Asparagus asparagoides (bridal creeper)

  Established plants can increase in size through spread of branching rhizomes bearing meristematic buds. Stansbury and Scott (1999) estimated from a landholder survey that patches of about 10 m² expanded radially by approximately 0.6 m year^-1. Situations where seeds were dispersed downstream by flood events have been observed (Graham and Mitchell, 1996).  

  Frugivorous birds are recognised as important contributors to seed dispersal in A. asparagoides (Stansbury, 1996, 1999, 2001; Raymond, 1999). At least 12 bird species, both native and exotic, have been recorded feeding on berries and potentially dispersing seeds in southern Australia (Stansbury, 2001). Ground-foraging birds, such as common blackbirds (

) and emus (Dromaius novaehollandiae) also contribute to secondary dispersal of fruits (Loyn and French, 1991; Raymond, 1999). Bird dispersal of seeds is highly stochastic and consequently dispersal is difficult to predict precisely (Siderov et al., 2006). The dispersal pattern is influenced by the behaviour of different bird species and habitat structure, since many birds fly to perch-trees to consume fruits. Stansbury’s (2001) diffusion model showed that A. asparagoides distribution at Bold Park in Western Australia was correlated with the flight pattern of silvereyes, with a mean distance for seed dispersal of 90.5 m. Occasional long-distance dispersal is also possible and may significantly increase rate of spread at a landscape scale. Based on silvereyes’ gut passage rates and flight speed, the maximum potential seed dispersal distance was estimated to be 12 km. Gradual short-distance bird dispersal of seeds along roadsides and wildlife corridors that connect disparate habitats has the potential to increase the rate of invasion of remnant vegetation patches at a local scale (Siderov et al.,2006).   A. asparagoides seeds are also dispersed by other animal vectors, such as rabbit (Oryctolagus cuniculus) and foxes (Vulpes vulpes) (Graham and Mitchell, 1996; Raymond, 1999).  

  Careless disposal of garden waste and earthworks (e.g., roadside grading) can spread rhizomes over considerable distances to start new infestations. Mud containing seeds that adheres to animals, vehicles and machinery is also believed to contribute to dispersal (Parsons and Cuthbertson, 2001), although this would be of relatively minor importance.  

  Deliberate movements of plants or propagules within and between countries occurred widely in the past for ornamental purposes. Rhizome fragmentation has been used locally to increase numbers of plants for sale, although such sale is now prohibited in Australia.

  Comparison of sequences of non-coding DNA regions from the chloroplast (trnL-trnF) and nuclear ribosomal (ITS1 and ITS2) genomes of South African and Australian accessions of A. asparagoides, identified the Western Cape Province of South Africa as the likely geographical source of founder populations in Australia (L Morin, CSIRO Entomology, Australia, personal communication, 2010). Chromosome number 2n = 20.  

  A. asparagoides’ flowers are bisexual and self-compatible. Cross-pollination has been demonstrated in hand-pollinated flowers, but the level of outcrossing versus selfing still remains to be determined (Raymond, 1999). Honeybees have been observed visiting flowers in large numbers, collecting both pollen and nectar (Raymond, 1999).   Shoots that emerge from well-established plants early in the growing season are the only ones that will produce floral structures and fruits, providing they can climb on a support (Raymond, 1999; Stansbury et al., 2007). Buds flower and wither within two weeks and, by the third week swelling of the ovary is usually visible. Fruit set is usually low < 50%) probably due to a combination of climatic or site-specific factors such as high intraspecific competition and stress among flowering shoots, extreme shading, herbivory on buds and flowers and mouldiness of developing fruits leading to abortion (Raymond, 1999).  

