Acacia longifolia
(golden wattle)

A. longifolia is a shrub or small tree native to Australia that has been deliberately introduced in various countries, mainly for dune stabilization and soil improvement (Dennill and Donnelly, 1991; Marchante et al., 2008; Stellatelli et al., 2014). It is recognized as an aggressive invasive weed in parts of its native range in Australia, and in some of the countries where it has been introduced; such as in South Africa, Spain and Portugal. Although is reported by PIER (2015) as invasive in California, USA, and was stated by Whibley and Symon (1992) as having established naturalized populations, it is reported as uncommon by Baldwin et al. (2012) and it is not listed in the California Invasive Plant Inventory (California Invasive Plant Council, 2016). It has a prolific seed production, and fast growth, facilitating its spread in suitable habitats (Rodríguez-Echevarria, 2010; Marchante et al., 2011). A. longifolia is associated with invasion events in New Zealand (Haysom and Murphy, 2003), and is recorded as invasive in Brazil (Instituto Horus, 2011).

A. longifolia affects the biodiversity and ecosystems by altering the microbial communities and by its high resource utilization, outcompeting native species (Marchante et al., 2008; Werner et al., 2010). It is included in the IUCN Global Invasive Species Database (GISD, 2015) and is reported as being costly to eradicate (EFSA Panel on Plant Health, 2015). 

Although there is substantial documented information about its spread and damage to ecosystems and biodiversity (Marchante et al., 2008; Werner et al., 2010) and being included in invasive lists, A. longifolia is still sold by nurseries and Internet sites as a desirable ornamental to be used on slopes, for screens and as a windbreak. It is advertised as being low maintenance, highly adaptable and fast growing. It can escape from cultivation and get established in suitable areas, from where it could spread because of its prolific seed production and rapid growth.

A. longifolia preferred habitats are full sunny, sandy coasts and dunes. Can be found also in woodlands, riparian zones, scrubs and grasslands (PROTA, 2015). In areas where it has become naturalised in Australia, it grows on roadsides, along watercourses, in swamps and in native bushland (Weeds of Australia, 2015). The species will do well in humid or warm humid temperate climate, extending into the Mediterranean climate. It is frost resistant down to -6°C and drought resistant, but needs at least 550 mm of rainfall (Werner et al., 2010). Will tolerate strong winds, but not maritime exposure. Grows best on well drained sandy soils and can grow in nutrient poor soils, as it able to fix nitrogen (PROTA, 2015). Will do well in fire prone areas, as fire will induce germination (Marchante et al, 2010). Seeds can handle high salinity, which contributes to its invasive ability in sand dunes (Morais et al., 2012a, b).

Genetics

The chromosome number reported for this species is 2n=26 (PROTA, 2015). A. longifolia does not show high phenotypic plasticity (Peperkorn et al., 2005).

Reproductive Biology

A. longifolia produces spicate flower heads with a violet-like scent. Each flower has many anthers. As other Australian acacias, it is pollinated by a wide variety of generalist insects, frequently locally native bees. Apis mellifera is reported as one of the pollinators. Although A. longifolia is an outcrosser, and it has a low ability to self-pollinate; in the absence of pollen vectors some self-fertilization will occur (Correia et al., 2014). A. longifolia is a polycarpic species, producing seeds annually throughout its life. The seeds are elliptic; 4-6 × 2-2.5 mm, shiny, and 20-30 mg. Seed viability is high (about 90%). Average 1000 seed weight: 16.81 g; protein content of 13.02% (PROTA, 2015). Germination tests results show 12% germination following 50 years of open storage at room temperature (PROTA, 2015).

Physiology and Phenology

A. longifolia is an evergreen tree growing to 9 m at a medium rate. The bark contains 15% tannins. It can resprout from the base. For the flower and fruit production the following is reported by PROTA (2015): Flowering in Australia: July-November. Flowering in South Africa: June-September. Fruiting in Australia: Summer. Fruiting in South Africa: November-December. In California, flowering occurs from January to April (Baldwin et al., 2012).

Associations

The species is capable of nodulating profusely, which aids the spread on poor soils (Rodríguez-Echeverría et al., 2009). In Portugal, Rodríguez-Echeverría et al. (2007) report that A. longifolia associates with the root-nodule bacteria Bradyrhizobium sp., which is believed to have been introduced from Australia with the acacia.

Environmental Requirements

A. longifolia prefers well-drained, light sandy loams and can grow in nutritional poor soils. Soils can be acid, neutral or basic (alkaline) and the plant will tolerate saline soil. It cannot grow in the shade. The species can resist strong winds but not maritime exposure. Mean annual temperature range is 10 to 19°C, although can live in areas up to 25°C. It is hardy down to -6°C (PROTA, 2015). It is tolerant to dry periods, frost and sea spray, but generally needs at least 550 mm annual rainfall to propagate (GISD, 2015).

Most of the seeds have an innate dormancy and need stimulation to induce germination, usually by fire (Dennil et al, 1993; Marchante et al., 2010). Heat increases germination of the A. longifolia seeds, which can sustain a maximum temperature up to 160° C for more than 20 min (Behenna et al., 2008).
 

