spiny amaranth

Amaranthus spinosus
infestation (Photo: Sheldon Navie)
habit (Photo: Sheldon Navie)
habit (Photo: Sheldon Navie)
reddish-coloured older stems (Photo: Sheldon Navie)
younger stem showing paired spines in the leaf forks (Photo: Sheldon Navie)
leaves (Photo: Sheldon Navie)
close-up of leaf with paler v-shaped marking (Photo: Sheldon Navie)
close-up of leaf underside with raised veins (Photo: Sheldon Navie)
flower cluster with elongated branches (Photo: Sheldon Navie)
close-up of male flowers, with yellow stamens (Photo: Sheldon Navie)
close-up of female flowers, with feathery styles (Photo: Sheldon Navie)
female flowers are also produces in smaller clusters in the upper leaf forks (Photo: Sheldon Navie)
young plant (Photo: Sheldon Navie)
Herb
Alternate
Simple
Green
Variegated
Green
Discoloured

A short-lived herbaceous plant growing up to 1 m tall. Its upright stems are often purplish or reddish in colour. Its leaves are sometimes variegated with paler v-shaped markings. Pairs of sharp spines are produced in the forks of the leaves. Separate male and female flowers are borne with each other in elongated clusters.

Common names 
Also known as: spiny amaranth, needle burr, spiny pigweed, thorny amaranth, thorny pigweed,
Family 
Amaranthaceae
Deciduous 
No
Flowering time 
year round
Native/Exotic 
Exotic
Origin 
This species is probably native to tropical America.
Notifiable 
No
Council declaration 
Class R – Reduce populations
Known distribution 

Widely naturalised along the north and east coast of Australia. Including northern territory, coastal Qld and northern NSW.

Habitat 

A weed of roadsides, disturbed sites, waste areas, footpaths, gardens, farmyards, pastures, crops and fallows.

Habit 

A robust, upright (i.e. erect or ascending), short-lived (i.e. annual), herbaceous plant usually growing up to 1 m tall.

Impact and control methods 
Stem and leaves 

The stems are green or reddish-purple in colour and can be hairless (i.e. glabrous) or sparsely hairy (i.e. puberulous). Two sharp spines (5-12 mm long) are located in the forks (i.e. axils) of the leaves. The leaves are alternately arranged with entire margins and stalks (i.e. petioles) 5-40 mm long. They vary from egg-shaped in outline (i.e. ovate) to almost oval (i.e. elliptic) in shape. The leaf tips can be either pointed, rounded or notched (i.e. acute, obtuse or emarginate). These leaves are often variegated with a pale v-shaped marking.

Flowers and fruits 

The flowers are small (1.5-2.5 mm long) with five 'petals' (i.e. tepals or perianth segments) and are borne in dense elongated clusters (i.e. terminal panicles). The first of these clusters is usually produced at the end of the main stem, and is often followed by others produced on the side branches. Flowering occurs throughout most of the year. The tiny fruit (i.e. utricles) are oblong, slightly flattened, and have a three-lobed tip with a transverse fissure. Each frut splits open when mature to release a single shiny black seed (about 1 mm across).