  A. asparagoides seeds have an after-ripening requirement that ensures they do not germinate too soon after they are produced. Passage of seeds through the gut of silvereye (Zosterops lateralis) birds was observed to reduce the after-ripening period and enhance germination compared to un-passed seeds (Raymond, 1999). Mature seeds can germinate over a wide range of temperatures (10–30°C) in both constant light or darkness, but germination is optimal at lower temperatures of approximately 15–20°C (Raymond, 1999). Willis et al. (2003), however, observed a strong inhibitory effect of light on germination when fluctuating light and temperature regimes were used. The effect of smoke and heat shock treatment (90°C for 10 min) on germination of seeds has been investigated, but results of different trials have been contradictory, highlighting the need for further work in this area (Willis et al., 2003).   Following rapid growth and establishment of shoots at the beginning of the growing season, A. asparagoides diverts some of its photosynthates towards tuber production. The above-ground biomass is positively correlated to its below-ground biomass (Kleinjan et al., 2004a). Once plants establish extensive below-ground reserves they can produce shoots at the onset of cooler temperatures, even in the absence of available external moisture (Kleinjan and Edwards, 1999; Raymond, 1999).   In South Africa, A. asparagoides adapts its phenology to different seasonal rainfall patterns (Kleinjan and Edwards, 1999). It behaves similarly in Australia, where its major distribution is in winter-rainfall dominant regions. In most areas, shoots emerge from the below-ground rhizome either slightly before or with the onset of the first late-summer or autumn rains. Seeds readily germinate throughout autumn and winter if conditions and microhabitat are adequate. Seedlings have a good chance of surviving over the dry summer period providing they have had time to develop some below-ground reserves (Raymond, 1999).   In Australia, flowering generally occurs in late winter to early spring (Raymond, 1999), while in South Africa flowers are produced as early as July (Kleinjan and Edwards, 1999). Fruits develop in the spring, mature into red berries in late spring to late summer, depending on the region, and can be retained on senescing plants for several months. Above-ground biomass begins to senesce in mid to late spring. The onset of senescence is earlier when plants are exposed to more direct sunlight. Shoots typically senesce from the apex down and take several weeks to die back completely. The below-ground biomass enables plants to survive until the next growing season.  

  Greater accumulation of litter was observed in habitats invaded by A. asparagoides compared to non-invaded native areas in South Australia (Stephens, 2005). This difference was probably due to litter becoming trapped in the climbing stems and foliage. Despite the significant changes in habitat and decrease in plant diversity due to invasion by A. asparagoides, Stephens (2005) found a very abundant and diverse arthropod and wasp community in both native and invaded habitats. Indeed, a higher number of soil/litter-associated pollinators of the endangered orchid, Pterostylis bryophila, were recorded at the invaded sites (Stephens et al., 2003). It appeared that A. asparagoides provided a favorable habitat for these specific pollinators and did not affect their movement or pollination efficiency within the invaded habitat. These study sites, however, were not completely colonized by A. asparagoides and consequently the plant may not yet have caused a habitat change significant enough to detect flow-on negative effects on the arthropod community.   Regeneration of native communities after removal of A. asparagoides may be slow where few propagules of native plants are left in the soil. Below-ground biomass may also impede seedling establishment of native species by occupying all of the space available in the substrate or by changing soil chemistry. Turner and Virtue (2006) observed that dead rhizomes and tubers were still present eight years after A. asparagoides had been killed. In another study performed in Western Australia, the initial decomposition of underground biomass was rapid with about 40% loss of dry weight occurring during the first three months after burial (Turner et al., 2006). Subsequent decomposition slowed dramatically with no significant change for the next six months.  

A. asparagoides is particularly vigorous in soils with high moisture content and areas high in nutrients. It grows best at sites with high levels of available nitrates, potassium and iron (Stansbury, 1999). A recent study in Western Australia showed that soils where A. asparagoides dominates have higher levels of available phosphorus than nearby non-invaded areas (Turner, 2008). It is unknown at this stage whether A. asparagoides originally invaded phosphorus-rich areas or if it is modifying the soil environment.

A. asparagoides can grow in areas where rainfall is sub-optimal providing it is irrigated (e.g. private gardens or citrus orchards), or in close proximity to watercourses or in moist gullies (Stansbury and Scott, 1999).

