Silver wattle
The bright yellow, pom-pom-like flower heads and feathery foliage of silver wattle is familiar to gardeners and florists under the confusing name ‘mimosa’; it should not be confused with members of the genus Mimosa. A cloyingly sweet scent, extracted from silver-wattle flowers, is also known as mimosa.
Silver wattle is native to south-eastern Australia and Tasmania but has been widely introduced to areas with warm temperate or Mediterranean climates. In Australia, silver wattle occupies different types of habits ranging from forests through woodlands to grasslands. Outside of its native range, silver wattle is, or has become, an invasive tree species, for example in the Western Cape of South Africa, south-western Europe and New Zealand.
In the early nineteenth century, soon after its discovery, the silver wattle was introduced to European horticultural collections and its ornamental potential was recognised. The grey-green foliage probably gives the tree its scientific name (dealbata means ‘white-washed’). The pattern of horticultural plants, introduced into Europe from across the globe, escaping gardeners’ care and becoming invasive species is familiar.
Once over the garden wall, the colonisation process begins. The plant must land in a place it can grow; most escapees will fail at this stage. If it can grow, the next steps are that the plant establishes itself, increases its population size and becomes naturalised. A naturalised species may spread, but becomes regarded as an invasive if it has detrimental ecological or economic effects.
Globally, Australian acacias are major contributors to lists of invasive species. Biological characteristics can be associated with a species’ potential to have invasive properties, although none are perfect predictors. For example, an ability to adapt to disturbed environments, such a riverbanks and agricultural fields, on potentially low-nutrient substrates. Moreover, species capable of producing large quantities of highly mobile, long-lived seed are also likely to be successful invaders.
Silver wattle has many such characteristics. It produces many dormant seeds that survive for decades in the soil seedbank. Moreover, one of the stimuli for these seeds to germinate is fire – a feature that is characteristic of Mediterranean climates. As a species capable of taking atmospheric nitrogen and converting into a form that the plant can use, it is well adapted to growing on low nutrient soils. Moreover, there is evidence that silver wattle can suppress the growth of other species through allelopathy – a process by which plants release toxins into the environment that kill competitors.
Further reading
Kull CA et al. 2011. Adoption, use and perception of Australian acacias around the world. Diversity and Distributions 17: 822-836.
Lorenzo, P et al. 2010. The genus Acacia as invader: the characteristic case of Acacia dealbata Link in Europe. Annals of Forest Science 67: 101.
Simberloff D 2013. Invasive species. Oxford University Press.
Stephen Harris