Weeds are plants that spread and persist outside their natural geographic range or intended growing areas such as farms or gardens. Weediness in Australia is often correlated with weediness of the plant, or a close relative, elsewhere in the world (Panetta 1993; Pheloung et al. 1999; Groves et al. 2005). The likelihood of weediness is increased by repeated intentional introductions of plants outside their natural geographic range that increase the opportunity for plants to establish and spread into new environments (eg escapes of commonly used garden plants) (Groves et al. 2005).

Modern Saccharum spp. hybrid cultivars do not possess many of the attributes commonly associated with problematic weeds such as seed dormancy, persistence in soil seed banks, germination under adverse environmental conditions and a short life cycle (Baker 1974; Keeler 1989; Keeler et al. 1996).

8.1 Weediness status on a global scale

An extensive compilation of the world’s weed flora is produced by Randall (2002). Most of the information contained in this book has been sourced from Australia and North America, but also includes numerous naturalised floras from many other countries. Randall (2002) lists twelve species of Saccharum which have been identified as having a documented weedy history. However, due to species reclassifications many of these species are now known by alternative names and are no longer in the Saccharum genus. The sugarcane parent species, S. officinarum is listed as naturalised, introduced, a casual alien, an economic weed and a quarantine weed in some countries. The other sugarcane parent species, S. spontaneum is listed as naturalised, introduced, a casual alien, an economic and environmental weed, a noxious weed and a quarantine weed in some countries. S. spontaneum is listed as one of the 104 most important world weeds by Holm et al (1997).The hybrid of these two species grown as cultivated sugarcane is listed as an Australian quarantine weed (species prohibited entry under a country’s quarantine regulations) (Randall 2002).

S. spontaneum is native to India and recorded as a weed in 33 countries. It has adapted to diverse environments throughout the world, ranging from tropical to subtropical regions, most commonly found in central and south-eastern Asia (Holm et al. 1997). S. spontaneum is a serious agricultural weed in Thailand, the Philippines, India and Indonesia where it competes vigorously on disturbed sites (Holm et al. 1997). It occurs in wastelands, fallow fields, marshes, on banks of streams and ponds, on sand dunes, along railroads and highways and in or around agricultural fields. Pure stands of S. spontaneum can be found in poor agricultural soils, degraded by fire and overuse (Holm et al. 1997; Hammond 1999). It is recorded as a noxious weed in the US (USDA 2010).

S. officinarum and Saccharum spp. hybrids have not been recorded as major weeds (Holm et al. 1997; USDA 2004; Berville et al. 2005) .

8.2 Weediness status in Australia

In Australia, sugarcane occurs almost exclusively in managed cultivation. In sugarcane growing districts, transient sugarcane plants may occur around fields, but there is no indication that these form self-perpetuating populations (Bonnett et al. 2007). Thus, sugarcane does not appear to be a problem as a volunteer weed (Berville et al. 2005).

None of six recognised Saccharum species (S. spontaneum, S. robustum, S. edule, S. barberi, S. sinensis and S. officinarum) are native to Australia. Only the two parental species of modern cultivars, S. officinarum and S. spontaneum, are recorded as naturalised in Australia (1990). Naturalised populations of S. spontaneum were recorded at several locations in QLD alongside the Mulgrave, South Johnston and Herbert rivers, and in Feluga and the Cardwell Range (Bonnett et al. 2008) as well as alongside the Daly river in NT (Magarey et al. 2007). The populations in Feluga and South Johnston were deliberately planted (Bonnett et al. 2008). Sampling of DNA from the populations has suggested that the plants at the South Johnston, Feluga, Cardwell Range and probably Mulgrave sites are clonally propagated; thus, growth of the populations is by vegetative, not sexual, reproduction (Bonnett et al. 2008).

S. officinarum is listed in the CSIRO Handbook of Australian Weeds as a minor weed found naturalised in some tropical and mediterranean climates in Australia (Lazarides et al. 1997). It has also been listed by Groves (2003) as present in QLD and NSW but not rated as a weed as it was either not a problem, or not present in agricultural areas. Australia’s Virtual Herbarium (Australia's Virtual Herbarium 2010) has records of S. officinarum collected in the NT and QLD, however, these may have been collected as representatives of a S. officinarum crop from commercial fields.

Molecular studies have shown S. officinarum to have low levels of genetic diversity compared to other Saccharum species (Sobral et al. 1994; Janno et al. 1999). Generally this species has less capacity to compete in the natural environment than S. spontaneum. However, due to its perennial nature, some populations escape from cultivation and can persist as long as there is sufficient moisture in the root zone. S. officinarum has thus become naturalised in some areas in Australia (Hnatiuk 1990).

Saccharum spp. hybrids are not recognised as weeds in Australia. They have lost many of the critical weedy attributes such as profuse tillering, adaptability to biotic stresses and resistance to pests and diseases that were present in the parental species from which the cultivated sugarcane hybrids were derived. As discussed in Section 4, most of the cultivated cultivars exhibit low fertility of both pollen and ovules, so flowers in commercial fields rarely set seed (James 2004). However, data from Bonnett et al (2008) suggests that viable seed production does occur at low levels in some commercial fields. Sugarcane seeds need optimum conditions for germination and survival of the resulting seedlings (see Section 4.4). These conditions may only occur sporadically in natural ecosystems, thus limiting the spread and persistence of sugarcane. There are isolated reports of seeds germinating naturally in the Mulgrave, South Johnston and Herbert sugarcane growing regions (17–18.5S) but not further south (Bonnett et al. 2010).

8.3 Control measures

Sugarcane may be killed by ploughing out the stools and then treating with herbicide (glyphosate) (Willcox et al. 2000). However, minimum tillage practices often result in inadequate eradication of the old crop (Leibbrandt 1993). The efficacy of glyphosate for killing sugarcane is affected by various factors such as cane being in active growth, cane cultivars, soil type and stage of cane growth (Turner 1980). Sugarcane grown in light soils is more susceptible to herbicide treatment than that grown on heavy soils. The plant is killed more easily when the height of the leaf canopy is between 0.4–0.75 m compared with older cane that has produced stalks (Turner 1980). Glyphosate is ineffective on recently cut ratoons until germination of buds is completed and tillering is advanced (Chedzey & Findlay 1985). Rain may also affect the efficacy of herbicide, so it is best used during the dry season (Owende et al. 1995). Research has shown that slashing of cane suppresses apical dominance and generally enhances chemical cane killing action on the regrowth. In addition, considerable improvement of eradication was also obtained when a mechanical under-cutter was used to shear the roots following herbicide application (Leibbrandt 1993).