Mimosa, Giant Mimosa, Giant Sensitive Plant, Thorny Sensitive Plant, Black Mimosa, Catclaw Mimosa, Bashful Plant

Quick Facts

Giant Sensitive Plant is native to tropical America and now widespread throughout tropical regions of the world.
In the Top End of northern Australia more than 80 000 hectares of native wetland vegetation has been replaced by Giant Sensitive Plant.
Once established, Giant Sensitive Plant is very difficult to control.
Each mature plant is capable of producing hundreds of thousands of seeds.
Prevent Giant Sensitive Plant spread by using effective quarantine, hygiene and monitoring, and by controlling feral animals.
Control Giant Sensitive Plant in small patches before it seeds by hand pulling, bulldozing or spraying herbicides.
Larger infestations should be sprayed from the air. Follow-up will be required.
Biological control agents help to control Giant Sensitive Plant in the long term.

What Does It Look Like?

What is it?

Giant Sensitive Plant (Mimosa pigra) is a shrub to 5 or 6 m tall, with scattered 5-12 mm long thorns along the stems and branches. Its touch-sensitive leaves are about 12-22 cm long, and consist of many fine leaflets (up to 45 pairs).

Its round flower heads (about 10-20 mm in diameter) are composed of about 100 individual pink or mauve flowers.

The fruit are clustered seed pods, with 10-20 produced in each flower head. The pods are flat, linear, 30-120 mm long, 7-14 mm wide and densely hairy with rusty-coloured bristles. They break into segments when mature, each segment containing an oblong shaped brown or olive-green seed (4-6 mm long, 2-3 mm wide). There can be anything from 14 to 26 seeds per pod.

For a more detailed taxonomic description see, for example, Lonsdale et al. (1995) and Cowan (1998).

Informative illustrations can be found in references given below, including Smith (2002), CRC for Australian Weed Management (2003), Navie (2004), Natural Resources & Water, Queensland Government (2006), Wilson & Lane (2006) and at the US Forest Service, Pacific Island Ecosytems at Risk (PIER) site (undated).

For further information and assistance with identification of Mimosa contact the herbarium in your state or territory.

Flower colour

Pink, Mauve

Growth form (weed type/habit)

Shrub

Where it currently grows? Preferred habitat

Giant Sensitive Plant favours humid and sub-humid tropical regions, growing on a variety of soils in moist sites such as riverbanks, coastal plains and floodplains (CRC 2003).

Are there similar species?

Giant Sensitive Plant is most likely to be confused with other species of the genus, Mimosa diplotricha and M. pudica all three having touch-sensitive leaflets and pink globular heads of flowers.

Mimosa diplotricha is a scrambling sub-shrub with long hairs and numerous prickles to 3 mm long (unlike Mimosa which has scattered and 5-12 mm long prickles) on 4-angled (not rounded) stems. Its seed pods are generally shorter and narrower and break into only 3-5 one-seeded segments rather than14-26 segments. The common sensitive plant differs from both of these species in that the leaves lack thorns and arise from one point rather than being arranged in pairs along the axis.

Giant Sensitive Plant also has some resemblance to other weedy Fabaceae such as Acacia farnesiana but are distinguished by not having sensitive leaves. The Coffee Bush (Leucaena leucocephala) superficially resembles Mimosa but it does not have prickles or sensitive leaves and its globular flower heads are creamy yellow.

For a more detailed comparison of the species of Mimosa and descriptions of other members of the Mimosoideae (Fabaceae) in Australia see Cowan (1998).

Why Is It A Weed?

What are its impacts?

Giant Sensitive Plant is a Weed of National Significance. It is regarded as one of the worst weeds in Australia because of its invasiveness, potential for spread, and economic and environmental impacts.

Agriculture: Giant Sensitive Plant forms dense stands that replace all native vegetation on the ecologically and economically valuable wetlands of the Top End of northern Australia. Giant Sensitive Plant invasion threatens the production, cultural and conservation values of wetlands, and reduces the scope for exploitation of resources by land users. Pastoralists are affected because the inedible and thorny Giant Sensitive Plant smothers and replaces grasslands, blocks access to stock watering points and hinders mustering. Additionally, the harvesting of bush foods by indigenous people is hampered by Giant Sensitive Plant (CRC 2003).

Native ecosystems: In environmental terms, nationally and internationally significant wetlands are threatened by Giant Sensitive Plant, which reduces the biodiversity of plant and animal life on the floodplains by out-competing native plants and reducing available habitat for native animals. Although currently limited in distribution, if left unchecked Giant Sensitive Plant has the potential to dominate wetlands across the whole of northern Australia (CRC 2003).

Cultural: Giant Sensitive Plant infestations can reduce access to areas for fishing and recreation. This is especially important as it affects the ability of indigenous groups to participate in cultural hunting and food collection such as turtles, water lilies and water chestnuts (Australian Government 2009). It also reduces accessibility to some indigenous sacred and cultural sites.

A more detailed analysis of the impact of Giant Sensitive Plant on the environment is available in Walden et al. (2004).

How does it spread?

