Invasive plants compete with native plants for nutrients, water and sunlight, and can quickly dominate areas into which they are introduced. Seed dispersal plays an important role in the introduction and establishment of invasive weed species in these new locations. Australia is troubled by weed invasion, with 28,000 alien species being deliberately or accidentally introduced since European settlement with c. 2,700 of these species becoming naturalized, 429 species being declared noxious and 20 being recently identified as ˜Weeds of National Significance" (WONS), with 17 of these present in Queensland. The range of infestation of these species is mostly extended by seed spread which occurs through different abiotic and biotic mechanisms. Among these, vehicular movement of seed is thought to play an important role, especially in Queensland.
To assess the role of small vehicles in the spreading of weed seeds, utility and sedan vehicles that had been in the field in south east and central Queensland were analysed for their weed seed loads. During these studies it was found that such vehicles can collect and transport a large number of germinable weed seeds (i.e. from 116 to 397 germinable seed per vehicle) coming from 25 to 59 species. The majority of these species were found to be introduced into Australia and some were of particular significance. For example, the seeds of parthenium weed (Parthenium hysterophorus L.) were found on vehicles that had been in the field in parthenium infested areas of central Queensland. Weed seeds were carried on almost all parts of the vehicles, either in mud or directly attached to vehicle parts with the majority of them found on the underside or on the back and front mudguards, and in the cabin.
To prevent weed seed spread by vehicles, cleaning procedures, including washing and vacuuming, are thought to be an effective management approach. As a result, more than 80 wash down facilities have been built in Queensland, but there are no data available to demonstrate their effectiveness. Therefore, studies were conducted in both controlled and field conditions, to assess their effectiveness and to determine if improvements could be made to them to increase weed seed removal. In controlled conditions, a high pressure-low volume cleaning system and a low pressure-high volume cleaning system were found to be equally efficient and able to completely clean baked mud in just 15 to 20 minutes. During the first 10 minutes of washing the removal of mud was slow, but in the later stages of cleaning the mud was much more rapidly removed. This was due to the time required to soften the dried mud before it could be removed from the vehicle.
Following these controlled experiments, trials were conducted at an automatic roadside, fixed wash down facility at Charters Towers, to determine its effectiveness in removing weed seeds from dirty utility vehicles that had undertaken field research in central Queensland. In these studies, the standard washing time recommended by the facility manager (i.e. 30 s) was compared to that of other times. It was found that the recommended time period at this wash down facility is quite ineffective in removing mud and seed from the utility vehicles. Only 14% of the mud and 11 seeds were removed from the utility vehicle after 30 s of washing. When this washing time was increased to 7.5 minutes, a significantly larger amount of mud (47%), and hence seeds, were removed from the utility vehicle. Moreover, washing for the shorter period of time usually only removed monocotyledonous weed seeds, while the removal of dicotyledonous weed seeds required at least 7.5 minutes washing time. Thus, the recommended washing time could not remove all of the weeds seeds, especially the dicotyledonous weed seeds, which include important weeds like parthenium weed.
In addition, the effect of pre-washing the vehicle was also investigated, to see its impact upon the removal of mud and seeds from utility vehicles. It was found that pre-washing for 5 s significantly improved the mud removal (69%), with higher numbers of seeds being removed during the 7.5 minute washing period. This is because pre-washing allowed the mud containing the seeds attached to the vehicle to be softened and loosened, following which it could be more easily removed. Moreover, other studies showed that different types of nozzles (i.e. fan, jet and spinning nozzles) varied in their effectiveness, and that spinning nozzles were more effective in removing mud and seeds from the utility vehicles.
Experiments were also conducted on other vectors, such as animals, fodder and seed lots, to investigate their potential contribution to weed seed spread. Dung samples were collected from three different locations in Queensland (i.e. Toogoolawah, Injune and Clermont). Dry and fresh dung samples contained an average of 287 and 262 germinable weed seeds per kg, respectively, which came from 55 species and represented 20 families. There was variation in the number of seeds in samples from different locations that were collected during different seasons. In addition, noxious weed seeds (e.g. parthenium weed) were identified in both dry and fresh samples of dung collected from Clermont in May 2010, when the parthenium weed plants were mature and shedding seed in the field.
Similarly, large numbers of weed seeds were found in the dung samples of different animals (e.g. cow, water buffalo, goat, sheep, horse and donkey) collected from Khyber Pakhtunkhwa Province in Pakistan in Winter 2010. All these animals were found to be involved in spreading a large number of weed seeds, which are all introduced to this Province. However, there were specific associations between certain weed species and certain animal species. For example, a large number of Poa annua L. seeds were found in horse dung samples, but not in the dung of goats and sheep. Thus, different types of animals spread the seeds of different weed species.
Experiments on weed seed spread in lucerne (Medicago sativa L.) and Rhodes grass (Chloris gayana Kunth) fodder were also conducted in Queensland. Lucerne and Rhodes grass fodder samples were collected from Laidley North, Monto and Biloela and were analysed for the presence of weed seeds. The seeds of numerous species of weeds were found to be spread in fodder, and the majority of these weed seeds were alien to Australia. In the lucerne hay collected from Monto, Biloela and Laidley North, all weed seeds identified were from introduced species, however no noxious weed seeds were present. In a separate experiment conducted in Khyber Pakhtunkhwa, large numbers of introduced species were found to be spread in wheat (Triticum aestivum L.) grain.
Factors such as their wide adaptability and high seed production, together with their broad distribution through a variety of vectors (e.g. air, animals, water, vehicles, agricultural and earth moving machinery, fodder and seed lots) have made certain kinds of weeds successful invaders in many parts of the world. Therefore, innovative techniques were also investigated that may be used to kill weed seeds in the soil prior to emergence, on vehicles, at wash-down facilities or in animal dung. Experiments conducted in the laboratory showed that dry weed seeds were only completely killed after exposed to temperatures as high as 100°C to 125°C. However, both large and small seeds could be completely killed when exposed to temperatures of 75°C if they were partially or fully imbibed. It was also found that this temperature could be generated in the field in the Queensland summer, in moist soil under clear plastic mulch. Solarization studies showed that all weed seeds were completely killed when kept under such conditions for 10 or more days either when they were at the soil surface or when they were buried to a depth of 2 cm. When exposed to increased levels of moisture, larger seeds were completely killed after 2 to 4 days of solarization and smaller seeds were killed after 6 to 8 days of solarization.
These findings demonstrate that the spread of weed seeds, and especially invasive weeds seeds, can be significantly reduced by using environmentally friendly preventative methods. In particular, this may be achieved by improving the design and recommended procedures of wash down facilities and by using solarization as a tool for killing weed seeds in the soil bed, in sludge or in animal dung.