Urena burr

Urena lobata
Herb
Alternate
Simple
White
Pink
Green

An upright and long-lived herbaceous plant or small shrub growing 0.5-3 m tall. Its younger stems are densely covered in star-shaped hairs, while its older stems become somewhat woody with age. Its alternately arranged leaves (4-10 cm long) have toothed margins and vary from being almost entire to shallowly three-lobed. Its pink flowers (about 1.5-2 cm across) are usually borne singly on short stalks in the upper leaf forks. Its fruit (6-10 mm across) are covered in numerous small hooked spines and split into several one-seeded segments when mature.

Common names 
Also known as: Caesar weed, Caesar's weed , pink-flowered Chinese burr, hibiscus burr, urena weed, Congo jute, bur mallow , aramina,
Family 
Malvaceae
Deciduous 
No
Flowering time 
Year Round
Native/Exotic 
Native
Origin 
This species is found throughout the tropical regions of the world (i.e. it has a pan-tropical distribution), but it was probably spread to large parts of this current range by aboriginal voyagers through its use as a fibre plant. It probably originated i
Notifiable 
No
Council declaration 
SIL – Special Investigation List
Known distribution 

Now almost always regarded as being naturalised in northern and eastern Australia (i.e. in northern Western Australia, the northern parts of the Northern Territory, northern and eastern Queensland and the coastal districts of northern New South Wales). Also naturalised in south-eastern USA (i.e. Louisiana and Florida), La Réunion, Papua New Guinea and on numerous Pacific islands (e.g. American Samoa, the Cook Islands, the Federated States of Micronesia, Fiji, French Polynesia, Guam, Niue, the Solomon Islands, Tonga and Hawaii).

Habitat 

A weed of waterways (i.e. riparian vegetation), swamps, forest margins, open woodlands, roadsides, waste areas and disturbed sites in tropical and sub-tropical regions.

Habit 

An upright (i.e. erect) and long-lived (i.e. perennial) herbaceous plant or small shrub usually growing 0.5-2 m tall, but occasionally reaching up to 3 m in height.

Impact and control methods 

Urena burr (Urena lobata) is regarded as an environmental weed in Queensland.

Stem and leaves 

Its younger stems are green and densely covered in star-shaped (i.e. stellate) hairs, while its older stems become woody with age. The alternately arranged leaves (4-10 cm long and 1.5-8 cm wide) are borne on stalks (i.e. petioles) 1-9 cm long. The lower leaves are egg-shaped in outline (i.e. ovate) or somewhat circular (i.e. orbicular) in shape and vary from being almost entire to shallowly three- or five-lobed. However the uppermost leaves are smaller and narrower (i.e. oblong or lanceolate), usually being only 4-7 cm long by 1.5-3 cm wide. All leaves have toothed (i.e. serrate or crenate) margins and pointed tips (i.e. acute apices). Both leaf surfaces are covered in star-shaped hairs (i.e. they are stellate pubescent), which are much denser on the undersides.

Flowers and fruits 

The pink flowers (about 1.5-2 cm across) are usually borne singly on short stalks (i.e. peduncles) 2-3 mm long in the upper leaf forks (i.e. axils), however sometimes two or three of these flowers may be present in a single leaf fork. The bases of the flowers are surrounded by several small leafy bracts (5-7 mm long) that are fused together at the base and resemble a calyx (i.e. epicalyx). The five true sepals, which are about the same size or slightly smaller and borne inside this structure, are also fused together at the base (i.e. into a cup-shaped calyx with five lobes). The five pink or reddish-pink petals (10-20 mm long) are usually slightly broader towards their tips (i.e. obovate). Each flower also has numerous stamens, that are fused together at the base, and a style that separates into ten tiny branches near the tip. The round or somewhat flattened fruit (6-10 mm across) turns from green to brown in colour as it matures and eventually splits into five one-seeded segments (i.e. mericarps). These fruit are covered in star-shaped (i.e. stellate) hairs when young and also have numerous small hooked spines (i.e. glochidia). Each of the spiny segments (4-6 mm long) contains a kidney-shaped (i.e. reniform) brown seed (2-3 mm across) that is almost hairless or somewhat hairy (i.e. glabrescent or pubescent).

Reproduction and dispersal 

This species reproduces by seed. The hooked spines on its fruit readily attach to animal fur and clothing. Seeds may also be dispersed by water, soil or contaminated agricultural produce.