Xanthium spinosum
(bathurst burr)

X. spinosum is a highly invasive plant classified as one of the world’s worst weeds, and is now widely distributed throughout many regions of the world, where it is a common agricultural and pasture weed and a declared noxious species in many countries. Originating in South America, it has spread widely, probably via its spiked seeds which attach to animals and clothing or are a contaminant of hay or other products. It produces prolific amounts of seed that germinate easily. X. spinosum can quickly dominate large areas, outcompeting crops, forage plants and native flora. Control is possible but requires significant effort. There is considerable ongoing research into various methods including biological control.

There appears to be disagreement over the native range of X. spinosum, though it is certain to have originated from South America. Some sources have suggested Chile, whereas others suggest a broader area that also includes at least Argentina, and possibly also other countries, which may be more probable. The native range proposed by USDA-ARS (2013) is used here, which includes Argentina, Bolivia, Chile, Ecuador, Peru, Uruguay and southern Brazil, though it also confirms that the limits of native range are obscure. Pitcher (1989) stated that X. spinosum had been introduced to at least 39 countries, though records for many more are available and the real figure may be higher still. It is widely distributed in the Mediterranean region, Europe, Australia, parts of Africa, South America and North America, though is only rarely found in the tropics (Holm et al., 1977).

X. spinosum has all the attributes that make accidental introduction very likely, and it has already been introduced to so many countries it is likely to spread further. Noting its recent arrival in western China, further spread into Central Asia may be expected in the future, for example.

X. spinosum can be found growing in a wide variety of habitats. Holm et al. (1977) described X. spinosum as mainly a pasture or meadow weed, growing along roads and in disturbed areas and abandoned fields, also being sometimes common around waterholes and along floodplains, canals, ditches, creek flats, river terraces, and other moist places. It can also be common weed in cultivated fields, stock yards and abandoned settlements. In New Zealand it is noted in ‘waste places, coastal sites, pasture, cultivated land, gardens’ (PIER, 2013), whereas in Namibia, light to moderate infestations are observed along rivers and in disturbed agricultural areas such as cattle pens (Bethune et al., 2004). In Australia Parsons and Cuthbertson (1992) state its wide distribution is due in part to its ability to adapt to a range of climatic conditions and to thrive where the soil has been disturbed, allowing it to grow in pasture, wasteland, sheep camps, as well as coastal areas, watercourses, dam banks and floodplains. Where it is recently invasive in western China, it is also reported to occur in desert grassland and oases (Song et al., 2012).

X. spinosum is recorded as a weed of cotton, maize, mungbean, sorghum, soyabean, sugarbeet, sugarcane, sunflower and tomatoes, as well as many other annual and perennial crops.

Genetics

Both species of Xanthium are tetraploid with a chromosome number of 2n=36 (Love and Dansereau, 1959). Mitch (1987) provided detailed information on the origin, taxonomy, biology, identification, utilization, distribution and spread of both X. spinosum and X. strumarium. Both species may set seed without fertilization of the ovule, resulting in local populations where plants are genetically similar and may also differ only slightly from other populations.

Reproductive Biology

X. spinosum prefers moist soil and so seeds tend to germinate in late spring to late summer after early rains. Emergence can occur from spring until late autumn, and new fruits (or burrs) are produced two to three months after germination and emergence (Pitcher, 1989). Burrs have two elongated cavities, each containing a single seed, and an average of 150 seeds are produced by each plant.

The two seeds, however, have different characteristics. The lower of the two seeds has a shorter dormancy period, typically germinating a few months after maturity, whereas the upper seed can remain dormant for as long as eight years (Pitcher, 1989). This difference in dormancy may be due to site- and season-specific reasons (Auld, 1993), although the relative permeability of the seed coat to oxygen may also contribute, and high temperatures (32-38°C) can also overcome dormancy (Pitcher, 1989). Seed dormancy was not affected by partial fruit coat removal (Auld, 1993).

Optimal seedling emergence occurred when seeds were buried at depths of 1-4 cm, and was significantly reduced when buried to depths of 8 cm and deeper, and seeds in fruits placed on the soil surface failed to emerge entirely (Auld, 1993). In controlled environment studies and in the field, flowering was found to depend on day length, with flowering time decreasing with decreasing day length. Auld (1993) concluded that seed dormancy, the wide amplitude of seasonal emergence, the ability to flower quickly in cohorts and the ability of seed to germinate towards the end of the growing season could all potentially contribute to this plant’s success. Most plants die in the late autumn or early winter due to frost.