Animal feed, fodder, forage

• Invertebrate food

Environmental

• Agroforestry
• Boundary, barrier or support
• Erosion control or dune stabilization
• Graft stock
• Landscape improvement
• Revegetation
• Shade and shelter
• Soil improvement
• Windbreak

Fuels

• Charcoal
• Fuelwood

General

• Botanical garden/zoo
• Ornamental
• Research model

Human food and beverage

• Seeds

Materials

• Bark products
• Dyestuffs
• Green manure
• Gums
• Mulches
• Wood/timber

Ornamental

• Cut flower
• Seed trade

Charcoal

Woodware

• Tool handles

Acacia ?oribunda, A. longissima, A. maidenii and A. mucronata are the closest relatives of A. longifolia, all having a curved phyllode apex that is acute to mucronate. Acacia oxycedrus is said to hybridize with several species including A. longifolia, A. ?oribunda and A. mucronata subsp. longifolia. (Brown et al., 2010). A. longifolia is similar to A. obtusifolia, but lacks resinous margins on the phyllodes and usually flowers during spring (Flora of Australia, 2015). 

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Public Awareness

A. longifolia is one of the plants featured in the Invasive Plants of Portugal (2015) internet site. The site provides information about invasive species in Portugal and has a downloadable sighting application, sighting maps to help locate the species and information on how to control them. It is also featured at the Invasive Species South Africa (2016), with existing legislation for its control and erradication and citing proposed alternative species for the people to plant instead.

Eradication

Marchante et al. (2011) discuss how the eradication of A. longifolia in the Portuguese dune ecosystems is an unrealistic goal, because the invasions are extensive, persistent, and produce substantial seedbanks. The levels of seed production measured by Marchante et al. (2010) are high despite major losses before and after entering the seed bank. These seeds can germinate in large numbers after removal of the canopy, repopulating the cleared areas and impeding the recovery of ecosystems. Marchante et al. (2009) suggest  prioritizing the control of recently invaded sites, as the recovery of both natural vegetation and soil is more likely. Marchante et al. (2010) also propose containment as the strategy for older thickets, removing plants in the surrounding areas, where new invasions occur. Targeting these areas is more manageable, which should reduce the abundance of the invader and restore some of the ecosystem.

Control

Physical/mechanical control

The following is recommended by Marchante et al. (2011) for the control of A. longifolia: to prioritize the removal of the species on recently invaded areas, also removing the thick litter layers to promote an increase in plant species richness and cover, and a decrease in susceptibility to reinvasion. Further management actions to supplement clearing operations are needed, such as propagation of native species and/or controlled fires to deplete the invasive species seed bank.

Biological control

In Australia, bitou bush litter (Chrysanthemoides monilifera, Asteraceae), introduced from South Africa, has been reported to negatively impact the distribution of A. longifolia, by affecting the seed production and germination and displacing the species from the fore- and mid-dunes. Although not used as a biocontrol method, more research should be made on the possible biocontrol use of this species (Weiss and Noble, 1984; Ens et al., 2009).

Dennill and Donnelly (1991) report two insects used as biological controls for A. longifolia in South Africa: Trichilogaster acaciaelongifoliae, a wasp which which produces galls that prevent the development of the inflorescences and suppress the vegetative growth, and Melanterius ventralis, a weevil which feeds on the seeds. Donnelly and Hoffmann (2004) report that M. ventralis complements the use of the Trichilogaster by destroying the seeds that are produced on the few inflorescences not affected by the insects. The Trichilogaster wasps are reported to have spread to plantations of the commercially important tree species A. melanoxylon (Dennill et al., 1993), but a review by Hill (2005) reports later studies suggesting that effects were negligible and temporary, and that no other non-target attacks have been reported in the field in South Africa.

Trichilogaster acaciaelongifoliae is being considered by the European Food Safety Authority for its introduction in Europe for the control of A. longifolia (EFSA Panel on Plant Health, 2015). They conclude that the insect could be introduced into the affected areas without having a major negative impact on other species. Hill (2005) discusses the potential for use of T. acaciaelongifoliae and M. ventralis for control of A. longifolia in New Zealand.

Chemical control

Herbicide treatment is suggested to supplement mechanical/hand removal of the species (PIER, 2015).

Ecosystem Restoration

Marchante et al. (2008, 2011) recommend the manual removal of A. longifolia on recently invaded areas, although recognizing that manually removing the invader is not sufficient to restore the ecosystem. Additional strategies are also required, which include: planting native species, removing the litter and depleting the invasive seed bank. Transplanting saplings of native species is suggested for the height advantage over the invasive seedlings. Rodríguez-Echeverría et al. (2015) suggest for Portugal the use of the two native coastal dunes legumes Cytisus grandiflorus and Ulex europaeus ssp. latebracteatus as good options for revegetation of areas where A. longifolia is removed. Galatowitsch and Richardson (2005) recommend for riparian areas to replant selected indigenous species to catalyze the recovery, stabilize the sites and close the canopies.

09/04/2016 Original text by:

Jeanine Vélez-Gavilán, University of Puerto Rico at Mayagüez