  Rapid growth of A. asparagoides in autumn and its climbing habit provide canopy dominance and make it highly competitive. Once an infestation is established, the amount of light reaching the soil surface is very low, thereby preventing other plants from persisting. Raymond (1999) reported that the amount of irradiance reaching the soil surface was reduced by 93% under a canopy of A. asparagoides foliage. Competitiveness of A. asparagoides is also due to the quantity of below-ground biomass that occupies most of the space available in the substrate. During the fruiting stage, established infestations can harbour large numbers of exotic birds (Raymond,1999; Stansbury, 2001), which are considered pests in Australia.   A. asparagoides has been identified as a threat to four endangered ecological communities in New South Wales (Coutts-Smith and Downey, 2006). Eleven additional ecological communities, including six already listed under the New South Wales Threatened Species Conservation Act 1995, are potentially threatened by A. asparagoides (Downey, 2006).  

  A. asparagoides is a highly competitive species that poses a direct threat to at least 17 native plant species in South Australia and New South Wales (Quarmby, 2006; Downey, 2006). In addition to the species listed in Downey (2006), the critically endangered orchid species Pterostylis bryophila and Pterostylis sp. ‘Halbury’ in South Australia are also considered under threat due to habitat loss and primarily A. asparagoides invasion (Quarmby, 2006). Furthermore, a recent assessment made in southeastern New South Wales, which involved an extensive consultation process, suggested that up to 52 additional plant species are potentially threatened by A. asparagoides invasion (Downey, 2006). The black grass-dart butterfly (Ocybadistes knightorum) is also listed as threatened by A. asparagoides in New South Wales (Coutts-Smith and Downey, 2006).

  A. asparagoides was widely grown as an ornamental plant from the mid-1880s to the early twentieth century, but its popularity declined substantially afterwards. It is not sold anymore in nurseries in Australia and it is legislated against in all states and territories (Morin et al., 2006a).  

  In South Africa, A. asparagoides is believed to have therapeutic attributes. A lotion prepared from its root biomass is used to bathe sore eyes (Guillarmod, 1971), although no detail on efficacy and extent of use is provided in the literature. Tubers have been seen being sold in the Cape Town market as a remedy for stomach problems, but again no detail is known of the recommended method of preparation or the dose rate (PB Edwards, CSIRO Biological Control Unit, University of Cape Town, South Africa, personal communication, 2006.  

  A. asparagoides’ fruits form part of the diet of a small number of Australian native birds (Stansbury, 2001), whose normal host plants may occur in low density due to habitat degradation or destruction. Its foliage is non-toxic to mammals. It is reported to form a significant part of the diet of tammar wallabies (Macropus eugenii) on Garden Island in Western Australia (Bell et al., 1987). Sheep and cattle have also been observed grazing on the foliage (Siderov and Ainsworth, 2004), but there is no information on its nutritional value.

 

  Since A. asparagoides was listed as a Weed of National Significance in Australia (Thorp and Lynch, 1999), major education campaigns have been undertaken to increase community awareness of the problems it causes and available methods of control (Weeds Australia, 2010).  

  Eradication of A. asparagoides at a local scale is only feasible for recently established infestations and prior to plants producing fruits. A. asparagoides has been the target of an eradication campaign for several years in Tasmania (Cooper and Warren, 2006), but the campaign’s effectiveness is now being questioned as new small infestations keep on emerging. An eradication campaign was also initiated in 2004 on Lord Howe Island off the coast of New South Wales and is intended to last for a minimum of 15 years (Le Cussan, 2006).  

  Since most A. asparagoides seeds are bird-dispersed within 300–500 m of the source (Stansbury, 2001; Siderov et al., 2006), it is recommended to control outlying plants or patches within a buffer zone of 500 m around the edge of a main infestation (Asparagus Weeds Best Practice Management Manual, 2006). Siderov et al. (2006) emphasised the importance of controlling A. asparagoides along paths and tracks within remnant vegetation because they act as a conduit for invasion. Alternatively, reducing the number, or modifying the spatial arrangement, of tracks within a reserve can be considered to limit seed dispersal within the area.  