Associations

In the USA, Xanthium species are sometimes found growing with Arctium species, a European immigrant weed genus common in the USA.

Environmental Requirements

X. spinosum has proved remarkably adaptable to a wide range of climates and other environmental conditions, with the exception only of very cold polar/tundra conditions and very hot and dry or moist lowland tropical climates. In tropical regions it is generally only found at high altitudes. It appears to show no specific requirements regarding soil type.

Materials

• Poisonous to mammals

Medicinal, pharmaceutical

• Traditional/folklore

X. spinosum differs from X. strumarium by having much narrower leaves that taper at both ends, shorter petioles, conspicuous three-pronged spines at the leaf base and ovoid burrs covered with hooked thorns.

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.

Cultural Control

Due to the burrs adhering to wool, the risk of transferring seed to new areas can be reduced by shearing sheep before moving them from infected to clean pastures (Nursey, 1959). In Australia, X. spinosum is one of the most serious weeds spread by livestock, which carry burrs from one centre to another, so early recognition is essential for effective control (Gardner, 1942). Due to its toxicity and unpalatability, grazing is not a useful control method.

Physical/Mechanical Control

Control is possible by early mowing, cultivation or crop rotation, or by cutting and burning before the burrs ripen. As this annual weed spreads entirely by seed, it can be relatively easily controlled by cultivation on arable land, and on pastures. Eradication should aim to prevent seed development for at least three years (Orchard, 1949; Parsons, 1973). Single plants can be easily pulled out or hoed. If removal is done after flowering, it is important to burn the plants to ensure that the seeds are completely destroyed (Pitcher, 1989). Due to the long dormancy of the seeds, periodic repeated physical control will be required.

Chemical Control

The most common type of chemicals used to control X. spinosum are the broad-leaf selective herbicides. Young plants are easily killed by one application of 2,4-D or MCPA applied to healthy, growing, non-flowering plants, whereas older plants need more than one application (Meadley, 1956; Orchard, 1949). The most effective time for the application of 2,4-D is reported to be at the 3-5 leaf stage of growth, when it is able to better penetrate the waxy coat on the leaves (Pitcher, 1989), or before the flowering stage in February-March (Nursey, 1959). Imazaquin is also reported to be effective (Weber, 2003), and in irrigated soyabeans in New South Wales, Australia, imazaquin gave significantly more control of X. spinosum than bentazone, but did not control X. spinosum when applied after the formation of the 3 pronged spines (Andrews, 1993). However, Andrews (1993) considered that imazaquin provided the greater season-long control and was therefore the preferred herbicide.

Biological Control

The potential of using insects as a biological control agent has been assessed in Australia, India, Pakistan and United States, but with limited success (Pitcher, 1989). Investigations were conducted into the possibility of biological control by infection with the fungus Colletotrichum xanthii, a fungus which causes stem lesions (Orchard, 1949). The rust Puccinia xanthii was also found to attack several parts of the plant, reducing plant transpiration, burr production and seed germination (Pitcher, 1989).

The most effective mycoherbicide to date is Colletotrichum orbiculare. It can be applied in a similar way to herbicide applications and causes leaf and stem lesions, killing the plant in just 14 days under optimal conditions (Auld et al, 1988; 1990). Auld et al. (1990) undertook field applications of aqueous spore suspensions of 10⁶ or 10⁷ spores/ml at four sites, including a dryland pasture, an irrigated soyabean field and two sites where the effect of artificial dew was assessed using X. spinosum in pots in the field. Effectiveness was 50-100%, with the best results (98-100%) on the dryland grazing site. Providing an artificial dew period of 18 hours resulted in 100% mortality compared to only 50% under the natural dew period of 0.3 hours, with effectiveness correlated to humidity levels. It was considered that the formulation of a C. orbiculare spore suspension into a product with a low evaporation rate could play a significant role controlling X. spinosum (Auld et al., 1990).

X. spinosum has not been studied as thoroughly as X. strumarium. Further research is required regarding effective cultural and biological control.

13/11/13 Original text by:

Nick Pasiecznik, consultant, France