Lantana (Lantana camara L.)

1 Introduction

Potential distribution mapping and climate change models suggest that lantana has the capacity to spread beyond its current range and to increase in density within its existing distribution. Consequently, without sustained commitment to its management, the cost to the Australian community is expected to increase significantly.

Widespread weeds like lantana offer a range of challenges to land managers due to the cost of control and logistics associated with their management. Based on current technologies and resources, it is unrealistic to expect that lantana can be completely controlled across its entire range. However, experience has shown that effective strategic management can ensure the impacts are reduced and our most valued environmental, economic and social assets are protected.

There are numerous industry, government, and community groups; as well as a large number of private individuals actively working to control the spread or reduce the impact of lantana. The challenges faced by these groups are significant; however, the cross-jurisdictional partnerships and vital support mechanisms developed through the delivery of this strategic plan can enable more strategic and coordinated management.

2 Background

2.1 The biology of lantana

2.1.1 Origins and varieties

Lantana has been an important part of the international horticultural industry since it was introduced to Europe from Brazil around 1636 (Howard 1969). Multiple subsequent introductions to Europe resulted in the creation of aggregate species of mixed parentage. As a result, the weed now known as L. camara is a highly variable hybrid plant. Worldwide there are several hundred wild and cultivated forms or varieties (Day et al. 2003). At least 29 varieties have become naturalised in Australia.

Despite this, recent research indicates that only shallow genetic variation exists among Australian L. camara varieties, and it is proposed that they originated from a single phenotypically diverse species from the plant’s native range (Watts et al. 2009). This view conflicts with the previous hybrid-swarm model (Sanders 2006) but supports the view that Australian weedy L. camara varieties should be considered one species.

Nevertheless, varietal differences can be quite significant from a management perspective. Morphological traits like leaf size, hairiness and plant structure can influence the effectiveness of herbicide control methods, and some lantana biological control agents are host specific to one or a small number of lantana varieties. In addition, management priorities may be determined on the basis of differences in levels of invasiveness and toxicity.

Varieties are identified based on morphological characteristics such as flower colour, structure of leaf hairs, and thorniness and length of bracts, however, in many instances, specialised expertise is required to reliably separate forms. To simplify the taxonomy for general use, weedy lantana varieties in Australia have been classified into five main groups based on flower colour: white, pink, pink-edged red, red and orange.

2.1.2 Physical description

Lantana has a shallow root system consisting of a short taproot with lateral roots branching out to form a root mat. It has square-shaped stems covered in short, recurved prickles. The leaves are opposite, 2–10 cm long, ovate to lanceolate, with toothed margins. They are generally bright green on the upper surface, and pale and hairy on the underside; however, leaf colour, size and shape are dependent on the variety of lantana and the availability of moisture and light (van Oosterhout 2004). The leaves and stems have a strong aromatic odour when crushed (Swarbrick et al. 1998).

An inflorescence consists of dense clusters of 20–40 flowers. Pairs of inflorescences form in the axils of opposite leaves. Tightly packed, angular flower buds open from the outside towards the centre of the inflorescence as they mature (van Oosterhout 2004). Flowers vary in colour from red–yellow, orange–pink and white depending on the lantana variety and maturity of the inflorescence.

2.1.3 Life cycle

Lantana flowers are pollinated by a range of insect species including butterflies, bees and thrips (Clemson 1985; Dronamraju, 1958; Goulon & Derwent 2004; Mathur & Mohan Ram 1986; Schemske 1976; Swarbrick et al. 1998). Cross-fertilisation is most common; however, some self-pollination may also occur (Day et al. 2003).

About half of the flowers will develop a single-seeded, fleshy berry. Berries are borne in clusters and ripen to shiny purple–black (Swarbrick et al. 1998). A single plant can produce up to 12 000 fruit per year (van Oosterhout 2004), although seed bank densities can average from 1.4–3.4 (Gentle & Duggin 1997, 1998) to 599–3674 seeds/m² (Vivian-Smith et al. 2006).