  Carr (1996) recommended initially focusing control efforts on low density infestations occurring in bushland with the highest conservation value before tackling more severely infested areas. Targeting control towards climbing plants that produce the most fruits within the core infestation may also help limit spread.  

  Gardeners are encouraged to prevent further spread by avoiding composting, mulching or dumping garden refuse containing A. asparagoides rhizomes from their properties. Dug-up plants should be placed in black plastic bags and left out in the sun for many months to kill rhizomes (Asparagus Weeds Best Practice Management Manual, 2006). Hygiene of earth-moving equipment is important to prevent spread of viable rhizome fragments that may be present in soil (Graham and Mitchell, 1996).  

Isolated young A. asparagoides plants with an underdeveloped root system can be pulled-out by hand when growing in loose soil (Carr, 1996). Manual removal of mature plants and their root system is only appropriate for small isolated infestations, particularly in areas of high conservation value (Carr, 1996; Asparagus Weeds Best Practice Management Manual, 2006). It is also used by growers in citrus orchards who are reluctant to use herbicides (Kwong and Holland-Clift, 2004). Follow-up control is often required because A. asparagoides regenerates from small pieces of living rhizome left behind in the soil. Removed root mats should be disposed of by deep burial (>2 m deep) or dried and burnt (Graham and Mitchell, 1996; Pike, 1996). Hand removing is time and labour intensive and consequently not suitable for severe and extensive infestations. Removal of well-developed mats of rhizomes and tubers can create excessive disturbance that hinders natural bush regeneration and facilitates colonisation by invasive plants.   Mechanical removal of above-ground biomass is effective at reducing seed production if performed before fruiting, but many repeated slashings are necessary to exhaust below-ground reserves. Slashing is sometimes carried out in citrus orchards around infested trees (Kwong and Holland-Clift, 2004). A. asparagoides is palatable to mammals (Bell et al., 1987; Carr, 1996; Siderov and Ainsworth, 2004) and thus livestock grazing is potentially an effective control method. Siderov and Ainsworth (2004) found no A. asparagoides occurring next to fenced areas of pasture where it was abundant and supported the implementation of controlled grazing in areas fenced off for revegetation to keep the invasion in check. Such an approach may be more efficient than regularly searching the area to remove newly-established seedlings, but may not be appropriate when the area is revegetated with native species palatable to livestock. Cooke and Robertson (1990) reported use of sheep to reduce A. asparagoides density prior to chemical control on Kangaroo Island, South Australia.   A. asparagoides is not killed by bushfires unless the fire is very intense. Hot summer fires can destroy a large proportion of rhizomes and tubers lying near the soil surface, but generally fire does not affect below-ground reserves located deeper in the soil (Carr, 1996; France, 1996). Land managers are encouraged to take advantage of a late summer or early autumn wild fire and implement chemical control since A. asparagoides is usually one of the first species to emerge in the burnt areas before regeneration of sensitive native species (Carr, 1996; France, 1996; Graham and Mitchell, 1996).  

