Section 3.1 Weediness of Saccharum spp. hybrids
Commercial cultivars of sugarcane are hybrids of S. officinarum and S. spontaneum. They are not recognised as weeds in Australia or anywhere in the world. They have lost many of the critical weedy attributes that were present in the parental species from which the cultivated sugarcane hybrids were derived (Holm et al. 1997). Most commercial cultivars of sugarcane are routinely harvested before flowering as the process of flowering leads to reduction in stem sugar content (Moore & Nuss 1987). As discussed above, when allowed to flower, sugarcane flowering is erratic and variable and sugarcane seed is short-lived (Rao 1980). Even if seed is produced, the chance of it germinating, surviving to reproductive maturity and spreading is low.
Volunteer sugarcane seedlings of modern cultivars are non-invasive in natural habitats and are likely to be controlled by natural herbivores during early stages of growth or be out-competed by other weeds. There has been one instance of volunteer sugarcane, of cultivar CP29-116, probably growing for 30-40 years in southern Queensland. These volunteer canes consist of only a few stools and have not spread further.
The potential for Saccharum spp. hybrids to become weeds in Australia is low because canes are harvested before they flower as mentioned above. Unaided dispersal of the sugarcane vegetative materials is unlikely due to the size and weight of the stem sections. Animal pests such as feral pigs have the strength to remove plant material and take them elsewhere. But there are no reports of feral pigs or other animal pests of sugarcane indulging in such activities. Establishment and spread of volunteers are unlikely as the seedlings of modern cultivars are highly susceptible to pests, weeds and therefore unlikely to establish and spread without human intervention. Un-germinated stem cuttings tend not to survive for long as they are rapidly degraded by soil micro-organisms due to their high sugar content.
Section 3.2 Weediness of Saccharum Species
None of five recognised Saccharum species (S. spontaneum, S. robustrum, S. barberi, S. sinensis and S. officinarum) are native to Australia. Some of these species are maintained within Australian sugarcane research stations as germplasm. Only two parental species of modern cultivars are recorded naturalised in Australia ie S. spontaneum and S. officinarum (Hnatiuk 1990).
3.2.1 Saccharum spontaneum
S. spontaneum is native to India and recorded as a weed in 33 countries and is 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). It is classified as a noxious weed in 42 states of the USA (USDA 2004). 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. 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). Pure stands of S. spontaneum can be found in poor agricultural soils degraded by fire and overuse (Hammond 1999);(Holm et al. 1997).
Because of higher genetic variability of polyploid species, S. spontaneum has more variable morphology and physiology, enabling it to develop higher phenotypic plasticity and effectively compete in disturbance prone and changeable habitats. S. spontaneum’s competitive advantage is due to its diverse reproductive abilities. S. spontaneum can reproduce by both vegetative propagation (layering, root and stem cutting) and seeding. It invades aggressively both underground and above ground by extending rhizomes, tillers (secondary shoots) and tertiary shoots, which together form thick stools or clumps (Pursglove 1972). The dense root mat of S. spontaneum makes it impossible for young seedlings to penetrate and emerge. S. spontaneum is highly variable, tolerating a broad range of soil types and moisture contents. It is shade tolerant and produces many small wind dispersed seed (Pursglove 1972). These weedy traits have not been incorporated into commercial hybrids.
Australia’s Virtual Herbarium (http://www.cpbr.gov.au/avh.html) records S. spontaneum collected in the Northern Territory, Queensland and northern New South Wales. Evidence suggests that currently S. spontaneum is not a serious weed in Australia, because surviving uncultivated populations have established in the Northern Territory for at least 50 years without further spread.
3.2.2 Saccharum officinarum
S. officinarum is cultivated as agricultural crop in many countries in Asia and other tropical climate countries. It has been grown as food crop in the Americas and Indies since the 18th century. It is thought that S. offiicinarum was originally selected by humans in Papua New Guinea. S. officinarum has escaped the agricultural area and naturalised in some areas but has not been recorded as a major weed in Australia (Hnatiuk 1990; USDA 2004) or elsewhere (Holm et al. 1997). Hnatiuk (1990) reported that S. officinarum is naturalised in Queensland and New South Wales. Lazarides et al (1997) recorded it as a minor weed found naturalised in some Tropical and Mediterranean climates in Australia. S. officinarum has also been recorded as a minor weed and/or a quarantine species in some countries (Randall, 2002) because it may pose a risk of quarantine disease transmission.
As a result of many years of cultivation, S. officinarum has essentially lost the capacity to invade in uncultivated habitats. 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. A few populations of S. officinarum have established outside agricultural areas in southern Queensland for 30-40 years. These populations consist of only a few stools and do not result in further spread.
There are a number of species within genus Saccharum which are recognised as weeds elsewhere such as Saccharum arundinaceum, S. bengalense, S. floridulum, S. narenga, S. procerum and S. ravennae (Randall, 2002). There are no records of these species in Australia (Hnatiuk, 1990; http://www.cpbr.gov.au/avh.html; Randall, 2002).