Acacia podalyriifolia (pearl acacia / vaalmimosa) is a listed Category 1b alien invasive plant species in terms of the 2014 Alien and Invasive Species Regulations of The National Environmental Management Biodiversity Act (NEMBA) (Act No. 10 of 2004). Land owners are compelled to control and eradicate it as far as possible from their properties. 

Origin: Pearl acacia, like all other wattle species, is native to Australia. In its home range, it is endemic to East and South East Australia, but has become invasive in its own country in the South West. It was introduced to South Africa as an ornamental tree for its bright yellow blossoms and silver-grey leaves (not to be confused with penny gum, popular with florists). It has established itself in Gauteng and is also recorded in the eastern provinces as well as the Western Cape. It is also naturalised in southern and eastern Africa and in other parts of the world, south-east Asia, parts of the Indian sub-continent, some Indian Ocean islands, South America and South Western USA. 

Pearl acacia
Photo: Louise Kritzinger

Description: A member of the large family Fabacea (pea family) and belonging to the subfamily Mimosaceae (thorn tree family), it is a thornless, evergreen shrub to small tree, between 3 to 6 m high (up to 10 m). Distinctively, the compound leaves are only present in young plants, replaced by phyllodes (modified leaf stalks that are flattened and widened, resembling simple leaves), which is an adaptation to better handle drought and nutrient deficient soils. ‘Leaves’ are sharp tipped and stems are bluish-greyish green, covered in dense white hairs. The showy flower heads are bright yellow globular clusters from June to August. The seed pods are short, wavy and hairy, becoming hairless and brown with maturity. Black seeds are long-lived, spread by birds, wind and water.  

Pods
Photo: Timmy, Inaturalist

Impact: Pearl acacia is considered a transformer plant, replacing indigenous vegetation. It is drought resistant and thrives in poor soils, favouring rocky sandstone hills and ridges, grassland, urban open spaces, riparian areas and other disturbed areas, impacting local biodiversity.    

Control methods: Pearl acacia can be chemically controlled via cut-stump application with triclopyr. It is important to apply herbicide as quickly as possible, before sticky resin is exuded. Seedlings can be controlled mechanically by hand pulling and young trees with a tree-popper. Least infested areas should always be prioritised first rather than targeting the centre of the infestation. The biological control agent (seed destroying weevil) identified by the Agricultural Research Council (ARC) has proven negligible effectiveness as a control method. Follow-up work is important, as seedlings left unattended will lead to rapid re-infestation. 

Phyllodes and flowers
Photo: Louise Kritzinger

Indigenous alternatives: Croton gratissimus (lavender feverberry / laventelkoorsbessie), Euclea crispa (blue guarri / bloughwarrie), Olea europaea, subsp. Africana (wild olive / olienhout), Maytenus undata (Koko tree / kokoboom), Brachylaena rotundata (mountain silver-oak / bergvaalbos), Terminalia sericea (silver clusterleaf / vaalboom), Buddleja saligna (false olive / witolien) and Tarchonanthus camphoratus (camphor bush / kanferbos).

Visit www.invasives.org.za for more information on alien and invasive plant and animal species. 

This article was sponsored by Bronberg Conservancy