  Three biological control agents for A. asparagoides, all of South African origin, have been released in Australia: An undescribed Erythroneurini leaf hopper in 1999, the rust fungus Puccinia myrsiphylli in 2000 and a Crioceris sp. leaf beetle in 2002 (Witt and Edwards, 2000, 2002; Kleinjan et al., 2004b; Morin and Edwards, 2006; Morin et al., 2006b). A Eurytoma sp. seed wasp was also considered a promising candidate agent, but host-specificity testing could not be performed dueto rearing difficulties and concerns raised about the conflict of interest with producers of cultivated asparagus seed. Large-scale releases of the leaf hopper and rust fungus were carried out in partnership with community groups, land managers and schools (Kwong, 2002; Batchelor et al., 2004; Overton and Overton, 2006).   Impact of the agents on A. asparagoides has been assessed in a series of glasshouse and field experiments (Batchelor and Woodburn, 2002; Morinet al., 2002, 2006b; Kleinjan et al., 2004a; Spafford Jacob et al., 2007; Turneret al., 2008a, 2010). The rust fungus has had a major impact in reducing A. asparagoides populations, particularly in moist coastal areas of Australia where climatic conditions are conducive to epidemics. The leaf hopper has also adversely affected A. asparagoides at some sites, but its populations have a tendency to fluctuate from year to year, hence limiting impact. Establishment of the leaf beetle has been extremely poor despite considerable release efforts and therefore this agent is not currently contributing to control.   Puccinia myrsiphylli was found in New Zealand in November 2005 near Auckland, New Zealand, probably as a result of an accidental introduction or natural dissemination of spores from Australia on wind currents (Waipara et al., 2006). Subsequent surveys and field assessments revealed that it was widespread throughout the range of A. asparagoides in northern New Zealand, often causing severe damage and premature defoliation (Harman et al., 2008).  

  Several herbicide trials have been conducted in the last 20 years (Cooke and Robertson, 1990; Dixon, 1996; Pritchard, 1996, 2002), identifying glyphosate, metsulfuron methyl and some related sulfonylureas as the most effective non-selective systemic herbicides against A. asparagoides. To achieve best results with chemical control, a spray program of at least three years is recommended. Repeat applications of herbicides are essential to kill plants that are missed in the first year, any new regrowth and seedlings, and to have a significant impact on below-ground biomass. A longer control program with herbicide applied every 2–3 years, rather than annually, may be a more efficient use of time and resources without compromising outcomes.   Best control outcomes have been achieved when metsulfuron methyl or glyphosate was applied onto actively growing A. asparagoides in the early to late-flowering stage (Dixon, 1996; Pritchard, 2002). Good control was also obtained with applications made at the flower bud to green berry stage (Pritchard, 2002). Spraying before flowering is advocated by some to allow better translocation to the rhizome (Cooke and Robertson, 1990) and to prevent off-target damage to indigenous species that emerge in spring (Carr, 1996).   Chemical control of A. asparagoides growing amongst citrus foliage is not generally practiced because of the high risk of damage to trees (Kwong and Holland-Clift, 2004).  

  A combination of biological and chemical control with other management tactics such as fire may be necessary in some areas to tackle A. asparagoides more effectively (Willis et al., 2003). Access to infested areas for spraying is considerably improved after a fire because most woody shrubs have usually been destroyed. Willis et al. (2003) suggested use of fire in autumn, after most annual shoots have emerged, and post-fire application of herbicide on regrowth to maximize depletion of below-ground reserves. Turner and Virtue (2009) assessed the impact of herbicide used following wildfire that burnt an area of mallee vegetation in South Australia infested by A. asparagoides. Ten years later there was almost 4 times less shoots of A. asparagoides in herbicide treated plots when compared to plots that were untreated at the time of the fire. There is no contra-indication to use biological control for A. asparagoides post-fire and agents are most likely to recolonize sites if they are present in unburnt nearby areas (Morin et al.,2006b).  

  Natural ecosystems may take many years to recover after an area is cleared of A. asparagoides (Turner and Virtue, 2006). Presence of bare ground after A. asparagoides is controlled increases the likelihood of re-invasion or colonisation by other invasive plants. Land managers can either rely on natural recovery processes or actively revegetate controlled sites with indigenous species. Pike (1996) observed natural regeneration of native plants, such as Hardenbergia comptoniana and Acacia pulchella in an area where A. asparagoides had been hand removed in Western Australia. However, Turner et al. (2008b) found that the exotic seed bank germinated more readily than the native species in areas that have been invaded by A. asparagoides and that additional restoration efforts would be needed after successful control.