giant sensitive tree

Mimosa pigra
Shrub
Alternate
Compound
Pink
Purple
Green

An upright prickly shrub with twice-compound leaves. Its leaves often have small prickles on their stalks and consist of 6-16 pairs of branchlets, each bearing 20-45 pairs of small leaflets. These leaves are sensitive and fold together when touched or during the night. Its flowers are pale pink or mauve in colour and arranged in fluffy globular clusters (10-20 mm across). Its flattened, elongated pods are covered in bristly hairs and break up into about twenty one-seeded segments when mature.

Common names 
Also known as: giant sensitive tree, sensitive tree, thorny sensitiveplant, bashful bush, catclaw mimosa, giant mimosa, mimosa,
Family 
Mimosaceae
Deciduous 
No
Flowering time 
Spring-Summer
Native/Exotic 
Exotic
Origin 
Native to Mexico, Central America, the Caribbean and tropical South America. Possibly also native to large parts of Africa, Madagascar and Mauritius (or an early introduction to these places).
Notifiable 
No
State declaration 
Class 1
Council declaration 
Class E - Early detection and eradication
Known distribution 

At present the distribution of this species is mostly limited to the northern parts of the Northern Territory. It has also been recorded more recently from the coastal areas of central Queensland (i.e. at Peter Faust Dam near Proserpine). Also naturalised in tropical Asia (i.e. Cambodia, Malaysia, Singapore, Thailand and Vietnam), south-eastern USA (i.e. Florida), the Galapagos Islands and Hawaii.

Habitat 

A weed of roadsides, waterways, drainage ditches, floodplains, swamps, seasonally flooded wetlands, lowland pastures and the edges of water bodies in wetter tropical and sub-tropical regions.

Habit 

An upright (i.e. erect), much-branched, prickly shrub growing 3-6 m tall and forming dense thickets.

Impact and control methods 

Giant sensitive tree (Mimosa pigra) is regarded as a significant environmental weed in the Northern Territory, and as a potentially significant environmental weed in Queensland and Western Australia. This species is one of the 20 Weeds of National Significance (WoNS) in Australia, largely due to its impact on natural ecosystems, and is actively weed managed by community groups in the Northern Territory.

Stem and leaves 

Young stems are greenish in colour, rounded (i.e. terete), have scattered prickles (5-12 mm long), and are covered with short stiff hairs. Older stems become woody and turn greyish in colour. The twice-compound (i.e. bipinnate) leaves are alternately arranged along the stems and borne on stalks (i.e. petioles). These leaves (20-31 cm long) often have small prickles present along their main axis (i.e. petiole and rachis). They consist of several (6-16) pairs of branchlets (i.e. pinnae), each about 5 cm long and bearing numerous (20-45) pairs of small leaflets (i.e. pinnules). The leaflets (3-12 mm long and 0.5-2 mm wide) are elongated (i.e. lanceolate) in shape and stalkless (i.e. sessile). They fold together when touched and also during the night.

Flowers and fruits 

The flowers are pale pink or mauve in colour and arranged in fluffy, globular clusters (10-20 mm across). One to three (usually two) of these small flower clusters are produced on stalks (i.e. peduncles) 2-7 cm long, emanating from each upper leaf fork (i.e. axil). Individual flowers have four tiny sepals (0.75-1 mm long), four inconspicuous petals (2-3 mm long), and eight prominent pinkish stamens that give the flower clusters their fluffy appearance. Flowering occurs mostly during summer and early autumn. The fruit is an elongated and flattened pod (30-120 mm long and 7-14 mm wide) that is covered in bristly hairs. The pods are borne in clusters (of 1-30), are straight or slightly curved, and are initially green in colour. As they mature they turn brown, and when fully mature they break up into about twenty (14-26) one-seeded segments. The seeds (4-6 mm long and 2-2.5 mm wide) are light brown, brown or greenish-brown in colour, elongated (i.e. oblong) in shape, and somewhat flattened (i.e. compressed).

Reproduction and dispersal 

This plant reproduces by seed and also vegetatively via adventitious roots and suckers from cut stems.The seeds survive being eaten by animals and are often spread in this manner. The one-seeded segments also readily become attached to animals, vehicles and clothing and can be dispersed in water and mud.

Similar species 

Giant sensitive tree (Mimosa pigra) is very similar to giant sensitive weed (Mimosa diplotricha var. diplotricha) and common sensitive plant (Mimosa pudica), which both also produce globular pink flower clusters. These species can be distinguished by the following differences: giant sensitive tree (Mimosa pigra) is a large shrub with an upright (i.e. erect) growth habit that has large much-branched leaves (i.e. 6-16 pairs of pinnae). It produces relatively large pods (3-12 cm long) that contain many (14-26) one-seeded segments.giant sensitive weed (Mimosa diplotricha var. diplotricha) is an upright (i.e. erect) shrub, creeper or climbing plant that has much-branched leaves (i.e. 4-9 pairs of pinnae). It produces relatively small pods (1-3.5 cm long) that contain a few (3-5) one-seeded segments.common sensitive plant (Mimosa pudica ) is a smaller plant, with a more low-growing (i.e. prostrate) habit, that has few-branched leaves (i.e. 1-2 pairs of pinnae). It produces relatively small pods (1- 2.5 cm long) that contain a few (1-5) one-seeded segments.