Weed Alert: Mimosa

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Mimosa (Mimosa pigra)

World Status | Spread | Identification | Control | Legislation

Mimosa is a branched, spiny, perennial shrub growing to 6 m in height. It forms dense thickets that replace native vegetation in northern Australia’s ecologically and economically valuable wetlands. It tolerates flooding, and can grow along roadsides, watercourses and seasonally inundated wetlands, and on a wide variety of soil types. It has the ability to invade ecologically sensitive areas and threatens indigenous cultural activities, and pastoral and tourism industries. Once mimosa becomes established, it dominates vegetation and is very difficult to control.

In the early 1890s the first mimosa plants were brought into the Northern Territory for the Darwin Botanic Gardens. A large infestation was identified in 1952 on the Adelaide River, 100 km south of Darwin. This infestation was thought to be the primary site for further infestations in the Northern Territory. Sand containing mimosa seed was removed from the area in the 1950s and used at commercial building sites in the north.

Mimosa now occurs on an estimated 85 000 ha of wetlands across northern Australia - an area climatically suited to this weed. It has spread to some of the main river systems in the Top End of Australia (Finniss, Reynolds, Mary Daly, East Alligator and South Alligator) and is also found in the Victoria River in the west and the Phelp River in Arnhem Land. An outbreak was discovered in 2001 near Proserpine in Northern Queensland - outside the projected distribution and indicating that mimosa may be able to survive in areas further south. In late 2009 an isolated patch was identified in a seasonal billabong near Kununnurra in Western Australia.

Mimosa has not been recorded in New South Wales (NSW). Note: it is not the same plant as the commonly-called mimosa bush Vachellia farnesiana which is widespread in parts of NSW.

World Status

Mimosa is a native of tropical America. It grows in moist areas in the humid and sub-humid tropics and has become a very serious weed of Africa, India, Southeast Asia and some Pacific islands.

Spread

The spread of mimosa occurs by seed. Large trees can produce up to 220 000 seeds per year. The seed pod segments (containing one seed per segment) float on water or attach to clothing or hair. Seeds can also be carried by water during floods, by animals (e.g. in cattle and horse manure or in mud attached to kangaroos and feral animals) and by humans (e.g. attached to clothing and machinery).

Identification

Mimosa has large thorns (5–10 mm long) on the stem and smaller thorns on the branches between the leaves. The greenish stems on young plants become woody with age. Mimosa has a branching taproot which can reach a depth of 1–2 m.

Key identification features

• Leaves are green and fern-like. They are made up of many fine leaflets that occur in pairs along the branches. The leaflets fold together at night or when they are touched.
• Flowers (1–2 cm diameter) are round and made up of 100 individual pink-mauve flowers.
• Seed pods (6–8 cm long) are olive-green and covered in numerous fine hairs. Each flower head produces 10–20 pods. The mature pods turn brown and break into segments. There is one oblong-shaped seed (4–6 mm long) per segment.

Control

Your local council weeds officer will assist with identification and information on control, removal and eradication of this weed. Infestations can be spread by inappropriate control activities.

Legislation

Mimosa is a Class 1 State Prohibited Weed across NSW under the Noxious Weeds Act 1993. It must be eradicated and land must be kept free of the plant. As a notifiable weed, all outbreaks must be reported to the local council within 24 hours, and the plant is prohibited from sale in NSW.

Acknowledgments

Adapted by AnDi Communications from the CRC for Australian Weed Management Weed Management Guide: Mimosa.
Reviewed by Peter Gray. Edited by Elissa van Oousterhout, Birgitte Verbeek

References

Lloyd SG & Vinnicombe TL (2010) Proceedings of the 17th Australasian Weeds Conference pp180-181.