2.5 mm wide, are surrounded by an involucre of fused bracts widely cupuliform, and hairy.  The bracts are filiforms, more or less as long as the corolla, they are closely tubulate with 5 short lobes. The ovary is atrophied. The female capitulums, 2 to 2.5 mm in diameter, are underpinned by a campanulate involucres, formed of fused bracts. The corolla is absent. They are obovoid, hairy or sometimes more or less smooth, surmounted by a long spout more than 1 mm long , with   4-5 tubers, about 0.5 mm long.
 
Fruit

The fruit is sub-globose achene which remains trapped in the bracts of the involucre.

Origin

Ambrosia tenuifolia is native of Uruguay, Paraguay and Argentina.
 
Global distribution
 
It is now widespread in South America and became a weed in many temperate, subtropical and tropical regions; widely naturalized in the Mascarene, South Africa and South-east Australia.

https://www.gbif.org/species/3110580