Germination rates are generally low and have been recorded between 6 per cent and 16 per cent (Vivian-Smith et al. 2006). Seeds normally germinate during the warmer months but will germinate throughout the year if sufficient soil moisture is available and temperatures are high enough.

Lantana seeds are mostly distributed by fruit-eating birds, but kangaroos, bearded dragons, sheep, goats, cattle, pigs, foxes, and possibly rodents also spread seed in their droppings (Day et al. 2003; Swarbrick et al. 1995). Lantana seeds have been dispersed up to 1400 m from the ingestion source by birds in the tropical rainforest environments of north Queensland (Westcott 2009), but seed is likely to be spread further in more open environments where birds travel further between roosts. The passage of seeds through the gut of animals is believed to improve germination rates, as there is some evidence that seed pulp delays or inhibits germination (Graaf 1987; Swarbrick et al. 1998).

Greenhouse-based trials indicate lantana seed survival of approximately 21.3 per cent after 36 months under natural rainfall conditions, and 27.2 per cent after 24 months when irrigated (Vivian-Smith & Panetta 2009). Computer modelling of seed survival projections suggest that a small percentage of buried seed may survive up to 11 years (Vivian-Smith & Panetta 2009).

Lantana plants can also reproduce vegetatively, sprouting from stems that take root, by layering, or through the planting of woody cuttings into moist soil.

Figure 1 Life cycle of Lantana camara

2.1.4 Environmental drivers of growth and invasion

Rich volcanic and well-drained clays are ideal soils for lantana but it will grow in sandy soils if there is adequate moisture.

Lantana does not grow well if it is waterlogged or shaded, in saline conditions or under prolonged drought (Thaman 1974; Winder 1980). Growth is also restricted in shallow soils with limited water-holding capacity, and in the hard, phosphate-deficient soils of open eucalypt forests (Swarbrick et al. 1998). The upper temperature limits of lantana are unknown, but the shoots are frost sensitive and plants do not tolerate consistent temperatures below 5 °C (Thaman 1974; Winder 1980).

After becoming dominant in small gaps or edges, lantana may gradually spread if the area experiences increased frequency and intensity of fires, which may kill understorey species (Swarbrick et al. 1995), or because of its climbing and strangling growth habit that causes further damage and reduction of the canopy (Alcova 1987). Lantana often dominates secondary succession, and allelopathic chemicals released into the soil prevent germination and competition from some other plant species (Gentle & Duggin 1997a, 1997b).

Activities that increase light intensity and soil temperature stimulate the germination of lantana seed. Consequently, lantana distribution and spread is closely linked to disturbance activities such as clearing, cultivation or road construction, inappropriate burning regimes, overgrazing or livestock trampling; and feral animal activity such as rabbit burrowing and pig rooting (van Oosterhout 2004).

2.1.5 ‘Sterile’ forms of lantana

Horticulturists have developed ornamental varieties that have been promoted as ‘sterile’; however, many still produce viable pollen, occasionally set seeds and are capable of spreading vegetatively.

Research indicates that some of these ornamental varieties are also capable of hybridising with weedy forms to produce viable offspring with increased genetic variability (Sples & du Plessis 1987). This could lead to the introduction of traits for increased flowering ability and drought and frost tolerance into the weedy population. It also has the potential to compromise lantana biological control programs by altering host susceptibility or recognition, and increasing the weed’s adaptability to new environments.

In most instances, the introduction of L camara varieties through the nursery trade has been successfully stopped. A 2009 Nursery and Garden Industry Australia survey found no species that are Weeds of National Significance (WoNS) on any of the 328 production/availability lists nationwide (Thomas & Kachenko 2009). In addition, records of illegal sale are rare and when identified local authorities deal with them rapidly.

Despite Australia-wide legislative restrictions on the sale and distribution of L. camara, removal of existing plantings is not enforced in most areas, and garden plantings continue to pose a risk to achieving long-term management outcomes.