Australian acacias are considered one of the most dangerous invasive plants at a global scale and one of the biggest environmental problems nowadays, displaying characteristics that lead to their proposal as model species of invasion. Acacia longifolia is a leguminous species native of Southeast Australia and Tasmania, particularly invasive in the European Mediterranean areas, Brazil, and South Africa. Two subspecies are described: A. longifolia subsp. longifolia e A. longifolia subsp. sophorae.
Acacia longifolia has elongated leaves (phyllodes) and flowers aggregated in characteristic yellow spikes, making it commonly known as “Sydney Golden Wattle”. It is a small tree or shrub with an average height of 5m.
Flowering of A. longifolia occurs in the winter and beginning of spring. Pods then form to release seeds in the summer. Seeds are about 5 mm long and covered in an extremely resistant black tegument.
The invasive capacity of A. longifolia is due to the large number of seeds it produces every year, and these seeds are able to remain dormant in the soil for several decades. Moreover, germination of seeds is promoted by the heat-shock provided by forestry fires, which are abundant and severe during the summertime. Besides the seeds, acacias also produce root nodules that can be as long as 2 cm where nitrogen-fixing bacteria reside, giving them a clear advantage in poor soils.
The control of invasion by A. longifolia is mostly done by manually cutting them down or by using herbicide. Additionally, biological control of acacia can be implemented by the release of the Australian wasp Trichilogaster acaciaelongifoliae, which forms galls in the flower buds preventing seed formation. Such has already been done is South Africa since 1983, and in Portugal is still being implemented since 2015.