Mimosa or Silver Wattle (Acacia dealbata, Link)
Acacia dealbata – N246 – Castelo de Vide – Alpalhão
Origin: South-eastern Australia (New South Wales, Victoria, and Tasmania)
Size: up to 20 metres height
The Acacia Mimosa it is a fast-growing evergreen tree, the leaves are glaucous grey-greenish to silver-green, bipinnate, with 10 to 26 pinnae pairs, which are divided in 20 to 50 pairs of leaflets. The flowers are bright yellow growing in panicles arranged in globular flower heads. The fruits are brownish-red compressed pods, with several seeds. The tree flowers between January to April.
In its natural range, can grow along water courses, dry forests and in moisty mountain forests, sometimes is bark is covered with a white lichen, which probably is responsible for the common name “silver”; the Latin name “dealbata” also means “covered in white powder”.
Propagation of Acacia Mimosa occurs by the production of root and coppice suckers and by seed, it produces thousands of seeds which remain viable in the soil for many, many years. Seeds can also be transported to other places by animals and water. The germination of the seeds is stimulated by fire. Can grow fast and form dense populations, stopping the growing of native species, diminishing the flow of water courses, and increasing erosion; the leaves are Nitrogen rich and can change the chemical properties of the soil, preventing native species to grow and support the growth of other Acacia species. This species is also drought tolerant and one of the first plants to grow in cleared land.
This Mimosa was introduced in warm temperate regions as an ornamental, for fixing soils and has a forestry plant. In Portugal it is thought to have been introduced in the XVIII Century, together with other Acacia species occupies an area between 30 to 60 thousand hectares. This plant has been introduced in many countries and it is considered an invasive species in South Africa, parts of India, Sri Lanka, France, Portugal, and Spain. In Portugal it is considered one of the worst invasive species.
The control and eradication of this species is extremely hard and expensive. Some countries start to use biocontrol or natural control, using species native from the same areas of the invasive species. This species can reduce for example, the number of seeds produce by these plants, although before it is necessary a long period of study to access the effects of these “natural controls” in the ecosystems.