Giant Sensitive Plant produces copious amounts of seed. A typical annual production of perhaps 9 000 seeds and as many as 220 000 has been recorded for a particularly productive individual (Lonsdale et al. 1995). Seeds are readily dispersed by humans, animals and water. At maturity, pods disintegrate into one-seeded segments, with the hairs on the surface of the pod ensuring that they are readily attached to fur and clothing. If consumed by animals, seed also passes unharmed through the digestive tract. Seed, in mud, can also be dispersed by vehicles. It is believed that sand contaminated with seeds was removed from the Adelaide River in the 1950s and used around the Top End in commercial building operations, leading to further spread of the species. However, on floodplains the most important dispersal method is simply water, which carries segments downstream (CRC 2003; NRW 2006).

What is its history in Australia?

While the evidence suggests that Darwin was the site of entry of Giant Sensitive Plant in Australia there is much doubt as to how, when and why it arrived. It is largely a matter of speculation, albeit well-reasoned. Research shows that Giant Sensitive Plant was definitely present in Darwin in 1936 and probably a weed in the Darwin Botanic Gardens by 1913. It was probably introduced into the Northern Territory in the 20 years prior to 1891. It may have been introduced to Darwin as a novelty (because of its sensitive leaves), as a contaminant of seed, or with animals (Miller & Lonsdale 1987). The presence of Giant Sensitive Plant in the Darwin Botanic Gardens in the 1890s, as suggested by some publications (e.g. Smith 2002; CRC 2003) is not confirmed and may be incorrect.

How To Manage It?

Best practice management

Preventing the introduction of and spread of Giant Sensitive Plant is the most effective and cheapest way of control (Australian Government 2009). Its seeds are easily spread and so implementing a hygiene program of people, vehicles, machinery and animals that have entered infested areas will help to slow the spread. Seed can easily be trapped in tyres, wheels and radiator grills

Chemical control: Larger sites can be sprayed with herbicides, and can be effective in eradicating Giant Sensitive Plant if enough time and resources are allocated. Herbicide is most effective when the plant is in its active growth phase (Australian Government 2009). Chemicals can be applied either by foliar or aerial spraying. Continuous use of chemicals on plants can lead to herbicide resistance.

Non-chemical control: Small outbreaks of Giant Sensitive Plant can be controlled by simple hand-pulling and grubbing. Note that because of seed-longevity – perhaps as much as 23 years in sandy soil but usually much shorter (Lonsdale et al. 1995) – it is necessary to revisit sites on a regular basis. Mechanical clearing of large infestations can occur after herbicide application. Chaining and chopper-rolling can also be used to clear large infestations.

Strategic use of fire can also be effective. This is most effective if the plants are treated with herbicide first. If not 90% of mature plants and 50% of seedlings can regrow after fire (Australian Government 2009). Fire can be used to stimulate seed germination that is then sprayed with chemicals.

Biological control: Long-term biological control offers the only cost-effective control of very large infestations. Since 1983 at least 13 insect and two fungal species have been released on Giant Sensitive Plant in Australia, with the most recent introduction – March 2007 – being a tiny beetle, Nesaecrepida infuscata, which attacks roots and leaves of the plant (Anon. 2007).

For more information on control methods, including details of biological control agents see, for example, Harley (1992), Parsons & Cuthbertson (2001), CRC for Weed Management (2003) and various papers in Julien et al. (2004).

For general information of the use of chemicals please visit the Australian Pesticides and Veterinary Medicines Authority at http://www.apvma.gov.au

Does it have a biological control agent?

YES. Thirteen biocontrol agents released with 11 establishing (Harvey et al 2023)

When does it grow? (lifecycle/growth calendar)

Giant Sensitive Plant can germinate year round if the soil is moist but not flooded. However, most germination takes place at the start and end of the wet season. Growth in seedlings is rapid, and flowering occurs between 4 and 12 months after germination. The main flowering season is January-March but flowering can be extended into the dry season under moist conditions. Seeding occurs approximately five weeks after flowering and fruits ripen after about three months. Most seeds germinate when first wetted although a tough, impermeable coating allows some seeds to remain viable in sandy soil for over 20 years. Giant Sensitive Plant grows extremely quickly and in ideal conditions infestations double in size every 18 months (CRC 2003).

Where Is It Found?

Which states and territories is it found?

NT, WA

What areas within states and territories is it found?

In Australia Giant Sensitive Plant is mostly confined to the Northern Territory, although it has been recorded in northern Western Australia near the NT border (Navie 2004). A small infestation of plants discovered in 2001 near Proserpine in Queensland has been removed, and there is active monitoring of the area and control of new emerging seedlings.

Following its introduction to Darwin, Giant Sensitive Plant was not particularly invasive and the populations were generally stable until the 1950s. However, about that time it had spread enough to reach its favoured habitat – open floodplains – and began to spread rapidly. Concerted efforts to eradicate the weed were carried out in the 1960s and early 1970s (Friend 2004), however monocultures of this plant now infest most major river systems in the Top End, from the Victoria River in the west to the Phelp River in south-eastern Arnhem Land and the Arafura Swamp to the north-east (Walden et al. 2004).